EFFECTS OF ESTROGEN AND TESTOSTERONE ON CIRCULATING THYROID HORMONE*

Abstract

A study was made to determine the effects of estrogen and androgen on circulating thyroid hormone. Administration of estrogen to athyrotic patients receiving adequate replacement therapy resulted in an increase in the level of serum protein-bound iodine (PBI), indicating that a functioning thyroid gland is not necessary for this effect. The increase in PBI concentration was greater in patients treated with thyroxine than in those treated with desiccated thyroid. Estrogen caused no elevation of the PBI level in hypothyroid patients receiving inadequate replacement therapy in whom the thyroxine-binding capacity of the serum was already increased. Estrogen also caused a prolongation of the thyroxine survival time (from 6.9 to 10.6 days, average), indicating a decreased rate of peripheral disposal of thyroxine. According to current information, the administration of estrogen results in: 1) an increase in the thyroxine-binding capacity of serum, 2) an increase in the concentration of PBI, 3) a decrease in the utilization of thyroid hormone, 4) a decrease in the basal metabolic rate, 5) an increase in the pituitary secretion of thyrotropin, and 6) an increase in thyroidal activity. Neither the thyroid nor the pituitary gland is essential for the first four steps, but each plays a role in the intact organism. The effect of testosterone therapy on the thyroid hormone was indicated by a consistent fall in the serum PBI level, but the mechanisms involved in this change were not clearly defined.