A hormone that inhibits the secretion of another hormone. The hypothalamus produces several hormones that inhibit the release of hormones by the anterior lobe of the pituitary (adenohypophysis). They include MSH-inhibiting hormone (which inhibits melanocyte-stimulating hormone), prolactin-inhibiting hormone, and somatostatin. These RIHs are released by neurosecretory cells into the hypothalamo-hypophyseal portal blood vessels, which convey the hormones through the pituitary stalk into the anterior pituitary.