FT3 IS HIGHER IN MALES THAN IN FEMALES AND DECREASES OVER THE LIFESPAN.

OBJECTIVE: Normal changes in free triiodothyronine (FT3), free thyroxine (FT4), and thyroid-stimulating hormone (TSH) levels over the lifespan and differences between sexes are not well documented, mainly because even the largest-scale studies available include relatively small cohorts. The aim of this study was to define age-related trends including sex differences based on reliable data.

METHODS: A large database including serum thyroid tests drawn in community clinics was studied. FT3, FT4, and TSH levels from 527,564 sera samples taken from patients age 1 year or greater were included. After highly extensive exclusion criteria applied to remove all samples that may have been taken from unhealthy people, 27,940 samples remained. These were stratified by decades of age and by sex.

RESULTS: FT3 decreases throughout life, significantly more so among females, with equalization between sexes with greater age. FT4 declines to a lesser extent, also more among females than among males. Among the very old, females have higher levels of FT4. In contrast, TSH declines until age 50 and then increases slightly in both sexes.

CONCLUSION: This study provides reliable data regarding trends in hormonal levels by age and sex, with the major finding being higher FT3 in males throughout life except in the very young and very old. These results have important implications for diagnosing and treating thyroid conditions.

ABBREVIATIONS: ANOVA = analysis of variance; BMI = body mass index; FT3 = free triiodothyronine; FT4 = free thyroxine.