anemia is a multifactorial condition whose prevalence increases in both sexes after the fifth decade of life , becoming a highly represented phenomenon in older adults . the estimated prevalence of anemia highly varies according to the study population , study setting , and health status , as well as the criteria to define anemia . the diagnosis is more frequently based on the world health organization ( who ) criteria reported in 1968 , as hemoglobin ( hb ) concentration < 12  g / dl and < 13  g / dl in nonpregnant women and men , respectively . data from the third national health and nutrition examination survey ( nhanes iii ) , by using the who definition , have estimated a prevalence of anemia of about 11% of community - dwelling older men and 10.2% of women , exceeding 40% among the oldest - old population ( 85 years and older ) . interestingly , the prevalence of anemia is greater among nursing home residents than in community - dwelling older individuals [ 2 , 5 , 6 ] . in the elderly , anemia is caused by different factors including nutrient deficiencies ( iron , folate , and vitamin b12 ) , occult hemorrhage , renal dysfunction , and chronic inflammation / diseases . however , data report that in about one - third of anemic older adults the determinants of this condition can not be clearly identified and anemia is defined as  unexplained  ( ua ) . anemia sine causa  is typically mild ( with hb levels approximately 1  g / dl lower than the who standard ) , featured by normal red blood cells size with no evidence for intravascular destruction or morphological features . all these characteristics make this condition not frequently recorded up to 75% of the cases . although mild anemia can be well tolerated in young and adult individuals , this condition in the elderly is a predictor of a progressive decrease in physical performance and muscle strength [ 10 , 11 ] , and an increased risk of recurrent falls , frailty status , cognitive impairment , and hospitalization [ 15 , 16 ] . moreover , large population - based prospective studies performed in community - dwelling older individuals identified anemia as risk factor for death , independently of multimorbidity [ 17 , 18 ] . an association between mild anemia and multiple adverse outcomes has also been shown among vulnerable class of individuals , such as those with chronic heart failure , chronic kidney disease , and diabetes mellitus . however , it is still unclear whether anemia has a causative role in the development of adverse events or is just a marker of the disease burden . efforts to understand the pathophysiology of the so - called anemia of aging have specifically targeted different physiological age - related changes including erythropoietin ( epo ) resistance and reduced proliferative capacity of bone marrow stem cells , stem cell aging , impaired renal function , myelodysplasia ( mds ) , and chronic inflammation with higher circulating levels of proinflammatory cytokines [ 8 , 23 , 24 ] . these contributing factors might lead to alterations in the red blood cell production and red blood cell survival . among the predisposing factors of anemia recent studies underline the role of hepcidin , an iron - regulating hormone , which at higher levels seems to favour both iron sequestration in reticuloendothelial cells and the alteration of gut ferroportin - mediated iron absorption . however , the role of hepcidin in the onset of ua is still controversial . interestingly , the age - related decline in androgen levels has been recently numbered among the factors involved in the pathogenesis of the so - called ua of the elderly [ 27 , 28 ] . epidemiological studies and several clinical trials have suggested a potential relationship between hb levels and the severity of androgen deficiency . serum insulin - like growth factor 1 ( igf-1 ) and insulin - like growth factor - binding protein 3 ( igfbp-3 ) , thyroid - stimulating hormone ( tsh ) , l - triiodothyronine ( t3 ) , and l - thyroxine ( t4 ) levels seem also to be involved in maintaining hb concentration under physiological and pathological conditions ( figure 1 ) . therefore , anabolic hormones , whose levels undergo a progressive decline during ageing , might represent a plausible cause of anemia when other causal pathways have been excluded . interestingly , the ageing model is frequently characterized by a simultaneous deficiency in anabolic hormones . dysregulation  is a more frequent phenomenon than a single hormonal derangement in older persons . this phenomenon mostly involves gonadal , adrenal , and somatotropic axes , with a subsequent decline in testosterone ( t ) , dehydroepiandrosterone sulfate ( dhea - s ) , and igf-1 concentrations , respectively , resulting in anabolic deficiency , an important determinant of accelerated aging process . there is recent evidence that anabolic hormones do not operate independently of each other and might have synergistic effects . it is very well known that t mainly acts as anabolic agent while gh exerts an anticatabolic affect . igf-1 can be also considered the cross - road of many stimuli including dheas and t . data from hagenfeldt and colleagues   suggest that androgen therapy is able to increase igf-1 levels , which may represent an important mediator of t action . a multiple hormonal derangement , rather than a single anabolic hormone deficiency , has been identified as a robust biomarker of health status in older persons . data from the aging in the chianti area ( inchianti ) study documented that the parallel decline in t , dheas , and igf-1 independently predicts 6-year mortality in older men . according to this concept a partial failure of a single hormone system might be compensated by one or more parallel systems without producing significant detrimental clinical effects , suggesting that the single hormonal derangement is a not specific predictive parameter in older individuals . data were especially focused on the erythropoietic properties of a single anabolic agent ( t , igf-1 , thyroid hormones , or estradiol ) , with limited evidence existing on the contribution of multiple anabolic pathways in the pathogenesis of anemia . thus , we performed an accurate review of the literature using pubmed until march 2015 , by selecting the observational human studies investigating the hormonal contribution ( testosterone , igf-1 , and thyroid hormones ) to anemia older subjects of both sexes . in this review , studies are presented according to the cross - sectional and longitudinal design and to the positive or neutral effects on different hematological parameters . we also analyzed the clinical trials having hematological indexes as the main or secondary outcome . the role of androgens on erythropoiesis is known since 1941   when , before the availability of recombinant human epo , they represented the main pharmacologic agents in the treatment of anemia of chronic and end - stage renal disease , as well as aplastic anemia [ 35 , 36 ] . 
