Objective: To investigate the prevalence of thyroid dysfunction and autoimmunity (TAI) and to determine age specific reference ranges in individuals= 60-year-old. Furthermore we investigated the impact of the age-specific reference ranges on the prevalence of thyroid dysfunction. Design: Retrospective analysis of laboratory data collected over six months in 2015, mainly from individuals consulting the outpatient clinic. Method: Data from 676 individuals were withheld, after having applied strict exclusion criteria to avoid confounders. After exclusion of individuals with TAI (TPO-abs >60 kIU/L) and/or outliers, data of 547 individuals were used to determine age-specific reference ranges. The prevalence of subclinical hypothyroidism (SCH) and subclinical hyperthyroidism (sch) was determined according to the reference ranges from the commercial assay and also according to the calculated age-specific reference ranges. From our study population. Results: From the 676 individuals included, 559 (83%) were= 60 year-old. The prevalence of sch and TAI was comparable between both groups (8.6% vs. 13.7% and 15.4% vs. 20.5% respectively). The prevalence of SCH was significantly higher in individuals >= 60 years, compared to that in individuals <60 years (14.5% vs. 5.4%; p < 0.001). The calculated 2.5 and 97.5 percentile for the age-specific TSH range was 0.24 and 4.4 mIU/L in individuals= 60 years. When these the prevalence of sch and SCH was then determined on the basis of the age-specific reference ranges, the prevalence of SCH significantly decreased in individuals 60 years (14.5% to 5%; p = 0.027) and it then became comparable with that in individuals <60 years (5% vs. 3%). Conclusions: The prevalence of SCH was higher in individuals 60 years, compared to that in individuals <60 years, but when age-specific TSH reference ranges were used, it was comparable between both study groups. In order to avoid misclassification in older individuals, it is important to use age-specific reference ranges in daily clinical practice (C) 2016 Elsevier B.V. All rights reserved.