Circadian rhythm of prolactin during the menstrual cycle

Ten women with regular cycles volunteered in a study of the relationship between PRL diurnal rhythm and menstrual cycle. A PRL surge during the afternoon between 2:00 and 8:00 P.M. was observed in the luteal and periovulatory phases, but not in the early follicular phase. The authors advise PRL measurement in the morning between 10:00 A.M. and 12 noon for diagnosing hyperprolactinemia, because morning PRL levels seem to remain constantly low during the menstrual cycle.