Effect of D-thyroxine on serum sex hormone binding globulin (SHBG), testosterone, and pituitary-thyroid function in euthyroid subjects.

Concentrations of serum sex hormone binding globulin (SHBG) and free testosterone (T) were examined in 10 euthyroid subjects (5 men and 5 women) before, during and after 30 days of the daily ingestion of 1 or 4 mg D-thyroxine (D-T4), the thyroxine analog that has only 1-15% of the calorigenic effect of L-thyroxine (L-T4). No changes in serum L-T4 or triiodothyronine (T3), serum cholesterol, SHBG, T, progesterone, estradiol-17 beta, or free T concentrations were observed in response to the 1 mg dose, but there was a slight elevation in the free thyroxine index (FTI) and a significant (p less than 0.02) suppression of the thyrotropin (TSH) response to thyrotropin releasing hormone (TRH). The 4 mg dose of D-T4 induced an increase in SHBG levels in all but one man. There was a significant negative correlation between the SHBG and percent free T (p less than 0.05) although the mass of free T did not change. As a group, the women responded with a greater increase in SHBG and decrease in percent free T than the men. Serum cholesterol decreased (p less than 0.01), all serum thyroid hormone values measured by RIA were increased (p less than 0.01), and the TSH response to TRH was completely suppressed. Despite these changes, the subjects remained clinically euthyroid. Concentrations of testosterone, progesterone and estradiol-17 beta remained unchanged. Serum luteinizing hormone (LH), which was evaluated in the men only, also did not change during the daily administration of 4 mg D-T4.