Circadian periodicity of serum prolactin concentration in man.

Immunoreactive human serum prolactin of pituitary origin has been measured by a radioimmunoassay developed for ovine prolactin. Blood samples were collected at four-hour intervals during a 24-hour period from 12 non-pregnant women, three pregnant women, and seven adult men. A circadian periodicity was found in serum prolactin concentration, with peak values during the night, between 1 a.m. and 5 a.m. for the non-pregnant women, and at 5 a.m. for the adult men. Mean serum levels of prolactin were 1.5 times higher in non-pregnant women than in men. In women investigated during the last month of their pregnancy the mean serum prolactin levels were 2.3 times higher than in the non-pregnant women, but there was no circadian periodicity.