 in vitro and animal studies have suggested a role for androgens in increasing erythroid mass , epo synthesis , and iron bioavailability . in bone marrow cultures , androgens stimulate hematopoiesis [ 3739 ] also improving erythroid and myeloid colony formation in semisolid cultures [ 40 , 41 ] . however , myelostimulating effects of androgens seem to target more mature erythroid progenitors rather than hematopoietic stem cells or immature progenitor cells . in human cell cultures , t exerts a direct role on bone marrow erythroblasts and red cell precursor survival [ 28 , 43 , 44 ] , by binding with a nuclear androgen receptor ( ar ) . interestingly , the pretreatment of bone marrow cells with cyproterone and flutamide , selective competitive blockers of nuclear ar , completely inhibits the beneficial effects of t on erythroid burst - forming units and colony - forming units . in mice , t administration increases hb levels and hematocrit as well as granulocyte and platelet numbers , with a greater effect in the oldest experimental study group [ 47 , 48 ] . most of the erythropoietic activities of t seem to be mediated by the stimulation of epo secretion [ 4951 ] and by the modulation of erythroid progenitor cells sensitivity to epo , overall resulting in an augment of the red cells production . in vitro studies have documented stimulatory properties for androgens on red cell synthesis , blocked by injection of anti - epo antibody . furthermore , erythroid progenitor cells from mice treated with t exhibited an increased proliferative response to epo [ 37 , 46 ] . however , a modest increase in epo concentrations has been also observed in patients who underwent total androgen blockade therapy ( short - term and long - term ) , after the decline in serum hb levels , independently of serum t concentration . there is evidence that t increases epo levels by modulating the hypoxia or hypoxic sensing , known to stimulate epo secretion . therefore , many effects induced by t ( including increased hb and hematocrit , increased red cell 2,3-bisphosphoglycerate , and increased muscle capillarity ) increase the net oxygen delivery to the tissue . alternatively , t could influence epo secretion via direct effects at renal peritubular fibroblasts level , establishing a new epo / hb a similar phenomenon is observed in post - transplant erythrocytosis , renal dysfunction , and some populations who live at high altitude . in contrast , other data describe a direct action of t in hypoxia - induced epo synthesis . these studies show no effect of t on epo transcription in hep3b cells , an epo - secreting cell line highly sensitive to hypoxic stimuli . however , mechanisms other than epo have been proposed to explain the role of t deficiency in the decline of both hb and hematocrit levels . animal [ 59 , 60 ] and human   studies reported that t may influence iron bioavailability and utilization . guo and colleagues   recently argued that the increased transferrin levels and reduced serum ferritin concentration could explain the increased hb concentration observed after t administration . however , t replacement therapy with t propionate seems more effective in stimulating erythropoiesis when chronic administration is established . it is very well known that serum transferrin concentration reflects the body iron status and erythroid transferrin uptake as well as total erythroid activity . an alternative hypothesis suggests a role for the suppression of hepcidin , the iron - regulating hormone factor , in the t - related erythrocytosis . testosterone might reduce hepcidin levels by decreasing inflammatory cytokines , especially interleukin-6 ( il-6 ) . elevated levels of il-6 are known to negatively influence erythropoiesis , also increasing the liver production of hepcidin through the hamp gene [ 7 , 65 , 66 ] . it is very well documented that t exerts anti - inflammatory properties by inhibiting the proinflammatory nuclear transcription factor kappa b ( nfb )   and the expression of the inflammatory mediator il-6 . bone marrow cell cultures treated with serum from patients affected by chronic disease exhibited a suppression of erythroid colony - forming units ( cfu - e ) . interestingly , this effect reversed after the administration of antibodies against tnf and ifn . in erythroid cells treated with t , an increase in both iron export from the spleen and iron availability for hb synthesis has been observed as consequence of the suppression of hepcidin and the upregulation of ferroportin ( the major cellular iron exporter ) . consistently , studies in mice with genetic alterations in iron regulatory genes , including inactivating mutations in the gene encoding hepcidin ( hamp ) , have shown a transient polycythemia . in humans , a number of epidemiological studies have hypothesized a relationship between serum t levels and the erythropoietic process ( table 1 ) . hemoglobin levels have been related to t status , with lower concentrations in severely hypogonadal men than in those moderately hypogonadal and eugonadal . therefore , the age - associated decline in t levels has been proposed to account for the decline in erythroid cell mass and the so - called unexplained anemia of older men . it is very well known that t serum levels decrease by 1% per year , and bioavailable t ( free plus albumin bound t ) by 2% per year , from the age of 35 . as a consequence , approximately 20% of men over 60 years old and 50% of men over 80 years old have serum t concentration below the normal range for young men . similarly , in women , t levels decrease from the age of 40 , approaching prior to menopause the 50% of those present at 30  yr . the most plausible causes of the reduced t levels observed during the aging process include impaired testicular responses to gonadotropin stimuli , coupled with incomplete hypothalamic - pituitary compensation for the fall in total and free t levels . in a recent population - based study , older men with unexplained anemia exhibited lower t levels when compared to their nonanemic counterparts . interestingly , a higher risk of anemia has been documented not only in men   but also in women   with low t levels . ferrucci and colleagues showed a significant linear relationship between bioavailable t and hb levels in 905 older participants of both sexes from the  invecchiare in chianti furthermore , low t levels were prospectively associated with an increased risk of anemia after a 3-year follow - up period . primary hypogonadism frequently occurs in elderly patients , while secondary hypogonadism is more commonly diagnosed in middle - aged men with multimorbidity including type 2 diabetes mellitus [ 71 , 104 , 105 ] . grossmann et al . in a cross - sectional cohort study , performed on 464 men with type 2 diabetes , demonstrated that low t levels were independently associated with anemia . accordingly , bhatia et al .   