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

Second Division, Department of Internal Medicine, Kyoto University School of Medicine 54 Shogoin Kawaharacho, Sakyo-ku, Kyoto 606, Japan

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 > 0.1), serum T₃ showed a significant inverse correlation with age (r = -0.28; P < 0.05). In contrast, no significant correlation was observed between rT₃ and age (r = 0.12; P > 0.1). Serum 3,3'–T₂) 3',5'–T₂, 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 < 0.01), -0.58 for 3',5'-T₂ (P < 0.01), and -0.47 for 3,5-T₂(P<0.01)

These findings suggest that sequential monodeiodinating activities in T₄ metabolism decrease with increasing age.

^* To whom requests for reprints should be addressed.

Received August 8, 1980.

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