The introduction of the assay of prolactin in human serum during the 19 70s resulted in the definition of the "hyperprolactinemia syndrome", i.e. menstrual disturbance, infertility and galactorrhea in women, and impotence with loss of libido in men. With the aid of highly effective inhibitors of the secretion of prolactin from the pituitary it then became possible rapidly to restore fertility and sexual function in a number of cases where the etiology of the patient's disorder had eluded the physician.
It was soon found that serum prolactin in some patients is heterogeneous. As regards size heterogeneity, whereas the molecular mass of pituitaryderived prolactin is about 23 kDa (199 amino acids in a single-chain polypeptide), part of the immunoassayable prolactin in some patients is larger, with molecular mass of about 50kDa, 150–170kDa or even more. These entities are sometimes referred to as "big" and "big, big" prolactin, respectively (1–9); also smaller