Randomised trial of iodine intake and thyroid status in preterm infants. Results of controlled double-blind study of thyroid replacement in very low-birth-weight premature infants with hypothyroxinemia. L-thyroxine treatment of preterm newborns: clinical and endocrine effects. Postnatal thyroxine supplementation in infants less than 32 weeks' gestation: effects on pulmonary morbidity.
Low levels of circulating thyroid hormones have been associated with poorer general and neurodevelopmental outcome in preterm babies and it has been speculated that the association is causal. Low levels of circulating thyroid hormone have been reported after inadequate intake of iodine in preterm infants being fed milk formula.
                To investigate whether increased iodine intake from supplemented preterm formula would improve thyroid hormone levels in preterm babies (this study) and hence improve neurodevelopmental status (planned subsequent study).
                A total of 121 preterm infants were entered into a randomised controlled trial of standard (68 microg/l) versus increased (272 microg/l) iodine in preterm formula.
                The two groups were comparable at recruitment. No evidence of an effect of the intervention on thyroid hormone levels was seen up to 41 weeks after conception.
                Calls for increased iodine content of preterm infant formulas are not justified by this study. The nature of hypothyroxinemia in sick very low-birth-weight (VLBW) infants was evaluated by assessment of the hypothalamic-pituitary axis and by the clinical response to thyroxine (T4) therapy. Twenty-three very low-birth-weight infants of gestational age 26 to 28 weeks, whose serum T4 concentrations were 4 micrograms/dL on two occasions, and thyrotropin less than 20 microU/mL, were included in a double-blind study. Following a thyrotropin-releasing hormone stimulation test, babies were given either T4 or placebo. Nine babies were thyrotropin-releasing hormone tested prior to therapy; four babies, two from each group, were tested 1 to 2 weeks after therapy. In 11 untreated babies, mean base line serum thyrotropin of 7.0 +/- 1.4 rose to 23.7 +/- 4.1 microU/mL in 30 minutes. This response was not significantly greater than the observed response in full-term babies, 23.7 +/- 4.1 v 16.6 +/- 0.97 microU/mL, respectively, P greater than .05. In two babies treated with T4 the thyrotropin response to thyrotropin-releasing hormone was completely suppressed. Serial serum T4 determinations showed normalization in both groups after a similar time interval. There was no beneficial effect of T4 therapy on growth of head circumference, length, or weight. Developmental data revealed no significant differences in the mental, motor, or gross neurologic outcome in the treated and nontreated infants after 1 year of follow-up. These observations imply that hypothyroxinemia in sick preterm infants is not a direct consequence of hypothyroidism. Despite the lack of demonstrable short-term beneficial effects of T4 therapy, follow-up studies are necessary to resolve the question of long-term benefits. Preterm newborns have low serum thyroxine (T4) levels compared with late-gestational fetuses. Low thyroid hormone levels are associated with increased severity of neonatal illness and neurodevelopmental dysfunction. We assessed the endocrine and clinical effects of increasing serum T4 levels in preterm newborns with a gestational age <31 wk. Forty newborns were randomized in a double blind protocol: 20 infants received a daily dose of 20 microg/kg L-T4 for 2 wk, whereas 20 control infants received saline. Serum concentrations of T4, triiodothyronine (T3), reverse T3 (rT3), thyroglobulin (TG), and TSH were measured weekly as well as serum levels of GH, prolactin, and IGF-I. After 2 wk, a TSH-releasing hormone (TRH) test was performed. Neonatal illness and outcome was evaluated by noting heart rate, oxygen requirement, duration of ventilation, development of chronic lung disease, oral fluid intake, and weight gain; a Bayley score was done at the corrected age of 7 mo. L-T4 administration induced a marked increase in serum T4 without apparent change in T3 levels, whereas the postnatal decline in serum rT3 was more gradual. L-T4 treatment was associated with a decrease in serum TG and TSH levels. TRH injection induced a definite rise in serum TSH and T3 in controls, but not in L-T4 treated newborns. Neither L-T4 treatment, nor TRH administration appeared to alter circulating levels of prolactin, GH, or IGF-I. In contrast to the pronounced endocrine effects, no clinical effects of L-T4 administration were detected. Transient hypothyroxinemia in premature newborns has been linked with poor neonatal outcomes. We designed this study to evaluate the effects of early thyroxine (T4) administration in the premature infant.
                A total of 49 newborns less than 32 weeks' gestation, were randomized in a double-blind, placebo-controlled trial. Within the first 48 hours of life, T4 (10 or 20 micrograms/kg; intravenous or through nasogastric tube, respectively) was administered for a total of 21 days. Chronic lung disease, the primary outcome variable, was defined by oxygen dependency at 28 days of life.
                The incidence of chronic lung disease, death, grade III or IV intraventricular hemorrhage, periventricular leukomalacia, and sepsis was not different in the placebo and treated groups.
                Early T4 supplementation in preterm newborns less than 32 weeks' gestation does not decrease the incidence of chronic lung disease or other complications of prematurity.
This review does not support the use of prophylactic thyroid hormones in preterm infants to reduce neonatal mortality, neonatal morbidity or improve neurodevelopmental outcomes. An adequately powered clinical trial of thyroid hormone supplementation with the goal of preventing the postnatal nadir of thyroid hormone levels seen in very preterm infants is required.