documented a lower hematocrit together with elevated plasma c - reactive protein ( crp ) concentrations in 70 patients with hypogonadotrophic hypogonadism and diabetes . thus , both low t and chronic inflammation could be identified as risk factors for mild anemia in men with type 2 diabetes . as widely documented , pituitary adenomas are another possible mechanism involved in the development of secondary hypogonadism . in a retrospective review of 197 male patients with pituitary macroadenomas and low t concentrations , ellegala et al . demonstrated an impairment of hematopoiesis arguing that t levels are independently correlated with hematocrit . it is very well known that the most frequent cause of hypogonadism among the general male population is the androgen deprivation therapy ( adt ) , commonly used in managing recurrent , locally advanced , and metastatic prostate cancer . interestingly , cancer patients undergoing adt are more frequently anemic [ 108 , 109 ] than those not on adt , further supporting the hypothesis that t is able to influence hb levels . patients with androgen deficiency related to orchiectomy or pharmacologic androgen ablation for prostate cancer typically have a drop of 1  g / dl in hb levels [ 110112 ] . interestingly , after a complete hormonal ablation and radiotherapy , this decrease can be seen as high as 2.5  g / dl . anemia associated with adt is also a frequent feature in men receiving combined hormone blockade . in this case , the anemia is usually normochromic , normocytic , temporally related to the initiation of androgen blockade , and reversing after combined hormone blockade discontinuation . finally , the association between t and decline in hepcidin levels in the elderly is still uncertain . in fact , no correlation between hepcidin and t levels was found in a large cohort of patients with unexplained anemia compared to matched , nonanaemic individuals and to patients with known causes of anemia . a meta - analysis of 51 intervention studies using different preparations of t showed a significant increase in both hb ( weighted mean difference ( wmd ) : 0.80  g / dl ; 95% ci : 0.451.14 ) and hematocrit ( wmd : 3.18% ; 95% ci : 1.355.01 ) . in particular , t administration proved effective in increasing hematocrit in older hypogonadal males ( table 1 ) . in a preliminary alternate - case controlled trial , older hypogonadal males ( mean age 77.6  2.3 ) with bioavailable testosterone levels less than 70  ng / dl , receiving testosterone enanthate ( 200  mg / ml ) intramuscularly every 2 weeks for 3 months , showed significantly increased levels of total and bioavailable testosterone concentration and hematocrit . steidle and colleagues , in 406 hypogonadal older men treated with two doses of aa2500 t gel ( 50  mg / day and 100  mg / day ) or t patch ( two patches delivering 5  mg t daily ) , reported increased haemoglobin and haematocrit values after 90 days of treatment , with greater levels in the topical testosterone gel . interestingly , snyder et al . showed that t patch treatment is an effective strategy to improve hematocrit , with an effect maintained during the 3-year follow - up period . the effectiveness of other molecular forms and different dosage of injectable t in improving hematopoietic parameters has also been studied . sih and colleagues showed a significant rise in hb concentration in men receiving injections of placebo or 200  mg of t cypionate biweekly for 12 months . . underlined that different formulations of t in hypogonadal men had different effects on t levels . in particular , the authors showed that t enanthate ( te ) 250  mg for 21 days was able to induce a higher elevation of hb by 21.1  2.6 the linear , dose - dependent increase in hb levels was also shown by coviello et al .   in healthy young and older men concomitantly receiving a long - acting gonadotropin releasing hormone ( gnrh ) agonist and graded doses of t enanthate . in this study , older people experienced a greater increment in both hb concentration and hematocrit but not in serum epo concentrations when compared to younger men . conversely , a double - blind placebo - controlled trial evaluating the effects of t patch therapy on bone turnover in 39 men ( range 4077 years ) showed that 5  mg / day of t therapy for 6 months led to an increase in hb level but not to significant changes in hematocrit . in anemic male patients , in particular , alexanian   documented that fluoxymesterone treatment in normal and anemic men ( dose of 40  mg / m ) , hypogonadal men , and anemic women ( dose of 10  mg / m ) drove a 5- to 10-fold greater increase in the urinary concentrations of epo . unlike animal models , humans are able to retain the responsiveness to androgens even after nephrectomy , probably because of the presence of extrarenal sources of epo . however , several studies failed to prove the effectiveness of transdermal androgen therapy in increasing epo levels in anemia of chronic kidney disease   or in healthy older men with serum t concentration < 475 ng / dl . in healthy young ( aged 1935 years ; n = 53 ) and older men ( aged 5975 years ; n = 56 ) , the administration of weekly graded doses of t enanthate ( 25 , 50 , 125 , 300 , and 600  mg ) over a 20-week period induced a significant increase in hematocrit together with decreased hepcidin levels . however , it should be underlined that erythrocytosis is a typical phenomenon occurring in diseases featured by high t levels [ 120 , 121 ] . therefore , the detection of erythrocytosis as the most common adverse event of t replacement therapy both in clinical practice and in t trials [ 122124 ] is highly expected . erythrocytosis may decrease organ perfusion and the rise of hematocrit levels has been linked to cardiovascular complications . based on these evidences , the endocrine society guidelines on androgen deficiency syndromes in men recommend monitoring haematocrit after 3 months from the initiation of t therapy and annually thereafter . however , the clinical benefits and long - term risks associated with t replacement therapy , particularly in prostate - related and cardiovascular - related adverse events , have not been adequately assessed in large , rcts involving older men . more recently , snyder and colleagues   have described a coordinated set of seven ongoing clinical trials , designed with the aim of determining the efficacy of t treatment in older men with low t and symptoms or signs of impaired mobility , diminished libido , and reduced vitality on objective measures of age - associated conditions , including anemia . convincing and definitive data are expected to come when these ongoing t trials will be completed . in the last five decades , further efforts have been made to outline the potential mechanisms underlying the relationship between androgens and hb and hematocrit . therefore , the usefulness of t therapy in anemic men exhibiting an age - related decline in t levels remains to date a controversial issue . estrogens levels , estradiol ( e2 ) and estrone ( e1 ) , are mainly the products of the aromatase - catalyzed t metabolism in men . similarly to what has been observed for t and igf-1 , these hormones undergo a progressive decline during the aging process , even with different trajectories compared to women . several investigations suggest potential erythropoietic properties for e2 . in particular , in vitro and in vivo studies have documented that e2 inhibits both hepcidin transcription and synthesis [ 129 , 130 ] . yeap and coworkers in a cohort of community - dwelling adult men ( mean age 50 years ) showed that e2 levels were positively correlated with hb . moreover , e2 , but not t , was also identified as an independent predictor of hb concentrations in 918 men aged 7081 from the mros ( osteoporotic fractures in men study ) . several lines of evidence however seem to support the hypothesis that e2 might modulate the hypoxia - induced epo expression [ 133 , 134 ] . a potential role for igf-1 in erythropoiesis has been hypothesized in