Interrelationships in the Regulation of TSH and Prolactin Secretion in Man: Effects of L-Dopa, TRH and Thyroid Hormone in Various Combinations*

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Abstract
The interrelationships in the regulation of thyroid-stimulating hormone (TSH) and prolactin (PRL) secretion in man were studied by following their response to drugs and hormones known to affect the serum concentration of either TSH or PRL. Administration of a single 0.5 oral dose of l-dopa to 12 patients with primary hypothyroidism produced a consistent suppression of the elevated TSH level in all patients. In contrast, l-dopa given to patient with known primary hypothyroidism, in whom thyroid hormone replacement therapy was discontinued for 3–4 weeks, produced suppression of TSH in only 2 out of patients, and there was no correlation between the basal TSH level and the responsiveness to l-dopa. The TSH response to l-dopa in a group of 13 euthyroid patients could not be interpreted due changes in TSH within the limits of the assay sensitivity. The TSH but not the PRL response to synthetic thyrotropin releasing hormone (TRH) was blunted early in the course of thyroid hormone replacement. After 2½ to 5 months, however, both TSH and PRL responses to TRH were virtually abolished. Administration of l-dopa 1 and 2 hr but not 3 hr prior to TRH abolished or diminished the PRL rise in response to TRH in patients with hypothyroidism of long duration. The TSH response to TRS, however, was unaltered by l-dopa given 1, 2, or 3 hr prior to TRH. The time course of both stimulatory and inhibitory responses of TSH paralleled those of PRL. These results suggest that TSH and PRL secretion in man is controlled by an interrelated pathway. If a common regulatory mechanism is involved, the pituitary thyrotrophs and lactotrophs have shown differential sensitivity to the common stimulatory and inhibitory substances.

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