Introduction The human prolactin molecule was first isolated in 1971. Prior to its discovery, scientists assumed that growth hormone mediated biological activities that were subsequently found to be prolactin dependent. Once its existence in humans was established, radioimmunoassays were developed and normal plasma concentrations defined. As a consequence, physicians began diagnosing hyperprolactinemia in women with previously unexplained amenorrhea. Prolactin is a polypeptide hormone secreted primarily by lactotrophs in the anterior pituitary gland. In humans, it has lactogenic, steroidogenic, and immunoregulatory functions. Hyperprolactinemia, simply defined as circulating plasma prolactin concentrations exceeding the upper limit of normal, may be caused by a variety of physiological, iatrogenic, or pathological conditions. Epidemiology The prevalence of hyperprolactinemia ranges from 0.4% in an unselected adult population to 17% in patients with polycystic ovary syndrome. A 5% incidence has been reported among women of reproductive age, increasing to 14% among women with secondary amenorrhea. Pituitary microadenomas have been found in 1.5–26.7% of autopsies. Macroadenomas are very rarely encountered in autopsy studies. Physiology Human prolactin comprises 199 amino acid residues with molecular mass of 23 kDa. Larger prolactin-like molecules are also found in the circulation. Big prolactin (48–56 kDa) and big big prolactin (100 kDa) are immunologically active but have less biological activity than the smaller prolactin.
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