experimental and animal studies , where igf-1 was able to positively influence various steps of the hematopoietic process . in particular , these biological activities seem to take place through igf-1 specific binding with two high - affinity membrane - associated receptors , both in precursors and mature erythrocytes [ 137 , 138 ] . by these mechanisms , igf-1 contributes to maintaining normal erythropoiesis , granulopoiesis , and lymphopoiesis [ 139 , 140 ] and may regulate several neoplastic haematopoietic processes . in vitro studies have shown that igf-1 directly stimulates the proliferation and differentiation of the late stage of primitive erythroid progenitor cells and/or early erythroid progenitor cells [ 137 , 141144 ] . moreover , circulating erythroid progenitors of hematopoietic cells from patients with polycythemia vera are hypersensitive to igf-1 treatment . both igf-1 and igf - binding proteins are synthesized and secreted by tc-1 murine bone marrow stromal cells . experimental studies have also clearly documented an effect of igf-1 on the expansion of the primitive multipotential cd34 + cd38 + stem cells [ 146 , 147 ] . it is very well known that cd34 + cd38 + stem cells account for the majority of bone marrow hematopoietic stem cell activity . therefore , a role for igf-1 similar to that of hematopoietic cytokines has been hypothesized . moreover , igf-1 administration in neonatal   or hypophysectomized animals   enhances erythropoiesis . at the molecular level , igf-1 seems also able to increase the expression of cell - surface transferrin - receptors by determining a redistribution from the intracellular compartment to the cell surface . there is further evidence that some of erythropoietic effects of igf-1 could also be driven by the interactions existing between igf-1 and igf-1 binding proteins ( igfbps ) with transferrin , which is known to be the major iron - carrying protein in serum . in vitro , igfbp-3 leads to a suppression of igfbp-3-mediated cellular proliferation and apoptosis , by interacting with transferrin . igfbp-3 is the storage protein for igf-1 , able to exert paracrine / autocrine actions on cellular growth via igf - dependent and igf - independent mechanisms . interestingly , several evidence indicate that some of igf-1 erythropoietic effects could be synergistic with epo and that igf-1 might replace epo as a stimulator of erythropoiesis [ 138 , 152 , 153 ] . in mice with chronic kidney disease - associated anemia , combined subtherapeutic doses of epo and igf-1 had a comparable effect to a single therapeutic dose of epo on hb concentrations . in hormone - depleted cell culture systems , physiological concentrations of igf-1 ( 0.51  ng / ml ) , in the presence of epo , stimulate erythroid cell growth and differentiation from bone marrow or peripheral blood . in 1989 brox et al . , by analyzing an anephric patient 's serum with nearly normal hematocrit and a low - to - normal epo levels , identified a specific 8-kd epo - like peptide factor , capable of stimulating late erythropoiesis . interestingly , congote et al . argued that this erythroid cell - stimulating agent has to be identified in the human igf-1 . however , the relationship between igf-1 and epo remains still unclear , with evidence yielding conflicting results . in male and female rats during accelerated growth igf-1 , but not epo , concentrations were linearly correlated with total iron incorporation into red blood cells . similarly , several lines of experimental evidence failed to report any increase in epo and erythropoiesis indices after igf-1 treatment . kling et al . , in suckling rats treated with igf-1 for 4 days , observed higher hb levels , red blood cell counts , and hematocrit , but no significant changes in plasma epo levels or reticulocyte counts or plasma iron and erythrocyte iron incorporation rate . consistently , short - time igf-1 administration did not promote epo secretion and erythropoiesis in rat models of protein malnutrition . finally , gh and igf-1 were capable of inhibiting epo secretion both in vivo and in vitro . igf-1 might also influence the hematopoietic process and the development of anemia by modulating inflammation . it has been also demonstrated that chronic inflammation , as a result of the unpaired balance between anti - inflammatory and proinflammatory mediators , could be implicated in the pathogenesis of mild anemia in the elderly . in fact , immune and endocrine systems interplay in maintaining homeostasis . from this point of view , igf-1 has been shown to play a regulatory action in a variety of immune events . if we assume that igf-1 plays an anti - inflammatory effect , decreased igf-1 concentrations may account for the increased levels of inflammatory molecules , which are known to negatively affect epo secretion and red cell precursor survival , ultimately resulting in anemia . in humans , igf-1 has been hypothesized as a mediator of anemia under physiological and pathological conditions ( table 2 ) . however , the involvement of igf-1 in maintaining hb levels has been particularly investigated only in the pediatric population . it has been estimated that about 30% of erythropoietic process in this age - group is influenced by the effect of igf-1 . in fact , the iron deficiency anemia is a frequent finding in children exhibiting an impaired gh - igf-1 axis . a positive correlation between hematocrit and serum igf-1 levels has been also documented in children with sickle cell anemia . igf-1 also seems to be a regulator of erythropoiesis in a pediatric population with short stature   and gh - deficient adults . decreased igf-1 levels have been documented in children with suboptimally treated transfusion - dependent -thalassemia compared to children with transfusion - independent -thalassemia . in these subjects , the correction of anemia was associated with a significant increase in igf-1 levels . the potential role of igf-1 in the regulation of human erythropoiesis has also been confirmed by studies performed in patients with laron syndrome ( primary igf-1 deficiency ) , where the infusion of igf-1 significantly raises the red blood cell parameters . interestingly , igf-1 seems also involved in the regulation of erythropoiesis in uraemic patients , especially when a severe secondary hyperparathyroidism coexists . the role of igf-1 in adult and older people affected by anemia of chronic kidney disease ( ckd ) has been also studied . in such cases , the onset of anemia depends on several factors , including the relative or absolute deficiency in epo production , the shorter survival of red blood cells , the presence of unknown inhibitors of erythropoiesis in uremic phase , hyperparathyroidism , accumulation of aluminium , and nutritional deficiencies ( iron , vitamin b12 , and folate ) . decreased igf-1 levels were also observed in adults and older patients affected by diabetes - related ckd ( dm - ckd ) . compared to those subjects with nondiabetic ckd , patients with dm - ckd experienced anemia at an earlier stage of the disease . kim and colleagues found significantly lower serum igf-1 levels and hb concentrations in a group of dm - ckd subjects than in matched ckd patients without diabetes . similarly , studies performed in healthy prepubertal and pubertal subjects suggested a positive correlation between hb concentration and both serum igf-1 and igfbp-3 levels . in female adolescents , aged 14 to 17  yr , low igf-1 concentrations