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Cross-sectional data on age and physique in relation to thyroid function test results were analyzed among 174 women aged 26-82 years who were examined in Illinois in 1974-80. There is little evidence for age changes in serum thyroxine (T4), T3 resin uptake (T3RU), free thyroxine index (T7), or estimated concentration of thy­roid binding globulin (TBG). Weight and weight/height are significantly correlated with T3RU (negative r’s) and TBG (positive r’s). Among a subgroup of 51 women who had radial bone mineral measurements, weight was a significant predictor of T3RU and TBG in multiple regression analysis while radial bone mass/bone width was not. These findings appear to represent an association of TBG with body fat, probably due to the effects of peripheral production of estrogens. Significant seasonal variations are evident in T4 and T7 among the 174 women, and analyses in two weight/height categories showed that seasonal variation in T3RU and TBG are evident only in the higher weight/height group. The latter findings are discussed in terms of obesity and thermogenesis.