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

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.