were associated with a greater incidence of iron deficiency anemia . in nondiabetic adult subjects with low levels of igf-1 , succurro et al . identified suboptimal igf-1 levels as a contributor to the mild anemia by showing a 2.49-fold increased risk in the group with the lowest igf-1 quartile . although the potential role of igf-1 in maintaining hb concentrations has been addressed in observational studies including subjects of different ages and diseases , very few studies have been carried out in the elderly . this is surprising , because hyposomatotrophism , characterized by typical gradual decline and alteration in gh secretion pattern and igf-1 production , is a frequent feature of the aging process . in fact , the daily secretion of gh undergoes a progressive reduction with age , esteemed around 14% per decade after puberty . as a consequence of this physiologic process , both igf-1 and igfbps serum levels tend to decline older men are at a igf-1 level below the lowest serum concentration of 16  nmol / l . in women , the age - related decline in gh - igf-1 axis activity is known to be a predisposing factor for several clinical adverse consequences , overall termed as  somatopause  . based on the existing evidence linking low igf-1 levels and anemia in other populations , it could be speculated that a reduction in igf-1 levels with aging might contribute to anemia . therefore , we believe that the onset of anemia in the elderly should be also considered in the context of somatopause . previous investigations in a representative sample of 938 older men and women aged 65 years from the inchianti study showed that igf-1 levels are independently and positively associated with hb concentration . in men , but not in women , some few additional studies were carried out in older individuals to address the relationship between igfbp-3 levels and indexes of erythropoiesis . nilsson - ehle and colleagues studied a community - based cohort of 297 older subjects aged 70 years and showed that igf-1 , but not igfbp-3 , levels influence hb concentrations independently of epo levels , sex , and health status . these data are consistent with those of a large study cohort of community - dwelling older individuals from the aging and longevity in the sirente geographic area ( ilsirente ) study , showing that those with igfbp-3 concentrations > 145.95  nmol / l had a significantly higher mean hb concentration compared with those in the lower igfbp-3 level group ( 13.4  1.4  g / dl versus 12.9  1.9  g / dl , resp . intervention studies testing the effect of gh therapy on indexes of erythropoiesis have been focused on gh - deficient children or adults with ckd . in these contexts , rcts seem to support the beneficial effect of gh on anemia [ 178 , 179 ] . rhgh administration in pediatric patients with idiopathic gh deficiency - related anemia resulted in normalization of hb concentrations and enhancement of erythroid colony growth . more interestingly , the correction of anemia by blood transfusion in subjects affected by -thalassemia and failure to thrive rapidly leads to an improvement in the gh - mediated igf-1 and igfbp-3 secretion . similarly , short - term continuous subcutaneous human gh administration ( rhgh 2  g / kg body weight per 0.1  ml / h for 72  h ) in anemic patients with chronic renal failure was able to ameliorate plasma epo levels and reticulocyte counts . in particular , igf-1 seems to play an important role in the regulation of erythropoiesis in patients with end - stage renal disease and erythrocytosis without an increased epo production . in a small rct conducted in malnourished dialysis patients , hb levels in malnourished elderly patients who underwent a 4-week low - dose rhgh administration ( 0.03  cg / kg 3 times per week ) . by contrast , a double - blind , placebo - controlled study of gh treatment in elderly patients on chronic hemodialysis did not show any effect of gh on hb levels . however , it should be underlined that the dose of gh highly differed between the two considered studies . finally , several trials were designed in order to address the effect of epo replacement therapy on anemia and igf-1 . however , the effect of epo therapy on igf-1 serum level is still debated . in uraemic patients , epo administration led to an enhancement of the gh response to the growth hormone releasing hormone ( ghrh ) . in hemodialysis patients , epo therapy was shown to be able to partly correct perturbations in the gh secretory axis , modulating the serum concentration of igf-1 and igf binding protein-3 [ 183 , 184 ] . the relationship between thyroid diseases and erythropoietic dysfunctions has been documented in experimental and human studies . in both hypo- and hyperthyroidism , studies have shown red cell abnormalities [ 186 , 187 ] as well as negative changes in myeloid cell lines . in particular , there is evidence that thyroid - stimulating hormone ( tsh ) , l - triiodothyronine ( t3 ) , and l - thyroxine ( t4 ) might exert a direct role in ensuring normal erythropoiesis [ 189191 ] . however , the potential underlying mechanisms by which thyrotropin and thyroid hormones influence anemia have not been fully elucidated . tsh could affect hematopoiesis by binding to a functional thyrotropin receptor ( tshr ) , which is found in both erythrocytes and some extrathyroidal tissues . interestingly , haematopoietic progenitor cells of hypo- and hyperthyroid patients showed a reduction of their proliferative potential together with negative changes in tshr and pro- and antiapoptotic genes expression . there is also in vitro evidence that both t3 and t4 are implicated in the regulation of hematopoiesis by influencing the erythroid precursor proliferative capacity . in rats with bilateral nephrectomy or subdued to bilateral ureter ligature , t3 and t4 were shown to be able to stimulate bone marrow erythropoiesis , especially at higher plasma levels of free active forms . in particular , t3 has been shown to enhance the release of growth factors from leukocytes and to potentiate the erythroid burst - forming unit ( bfu - e ) proliferation [ 195 , 196 ] . it may be involved in the control of growth and apoptosis of hematopoietic cells and bone marrow tissue . moreover , t3 seems also to influence the proliferation of erythroid cells together with inhibitory properties on epo - induced differentiation . sullivan and mcdonald showed a direct 2-adrenergic receptor - mediated stimulation of red cell precursors by t4 . at the molecular level , the effects of thyroid hormones on erythropoiesis seem to be mediated by the t3 binding to specific nuclear receptors , that is , tr and tr , which are encoded on separate genes . the endogenous receptor alpha ( c - erba / tr ) and the closely related retinoic acid receptor  ( rar ) have been shown to play a role in the regulation of normal erythroid differentiation . interestingly , an alteration in c - erba / tr content seems to negatively affect the balance between erythroid proliferation and differentiation in chicken erythroblasts . moreover , a mutated form of c - erba / tr receptor ( namely , v - erba ) was responsible for the development of avian erythroleukemias in animals . these data are also supported by studies in tr knockout mouse , where negative changes of fetal and adult erythropoiesis , with reduced numbers of erythrocyte progenitor cells and an impaired erythroid maturation , occurred [ 202 , 203 ] . it is very well known that