Age-Related Changes of Serum 3,3′-Diiodothyronine, 3′,5′-Diiodothyronine, and 3,5-Diiodothyronine Concentrations in Man

To evaluate the effects of aging on T4 metabolism, serum 3,3'diiodothyronine (3,3'-T2), 3',5'-diiodothyronine (3',5'-T2), and 3,5-diiodothyronine (3,5-T2) as well as T4, T3, and rT3 concentrations were determined by RIAs in 81 normal subjects, aged 24-81 yr. 3,3'-T2, 3',5'-T2, and 3,5-T2 antisera were prepared by immunizing rabbits with the protein conjugate of each diiodothyronine. Since the 3',5'-T2 antiserum cross-reacted with rT3 to the extent of 1.8%, correction was made by subtracting 1.8% of the rT3 value from the measured 3',5'-T2 value. The other 2 antisera had no noticeable cross-reactivity with various thyroid hormone derivatives. Although no significant relation existed between T4 levels and age (r = -0.14; P greater than 0.1) serum T3 showed a significant correlation with age (r = -0.28; P less than 0.05). In contrast, no significant correlation was observed between rT3 and age (r = 0.12; P greater than 0.1). Serum 3,3'-T2, 3',5'-Tw, and 3,5-T2 values (nanograms per dl) all had significant inverse correlations with age, the coefficients of correlation being -0.38 for 3,3'-T2 (P less than 0.01), -0.58 for 3',5'-Tw (P less than 0.01), and -0.47 for 3,5-T2 (P less than 0.01). These findings suggest that sequential monodeiodinating activities in T4 metabolism decrease with increasing age.