A quantitative difference between immunologically and biologically active prolactin in hypothyroid patients

Under certain conditions, the pituitary may secrete PRL species displaying diverse immunological and biological activities. To study this phenomenon further, we compared PRL determined by RIA and radioreceptor assay (RRA) in 25 hypothyroid patients (TSH, greater than 10 microU/ml) and in 49 endocrinologically normal patients. In the normal women, the ratio of the PRL mean determined by RRA (67.3 ng/ml) to that determined by RIA (13.5 ng/ml) was 4.99 +/- 0.435. By contrast, in hypothyroid patients, PRL determined by RRA was significantly lower than the value determined by RIA for each patient, and the ratio of means (RRA to RIA) was 0.782 +/- 0.067. The mean PRL determined by RRA (52.2 ng/ml) was not significantly different from the mean of control patients (P greater than 0.5). PRL determined by RIA was elevated in all hypothyroid patients (range, 24-237 ng/ml), and the mean (by RIA) (64.1 ng/ml) was significantly elevated above the mean PRL (by RIA) of controls (P less than 0.000001). Thus, in the chronically hypothyroid patients, PRL is characterized as possessing normal receptor-binding activity and augmented immunological activity.