Effect of Methyl Testosterone on Thyroxine Metabolism and on Triiodothyronine Kinetics

Several parameters of thyroid function were investigated in 16 euthyroid young adult males before and after a 3–4 week period of administration of 20 mg sublingual methyl testosterone (MT) daily. MT decreased the maximal binding capacity of thyroxine binding globulin (TBG) and increased maximal binding capacity of thyroxine binding prealbumin (TBPA). There was an associated decrease in serum thyroxine (T4) iodine concentration, and an increase in 131I-triiodothyronine (T3) resin sponge uptake. The paper electrophoretic distribution of 131I-T4 among TBG, albumin and TBPA moieties was altered; there was a decreased proportion of label migrating with TBG and an increased proportion with TBPA, and albumin during the period of MT administration. The proportion of plasma free T4 increased significantly during MT administration, but equilibrium free T4 concentration was not altered. Studies of 131I-T4 and 125I-T3 kinetics were conducted simultaneously. T4 distribution space was not altered by MT but fractional T4 degradation rate increased significantly (decreased half-life). Daily T4 turnover decreased about 13%; this decrease may have been related to the mild hepatotoxicity indicated by significant BSP retention in 6 of 6 subjects tested. The volume of distribution and fractional degradation rate of 125I-T3 were not influenced by MT at a time when available TBG-T4 binding sites had decreased 18%. These data would suggest that these parameters of equilibrium T3 kinetics are largely independent of the concentration of available TBG-T4 binding sites. The concentration of T4 available to tissue binding sites was probably not altered in the present study. Equilibrium thyroidal radioiodine clearance was unchanged by MT, suggesting that equilibrium TSH secretion also was unaltered.