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Increased Plasma Gonadotropin Levels in Spontaneous Hyperthyroidism Reproduced by Thyroxine But Not by Triiodothyronine Administration to Normal Subjects^*

EVA MARIE ERFURTH and PAVO HEDNER

Department of Internal Medicine, University Hospital of Lund S-221 85 Lund, Sweden

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 µg 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. Dopamihergic 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.

^* This work was supported by a grant from the Royal Swedish Academy of Sciences.

To whom requests for reprints should be addressed.

Received May 15, 1986.

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