Asymptomatic 'big' hyperprolactinemia in two men with pituitary adenomas.

'Big' and 'big-big' hyperprolactinemia, the presence of increased serum concentrations of high molecular weight (50-60 and 150 kDa respectively) prolactin forms, has mostly been reported in women with idiopathic hyperprolactinemia and normal hypothalamic-pituitary ovarian axis function. It has been suggested that both 'big' and 'big-big' prolactin species are biologically less active than the 22 kDa form predominating in normal individuals. We report the cases of two men with pituitary adenomas who were secreting significant amounts of 'big' (50-60 kDa) prolactin documented by Sephadex G-100 column chromatography. Both patients reported normal sexual function despite high prolactin levels. Results of nocturnal rigidity and tumescence testing were normal, confirming that significant hyperprolactinemia was not interfering with either patient's sexual function. 'Big' hyperprolactinemia should thus be suspected even in male patients with prolactin-secreting pituitary adenomas who maintain adequate sexual function in the presence of high prolactin levels.