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Suppression of Serum Thyrotropin (TSH) by L-Dopa in Chronic Hypothyroidism: Interrelationships in the Regulation of TSH and Prolactin Secretion¹

Thyroid Study Unit, Department of Medicine, University of Chicago Chicago, Illinois McGill University Clinic, Royal Victoria Hospital Montreal, Canada

Address request for reprints to Dr. Refetoff, 950 East 59th Street, Chicago, I11. 60637.

Synthetic thyrotropin releasing hormone (TRH) stimulates the release of human thyrotropin (TSH) and prolactin (HPr). Though acute administration of L-dopa inhibits HPr release, it has failed to produce discernible changes of serum TSH in euthyroid men. We have examined the effect of acute administration of L-dopa on TSH secretion in 8 women with primary hypothyroidism of at least 1 yr duration. Serum TSH decreased with 24–60% of the basal value at 2-4-hr after L-dopa administration. The mean decrease for the entire group was 36.9% ± 5.3 SEM at 4 hr. Changes in serum HPr occurred in parallel with those of serum TSH. Treatment with 100 ng of T₃ per day for 1-1/2 to 2 days produced a gradual diminution in basal serum TSH levels and resulted in a decreased absolute serum TSH but not serum HPr response to TRH. L-dopa had no effect on the peripheral metabolism of T₄. The observed suppressive effect of L-dopa on serum TSH in hypothyroid patients might be due to an inhibition of endogenous TRH secretion, a direct inhibitory action on the pituitary or a stimulatory effect on the hypothalamic prolactin inhibitory factor (PIF) or thyrotropin inhibitory factor (TIF). The postulated TIF might be a substance identical to PIF. However, since a dissociation in the serum TSH and HPr responses to TRH was observed after short term treatment with T₃, a common regulatory mechanism for HPr and TSH secretion, if existant, would a priori have a differential sensitivity.

¹ Supported in part by USPHS Grants RR-55 and AM-15,070.

² Fellow, Damon Runyon Memorial Fund for Cancer Research.

Received June 6, 1972.

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