Circulating hormone weighs approximately 23,000 mw. Approximately three quarters of circulating prolactin is non-glycosolated monomers. These polymers have a lower bioactivity and slower clearance than the monomeric form but can be detected by the radioimmunoassays frequently used in the laboratory evaluation of prolactin. These circulating polymers may explain the presence of eumenorrhea and fertility despite hyperprolactinemia in some women. The neuroendocrine regulation of prolactin is complex, but a thorough understanding of the regulatory mechanisms of prolactin release provides an important foundation for the understanding of the clinical pathology.
Prolactin is the only pituitary hormone to be under tonic inhibitory control.
This is demonstrated by the fact that interruption of the pituitary stock will result in decreased secretion of all other pituitary hormones, but increased secretion of prolactin. Dopamine secreted from neurons in the the hypothalamus is the primary inhibitor of prolactin secretion from the pituitary gland. Dopamine acts through the D-2 receptor and uses a cyclic AMP as a second messenger. In addition to dopamine control, there are a number of other substances which have been shown to effect prolactin release. The effect of thyrotropin releasing hormone on the release of prolactin is quite pronounced.