thyroid hormones exert a stimulatory action on the 2 - 3-diphosphoglycerate concentrations , which is responsible for the delivery of oxygen to the tissues [ 185 , 186 ] . interestingly , both lower plasma epo levels and bone marrow repression have been found in hypothyroid anemia . in human hepatoma cell line , t3 and t4 were able to stimulate erythropoiesis by hypoxia - induced epo formation in the kidney in a dose - dependent fashion . thyroid hormones have been proposed to influence epo gene expression via the induction of hypoxia - inducible factor-1 alpha ( hif-1 ) synthesis . according to this molecular mechanism , thyroid hormones might act in concert with hypoxic accumulation and activation of hif-1 . however , several lines of evidence strongly argued against the hypothesis that anemia of hypothyroidism is to be exclusively attributable to insufficient epo levels . in fact , notwithstanding the frequent detection of a normocytic normochromic anemia in hypothyroid patients related to the impaired red blood cell mass and the erythroid progenitors hypoproliferation , a normal erythrocyte life span can be observed . the reduced iron utilization , the ineffective erythropoiesis and , in long - standing severe hyperthyroidism , malnutrition   have been proposed as additional causal factors leading to anemia of thyroid disorders . a recent review indicates that thyroid diseases , hypothyroidism , and hyperthyroidism ( overt and subclinical ) might cause anemia by modifying the coagulation - fibrinolytic balance . the potential relationship between thyroid hormones and iron there is evidence that thyroid hormones might increase iron absorption and iron incorporation into erythrocytes , with studies showing increased tsh and lower serum t3 and t4 levels   in iron - deficient subjects . conversely , there is evidence that iron deficiency might negatively affect thyroid metabolism by decreasing oxygen transport ( impairing nuclear t3 binding and thyroid peroxidase activity ) . a large body of observational evidence suggests an association between thyroid hormones and anemia ( table 3 ) . in particular , pernicious anemia is a frequent finding of hyperthyroidism and thyroiditis , diagnosed in as much as 2060% of affected patients . anemia of hypothyroidism is more frequently normochromic normocytic , hypochromic microcytic , and macrocytic , with a severity that varies according to hypothyroidism degree . in contrast , hyperthyroid individuals quite commonly exhibit a concomitant increase in red cell plasma volume and erythrocytosis [ 207 , 214 ] . interestingly , morphological features of erythrocytes are similar to that of hypothyroidism . in 1975 , fein and rivlin found a strong association between graves ' disease and the risk of anemia . subsequently , horton and colleagues   observed lower red blood cells number in the peripheral blood of patients who underwent thyroidectomy . in hyperthyroid patients , kawa et al .   showed higher red blood cells counts , hb concentration , and hematocrit whereas lower erythrocyte count and hb concentrations were found in those who are hypothyroid . a retrospective analysis estimated that anemia is a frequent finding in thyroid disease patients , occurring in up to 40.9% and 57.1% of those hyperthyroid and hypothyroid individuals , respectively . moreover , the authors showed lower mean corpuscular hemoglobin ( mch ) and mean corpuscular hemoglobin concentration ( mchc ) and higher mcv in hypothyroid individuals . there are several investigations supporting a tight relationship between alterations in bone marrow function and immunological thyroid disorders . a retrospective analysis involving 388 patients with immunological and nonimmunological thyroid disorders ( age range 1489 years ) undergoing thyroidectomy showed lower white and red blood cells values , as well as hemoglobin concentrations in the immunological group . lima et al . by describing four cases of severe pancytopenia in a cohort of patients with thyrotoxicosis by graves ' disease , concluded that thyroid evaluation should be performed in those with pancytopenia even in absence of specific thyroid symptoms . additional evidence also supports a significant association between thyroid hormones and hematological parameters in euthyroid subjects . bremner and coworkers in a population - based cohort of 1011 euthyroid older individuals showed that free t4 and free t3 levels , but not tsh , were positively and linearly associated with hb concentration , erythrocyte count , and hematocrit . however , no adjustment for potential confounders including nutritional parameters ( vitamin b12 , folic acid , and iron ) and renal function was applied . similarly , schindhelm and colleagues in a study population cohort of 708 euthyroid subjects ( aged 5585 ) from the longitudinal aging study amsterdam demonstrated that free t4 was significantly associated with hemoglobin , hematocrit , and erythrocyte count , whereas tsh was not . the relationship between tsh , free t4 , and erythrocyte parameters was further assessed by lippi et al . in a retrospective investigation in 1050 euthyroid outpatients aged 50 years or older . in spite of the lack of association between tsh , ft4 , and hb or hematocrit , the authors found a significant and positive association between the red blood cell distribution width ( rdw ) values and thyroid hormone concentrations . ft4 has been also positively correlated with the number of red blood cells and hb concentration and inversely correlated with mcv and mch . the association of free t4 with erythrocyte indices , including hb , red blood cell count , mcv , rdw , and ferritin concentration , has been further assessed in euthyroid adults with or without c282y homozygosity from the haemochromatosis and iron overload screening study . in this study , free t4 was positively correlated with hb concentration and negatively correlated with rdw in those individuals without c282y homozygosity . finally , the association between ft4 and anemia was analyzed in relation to ethanol consumption , given the known relationship existing between alcoholism and thyroid function . in a study cohort of 843 euthyroid men aged 3089 years from the nagasaki islands study , a significant inverse association between ft4 and risk of anemia was found only in those who did not consume ethanol . however , in spite of the evidence supporting the relationship between anemia and overt thyroid disease , there is a limited number of studies targeting subclinical thyroid conditions , especially in older individuals . this is surprising because subclinical thyroid diseases , defined by circulating concentration of free t4 and free t3 within their respective reference range , in the presence of abnormal circulating concentration of tsh , are more prevalent in the elderly compared to adult or young populations . in this specific age - group , the prevalence of hyperthyroidism ranges from 0.5 to 3% , whereas subclinical hyperthyroidism affects 1 - 2% and 7 - 8% of subjects in iodine - sufficient and iodine deficient areas , respectively [ 220 , 221 ] . this is the result of physiological changes in thyroid hormone production , metabolism , and action occurring during the aging process , similarly to what has been described for t and igf-1 . in particular , serum tsh and total and free t3 levels progressively decline with age whereas serum total and free t4 levels tend to remain more stable . this term describes