Studies on human prolactin physiology

Although the clinical and experimental data were in favour of the existence of prolactin in humans like other vertebrates, as a pituitary hormone distinct from growth hormone, its presence remained contested until recent years. The predominant influence of the human hypothalamus on prolactin secretion is inhibitory. Circulating prolactin shows diurnal variations, which are not synchronized with that of TSH or ACTH; the prolactin rhythm is abolished during the last trimester of pregnancy and in patients with prolactin secreting tumors. Estrogens appeared to be less marked stimulators of prolactin secretion in man than in animals, although serum prolactin levels follow a pattern similar to that of endogenous estrogens during the normal menstrual cycle and during pregnancy. After delivery, basal prolactin levels declined progressively. In women under long term medroxyprogesterone acetate treatment, the immunoreactive serum prolactin was within the normal range of cycling women. Prolactin is found in appreciable amounts in amniotic fluid and in the serum of newborn infants. Synthetic LH and FSH releasing hormone did not change circulating prolactin levels in normal humans. A possible luteotrophic action of human prolactin in synergism with LH cannot be excluded.