Endocrine Abstracts

Background: Prolactin levels are affected by stress, and in patients with moderate hyperprolactinaemia, a repeat test and/or a resting prolactin has been recommended, but there are very few data addressing the utility of these additional measurements.

Aim: To study the value of: A) Repeat measurement and B) Resting measurement of serum prolactin in mild to moderate prolactin excess (510–7500 IU/L).

Methods and subjects: Case note review of 75 patients referred for evaluation of high prolactin (‘referral prolactin’, P0). All patients had a repeat sample done at our investigation unit immediately following insertion of a forearm cannula (‘repeat prolactin’, P1) and a second sample drawn after 120 min bed rest (‘resting prolactin’, P2). P1 and P2 values were corrected for macroprolactin to achieve an estimate of monomeric prolactin. 14/75 patients who were on dopamine receptor antagonists were excluded from the analysis.

Results: P0 median was 1229, range 561 to 5949 IU/L. P1 was normal in 17/61 patients (27%), one due to macroprolactin; amongst these 17, P0 median was 892 IU/L (range 616–1800). Of the remaining 44, eight showed normalisation of resting prolactin, P2; amongst these the P0 median was 1445 IU/L (range 561–2412). However, the highest P1 in those in whom hyperprolactinaemia was excluded was 1062 IU/L.

Conclusions: 41% of patients with elevated prolactin normalised on further testing. Most of these false positive results would have been corrected by a simple repeat sample. A resting prolactin is probably unnecessary if a simple repeat value is twice the upper limit of normal.