a condition with low serum t3 concentrations , abnormal t4 to t3 conversion and higher concentrations of reverse t3 ( rt3 ) ( the inactive metabolite of t4 ) without any obvious sign of thyroid disease . low t3 syndrome has been associated with a poor health status . in a retrospective cross - sectional analysis conducted in 600 untreated and treated subclinical hypothyroid and primary hypothyroid patients ( aged 2560 years ) , bashir and colleagues showed negative changes in different hematological indices ( hb , rbc , mcv , hct , rbc% , and rdw ) only in those untreated individuals with subclinical and primary hypothyroidism . several lines of evidence apply to the specific setting of subclinical hypothyroidism and iron deficiency anemia . in a case - control study conducted in 57 subclinical hypothyroid women and 61 euthyroid controls , despite the prevalence of iron deficiency , anemia did not significantly differ between the two groups , while mean serum iron and ferritin concentrations were lower in the group with subclinical hypothyroidism . consistently , bremner et al . underlined that iron utilization and/or transport might be impaired in subclinical hypothyroid conditions , with lower serum iron concentrations and transferrin saturation in the subclinical hypothyroid compared to the euthyroid . there are also several data supporting the hypothesis that anemia in patients with autoimmune thyroid diseases is more related to a cobalamin deficiency for concomitant autoimmune gastrointestinal diseases . in fact , these group patients exhibit a higher frequency of chronic unexplained anemia ( defined as anemia not related to evident or occult bleeding and/or to erythropoietic disorders ) . it has been demonstrated that anemia of cobalamin deficiency due to an occult chronic atrophic gastritis can occur in multiple autoimmune processes , which include autoimmune thyroid diseases . however , to date , a screening for either b12 and/or folic acid deficiency in hypothyroid subjects or those with subclinical thyroid function is not routinely recommended [ 95 , 213 ] . mehmet et al .   did not observe any difference in vitamin b12 and folic acid concentrations between subjects with subclinical and overt hypothyroidism , compared to healthy controls . similarly , lippi and colleagues , in a retrospective study performed in 946 hyperthyroid or hypothyroid outpatients aged 2187 , reported differences in neither folic acid nor vitamin b12 concentrations . this is consistent with data from caplan et al . in a study population composed of 56 hypothyroid , 47 hyperthyroid patients , and 103 age- and sex - matched healthy controls . the lack of an association between vitamin b12 and thyroid hormone status was similarly confirmed by stella et al . . the authors showed that , in a cohort of 279 older adults , levels of serum vitamin b12 were independent of serum tsh quartile distribution . there are very few intervention studies that have tested the effects of thyroid hormone replacement therapy on hb concentrations and erythrocyte parameters ( table 3 ) . in a study involving 202 hypothyroid patients , levothyroxine treatment normalized anemia by also reducing the degree of anisocytosis in 25% of cases , in the presence of normal vitamin b12 , folic acid , and iron concentrations . in young and nonobese untreated subclinical hypothyroid patients , nekrasova and colleagues   documented an increased frequency of anemia that was normalized after starting substitution therapy . similarly , in 70 iranian individuals with primary hypothyroidism , three months of levothyroxine administration induced a significant improvement in different hematologic parameters . however , in a randomized , double - blind placebo - controlled trial performed in 63 women ( mean age 58.5  1.3 years ) with subclinical hypothyroidism , christ - crain and colleagues did not show any significant change in hb concentration and hematocrit after 48 weeks of levothyroxine treatment ( 85.5  4.3  g / day ) , despite demonstrating an increase in epo levels . additional evidence suggests that a combined treatment with levothyroxine and iron salts is more effective in inducing an improvement in hb and ferritin concentrations than levothyroxine or iron salts alone . data from a recent 3-month rct including 60 patients with subclinical hypothyroidism and iron deficiency anemia interestingly showed a greater improvement in several hematological parameters in the treatment arm receiving both medications than in the control arm receiving only one of the two . showed that the t4 coadministration with iron ( 240  mg / day oral iron plus 75  g / day levothyroxine ) , compared to iron in monotherapy ( 240  mg / day oral ) , was responsible for a fivefold increase of hb concentration as well as better blood count indexes [ a mean of 0.4  g / dl in the iron group versus 1.9  g / dl in the iron / levothyroxine group ] . interestingly , isolated evidence indicates that iron supplementation may have a beneficial effect on thyroid function , even in iron - deficient patients without iodine deficiency . however , the conclusions of these studies highly differ from those published by gokdeniz et al .   where iron replacement in patients with iron deficiency anemia and normal thyroid function led to a significant decrease in tsh level and to increased free t4 levels . therefore , there is very limited evidence suggesting that subclinical and overt thyroid diseases are implicated in the genesis of unexplained anemia . moreover , there is much uncertainty on whether thyroid hormone replacement therapy might be considered an effective strategy to improve anemia . ageing model is more frequently characterized by a simultaneous deficiency in anabolic hormones with  multiple hormonal dysregulation  rather than a single mild hormonal derangement . an imbalanced anabolic - catabolic equilibrium , which favours catabolism , has been hypothesized as the key pathway of the accelerated aging process . there is recent evidence that anabolic hormones do not operate independently of each other and might have synergistic effects . it is very well known that t mainly acts as anabolic agent while gh exerts an anticatabolic effect . igf-1 can be also considered the cross - road of many stimuli including dheas and t . hagenfeldt and colleagues concluded that androgen therapy is able to increase the level of igf-1 , which may represent an important mediator of t action . maggio and colleagues showed that age - related decline in anabolic hormone levels is a strong independent predictor of 6-year mortality in older men . interestingly , multiple hormonal deficiencies represent a more robust biomarker of health status in older persons than a deficiency in a single anabolic hormone . this concept is of paramount importance because a partial failure of a single hormone system might be partially or fully compensated by one or more parallel systems without producing a significant detrimental clinical outcome . this phenomenon might explain why symptoms associated with the single hormonal derangement are often not specific . thus , clinical and research efforts to understand the hormonal contributors to anemia should account for all the overall anabolic hormonal status . anabolic hormonal pathways could be linked to each other to some extent in influencing different steps of the erythropoietic process . however , very few studies have investigated the potential interaction