Thyrotropin-Releasing Hormone Is Not the Sole Physiologic Mediator of Prolactin Release during Suckling

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Abstract
Suckling elevated serum prolactin but had no effect on circulating thyrotropin, tri-iodothyronine and thyroxine in seven post-partum women. Injections of 6 and 300 μg of thyrotropin-releasing hormone raised serum concentrations of both prolactin and thyrotropin, but not of tri-iodothyronine and thyroxine. One or 2 μg of thyrotropin-releasing hormone had no effect on either prolactin or thyrotropin, although in three subjects, 6 μg caused similar relative increases in both serum prolactin and thyrotropin. Lactotrophs and thyrotrophs in the post-partum pituitary gland appear to be similarly responsive to exogenous thyrotropin-releasing hormone, and the failure to observe thyrotropin release with suckling is not due to blockade of the response of thyrotrophs by high levels of circulating thyroid hormones. The selective increase in serum prolactin induced by suckling, therefore, is not caused solely by release of endogenous thyrotropin-releasing hormone. (N Engl J Med 290:1162–1165, 1974)