Endocrine Abstracts

Macroprolactin(mPRL)is a high molecular weight form of prolactin(PRL)most commonly due to formation of a complex with IgG.The PRL in mPRL remains reactive in immunoassays for PRL and the complex has a longer plasma half life than monomeric PRL.mPRL is a common cause of hyperprolactinaemia but its origin is not known and the complex has minimal bioactivity in vivo.The clinical significance of hyperprolactinaemia due to mPRL is that it can cause diagnostic confusion and lead to innapropriate treatment.We report an unusual case of hyperprolactinaemia and macroprolactinaemia in association with prolactinoma.

A 23 year old woman presented with post-pill amenorrhoea.Endocrine investigation revealed total serum PRL 11,847 mU\/L,estradiol <37 pmol\/L,normal thyroid function and normal cortisol response to synacthen.Pituitary MRI showed an adenoma.However chromatographic analysis showed that 40% of the total serum PRL immunoreactivity was due to mPRL.Stimulation with TRH showed a normal TSH response but no change in monomeric or mPRL over 3 hours.Following a single dose of bromocriptine(2.5 mg)serum PRL fell with monomeric PRL reaching a minimum of 35% of basal at 6 hours and mPRL declining more slowly to a minimum of 74% of basal at 24 hours.Over 48 hours monomeric PRL and, more slowly, mPRL returned to pre-treatment levels.Similar changes were observed during long term treatment with cabergoline;after 3 months total PRL remained elevated(1380 mU/L) due to mPRL(61% of total).

These results are consistent with the formation of mPRL in serum in response to pituitary secretion of monomeric PRL.mPRL can occur as a chance phenomenon in association with pituitary pathology and can confuse interpretation of the response to treatment with dopamine agonists