Background: In patients with hyperprolactinemia, the thyrotropin-releasing hormone (TRH) stimulation test is widely applied to distinguish prolactinoma from other causes of hyperprolactinemia. In the present study, we established reference values for the plasma concentration of prolactin (PRL) and its response to TRH.
Methods: Basal PRL and the PRL response to 400 micrograms TRH i.v. was determined in 50 subjects recruited from the general population, equally distributed according to sex and age between 20 and 69 years. PRL was determined by a fluoroimmunometric assay. Reference values are given as the observed range.
Results: Plasma concentrations of PRL were 4.0-25 micrograms/l (median: 10.0 micrograms/l) in women and 0.5-19.0 micrograms/l (median: 8.5 micrograms/l) in men (p = 0.11). The peak PRL concentration after stimulation with TRH was slightly higher in women (median: 51 micrograms/l) than in men (median: 41 micrograms/l; p = 0.04) and was reached at t = 20 min in all subjects. The relative increase in plasma PRL (median: 440%) did not show a statistically significant effect of age or sex. In 12 subjects (24%), the relative increase in plasma PRL was lower than 250%, which has traditionally been considered the minimum cutoff for a normal response. There were no effects of smoking and alcohol, but regular ingestion of liquorice was associated with lower basal (p = 0.03) and lower stimulated (p = 0.05) plasma concentrations of PRL.
Conclusions: The present study provides reference values for basal and TRH-stimulated plasma concentrations of PRL.