Effects of hyperthyroidism on binding proteins for steroid hormones

Sex hormone binding globulin (SHBG) and corticosteroid binding globulin (CBG) binding capacities were examined weekly in eight normally cycling women and three women taking birth control pills during a 5-week baseline period and after daily ingestion of 75 micrograms of L-triiodothyronine (T3) for 30 days. The SHBG binding capacity increased whereas the CBG binding capacity decreased after T3 therapy. The binding capacities of proteins for steroid hormones were measured in 18 hyperthyroid subjects (Graves' disease) prior to and after 3 months of antithyroid drug therapy. SHBG binding capacity in hyperthyroid men or women was higher, and CBG binding capacity lower than those in euthyroid subjects. Thus, during hyperthyroidism, binding capacities of sex hormone binding globulin and corticosteroid binding globulin vary in opposite directions. A statistically significant correlation between the ratio of the sex hormone binding globulin to the corticosteroid binding globulin and triiodothyronine levels was found (P less than 0.01). Therefore the ratio of the sex hormone binding globulin to the corticosteroid binding globulin might be potentially useful as a biochemical index of thyroid hormone action in peripheral tissues.