between anabolic hormones ( such as t , e2 , sex hormone binding globulin ( shbg ) levels , and gh / igf-1 axis ) and erythropoiesis . the effects that a combined androgen and estrogen administration might have on some indexes of erythropoiesis were evaluated by rochira and colleagues in two adult men with aromatase deficiency . these authors used a human model of congenital estrogen deficiency to avoid the concomitant presence of a functioning aromatase enzyme , able to convert exogenous androgens to an intact hypothalamic feedback . the authors found that t , but not e2 administration , was able to increase hb concentrations , hematocrit , and red cell count . thus , it might be assumed that t aromatization to e2 is not involved in the stimulation of erythropoiesis . the interrelationship between serum t , shbg , and e2 and hematocrit has been cross - sectionally evaluated in 1273 men aged 20 years enrolled in the nhanes iii survey ( 19881991 ) . in this study , low free t and high shbg levels were associated with low hematocrit levels , whereas total and free e2 levels were directly associated with hematocrit . nevertheless , in 492 middle - aged and older men , yeap et al . observed that total and free t , but not shbg , were significantly associated with hb levels , even after adjustment for confounders . some of t activities on erythropoiesis could also be linked to the gh / igf-1 anabolic pathway . there is a significant evidence that these two hormones stimulate each other 's production and potentiate their respective anabolic or anticatabolic effects . in normal   and hypogonadal men , t administration was able to increase serum igf-1 levels . moreover , the combined administration of gh and t resulted in greater anabolic efficacy compared to the single hormone administration [ 236 , 237 ] . however , despite the observed increase of igf-1 levels in men receiving adt for localized prostate cancer , colloca et al .   showed a decline in the rbc count and hb concentration after adt treatment . carlson et al . did not appreciate any significant change in hormonal parameters including prolactin , testosterone , lh , fsh , tsh , free t4 , t3 , gh , and igf-1 , before and during epo therapy in a small cohort of older adults with end - stage renal disease under hemodialysis . finally , there is evidence that hormonal or growth factors , including insulin , igf-1 , igf-2 , egf , and basic fibroblast growth factor-2 ( fgf-2 ) , might influence epo gene expression via the induction of hypoxia - inducible factor-1 alpha ( hif-1 ) synthesis [ 240243 ] . according to this molecular mechanism , these factors might act in concert with hypoxic accumulation and activation of hif-1 , suggesting a common pathway underlying the hormonal regulation of erythropoiesis . in the last five decades , the erythropoietic role of anabolic hormones on anemia has been particularly investigated in a large number of studies . different molecular mechanisms by which androgens , igf-1 , and thyroid hormones can affect the hematopoietic process have been identified . however , the interaction between different anabolic hormonal pathways and the contribution of multiple hormonal dysregulation to the pathogenesis of the mild anemia of the elderly has been poorly addressed . a very limited number of epidemiological studies have tested the role of the age - related decline in anabolic hormones as contributing factor of anemia , especially in older individuals . notwithstanding the fact that rcts testing the effects of a single hormonal replacement therapy on different hematological parameters yielded more convincing results , these studies have been focused on cohorts with a wide range of age and multimorbidity distribution . in particular , hb levels have been significantly correlated with the severity of androgen deficiency and t status . the rcts and registries testing the effects of t therapy on hematological parameters have shown that erythrocytosis is the most common adverse event of testosterone replacement therapy . however , the usefulness of t therapy in anemic men exhibiting an age - related hypogonadism remains to date a controversial issue . the balance between long - term clinical benefits and risks of t replacement therapy , especially at prostatic and cardiovascular level , has not been adequately assessed in large , randomized clinical trials involving older men . igf-1 has been hypothesized as cause of anemia under pathological conditions including a wide range of anemic disorders in observational studies . igf-1 is a regulator of erythropoiesis in children with short stature , adults with gh deficiency , and those affected by chronic kidney diseases . a positive and significant correlation between hb concentration and both serum igf-1 and igfbp-3 levels has been also observed in few studies conducted on healthy prepubertal and early pubertal subjects , nondiabetic adult subjects , and older individuals . although intervention studies testing the effect of gh therapy on indexes of erythropoiesis in several diseases seem to support the beneficial effect of gh administration on anemia , there is limited evidence in healthy individuals . finally , a large body of observational evidence suggests an association between overt thyroid hormones anemia and erythrocyte abnormalities . however , there is limited evidence targeting subclinical thyroid conditions , which are more frequently observed in the older population . very few intervention studies have tested the effects of thyroid hormones replacement therapy on hb concentrations and erythrocyte parameters , with most of them showing no beneficial effect of levothyroxine treatment . in conclusion , despite the biological rationale for considering the decline in anabolic hormones and alteration in thyroid hormones as determinants of mild anemia of aging , the usefulness of hormonal replacement therapy for treating anemia remains to date a controversial issue .
anemia is a multifactorial condition whose prevalence increases in both sexes after the fifth decade of life . 
 it is a highly represented phenomenon in older adults and in one - third of cases is  unexplained . 
 ageing process is also characterized by a  multiple hormonal dysregulation  with disruption in gonadal , adrenal , and somatotropic axes . 
 experimental studies suggest that anabolic hormones such as testosterone , igf-1 , and thyroid hormones are able to increase erythroid mass , erythropoietin synthesis , and iron bioavailability , underlining a potential role of multiple hormonal changes in the anemia of aging . 
 epidemiological data more consistently support an association between lower testosterone and anemia in adult - older individuals . 
 low igf-1 has been especially associated with anemia in the pediatric population and in a wide range of disorders . 
 there is also evidence of an association between thyroid hormones and abnormalities in hematological parameters under overt thyroid and euthyroid conditions , with limited data on subclinical statuses . 
 although rcts have shown beneficial effects , stronger for testosterone and the gh - igf-1 axis and less evident for thyroid hormones , in improving different hematological parameters , there is no clear evidence for the usefulness of hormonal treatment in improving anemia in older subjects . 
 thus , more clinical and research efforts are needed to investigate the hormonal contribution to anemia in the older individuals .