Endocrine Abstracts

It is recognized that a substantial proportion of patients found to have raised prolactin (PRL) level on a single testing is subsequently found to have normal PRL levels. Though this is often attributed to stress or medication factor, it should be noted, that PRL secretion displays diurnal variation with a nocturnal peak in the late-night/early morning hours. In our study we tested the hypothesis, that in some patients raised PRL levels in the morning may reflect individual diurnal variation of PRL secretion.

Subjects & Methods: We assessed serum PRL concentrations tested at 2.00, 5.00, 8.00, 11.00, 14.00 and 17.00 hours in 55 female patients (age (mean±SD) 33.07±13 years, BMI 25.3±5.2 kg/m2) referred to our Department with the history of galactorrhoea and/or menstrual irregularities.

Results: In 12 of 55 patients (21.8%) we confirmed significantly raised PRL levels on serial sampling, with evidence of either macro- or microadenoma on pituitary MRI scan in 6 patients (10.9%). In the remaining 43 patients (78.2%) PRL levels were significantly higher at 2.00 ((mean±SEM) 35.59±2.21 ng/mL), 5.00 (32.87±2.22 ng/mL) and 8.00 hours (30.65±2.19 ng/mL) than at 11.00 (18.62±2.27 ng/mL), 14.00 (21.18±2.39 ng/mL) and 17.00 hours (22.05±2.24 ng/mL), respectively, p<0.05. With the upper normal limit of our assay around 29 ng/mL we found PRL levels above this limit in 11 out of 43 patients (25.6%) at 8.00 hours. All of those subjects were subsequently found to have normal PRL concentrations later during the day.

Conclusion: Significant proportion of patients have PRL concentrations above the upper limit on the reference range if blood sample is taken around the opening hours of the laboratory. This might be explained by individual diurnal variation of PRL concentrations, when sampling is performed on the descending arm of the nocturnal peak of PRL secretion. As finding of raised PRL concentration usually generates additional, and often unnecessary, investigations, we postulate that sampling for PRL should be delayed till mid-morning