Increased plasma gonadotropin levels in spontaneous hyperthyroidism reproduced by thyroxine but not by triiodothyronine administration to normal subjects.

In nine hyperthyroid women studied in the follicular phase of the menstrual cycle basal plasma LH concentrations and LH and FSH responses to GnRH were increased compared to those in nine normal women. The increased gonadotropin levels could not be related to changes in body weight or plasma estradiol or serum sex hormone-binding globulin levels, and they approached normal levels during or after treatment of hyperthyroidism. When normal women were treated with T4 (0.5 mg daily for 6-10 days), their mean serum free T4 level increased to about 60% of that in the hyperthyroid patients, while their serum T3 levels increased to a lesser degree. During T4 administration these women had gonadotropin changes similar to those of the hyperthyroid patients. When the normal women took T3 (60-120 micrograms for 6-8 days), their serum T3 level rose almost to the level in the hyperthyroid patients, but basal and GnRH-stimulated LH and FSH remained close to control levels. Dopaminergic blockade with metoclopramide did not alter the gonadotropin response to GnRH in any subject group. We conclude that serum T4 is of greater importance than serum T3 for at least short term gonadotropin regulation.