A study of age-dependent changes in thyroid function tests in adults.

Total serum thyroxine (T4), triiodothyronine (T3), T3 resin uptake (T3U), thyrotrophin (TSH), and reverse T3 (rT3) were measured in 209 healthy adults 20--89 yr old. Mean T4 values for men were stable throughout life, but in females under age 60, T4 values were significantly higher than in older women. Values for T3U in males were significantly higher than in females throughout all decades, although females had a significant increase in T3U after age 60. TSH values increased significantly in females over age 60. Throughout all decades, males had stable TSH levels that were slightly higher than the female results before age 60 and lower thereafter. Mean serum T3 declined similarly for both sexes with increasing age, although not to the extent previously reported. Men had significantly higher mean rT3 values over all decades than females, although female rT3 levels decreased after age 50 whereas males maintained stable values. The physiologic reasons for these findings may be due to sex-related changes in binding proteins and alterations in metabolic clearance rates, production, and degradation of these hormones with increasing age.