Which anterior pituitary hormone lacks a stimulatory hypophysiotropic hormone?

• A. Prolactin
• B. Growth hormone
• C. Vasopressin
• D. ACTH

Answer: (A)

Anterior pituitary hormone release is governed by hypothalamic releasing and inhibiting factors, and prolactin stands out because its secretion is controlled mainly by inhibition rather than a defined stimulatory factor. Dopamine from hypothalamic neurons tonically inhibits lactotrophs, so prolactin levels rise primarily when this inhibition is lifted. There isn’t a strong, dedicated stimulatory hypophysiotropic hormone for prolactin, and any stimulatory input (like from TRH) is modest compared to the clear stimulatory controls seen for other pituitary hormones.

In contrast, growth hormone is stimulated by GHRH and tempered by somatostatin, ACTH by CRH, TSH by TRH, and the gonadotropins by GnRH. Vasopressin, meanwhile, is a posterior pituitary hormone, not regulated by hypothalamic hypophysiotropic releasing hormones in the same way as anterior pituitary hormones.