Endocrine Abstracts

Background: Prolactin is found in serum in different molecular forms, differing in molecular size. Macroprolactin is a complex of prolactin with immunoglobulin and has limited biological activity due to failure to cross capillary wall because of its high molecular weight complex.

Objective: To assess the response of the inactive macroprolactin and the biologically active monomeric prolactin, in patients with hyperprolactinaemia, during TRH and metoclopramide tests.

Design: Eleven patients (8 females) with hyperprolactinaemia had a TRH test and eight (6 females) had a metoclopramide test. The macroprolactin and monomeric prolactin levels were assessed at baseline and at 20 minutes. Adjusted prolactin and macroprolactin levels were measured following standard PEG precipitation technique.

Results: The mean response, from baseline, of macroprolactin to TRH stimulation was 17.7%(Range: −40% → +92.5%) and the mean response of monomeric prolactin was 160% (Range: −0.5% → +485%), P<0.01. Following the metoclopramide challenge the mean rise, from baseline, of macroprolactin was 69.2% (Range: −1.2% → +199%) and the rise of monomeric prolactin was 297%(Range: +28.3% → +1164.6%), P=0.07.

Conclusion: These data demonstrate that there is rise of the biologically inactive macroprolactin, which is less than that observed in the bioactive monomeric prolactin, following both the TRH and metoclopramide challenge. This would support the hypothesis that monomeric prolactin is the principal secretory product from lactotrophs and that macroprolactin is a post-secretory complex with immunoglobulin.