Is Morning Prolactin Level Sufficient for Diagnosis of Mild Hyperprolactinemia?

Background: Taking into consideration the pulsatile secretion of prolactin, a single prolactin measurement may not be adequate to confirm abnormal prolactin level. Objectives: The present study aimed to compare prolactin serum levels in the morning and in the evening in women suspected of hyperprolactinemia. Methods: This descriptive cross-sectional study recruited women presenting to the laboratory for measurement of prolactin. In patients with symptoms of hyperprolactinemia and elevated early morning serum prolactin, a new sample was obtained in the evening. Women with a clinical history of hyperprolactinemia, pituitary adenoma and galactorrhea were excluded. Prolactin was measured by chemiluminescence method. A P value less than 0.05 was defined as statistically significant. Results: From 109 women with morning hyperprolactinemia, 52 (41.71%) had normal evening prolactin levels and 57 (58.29%) had high prolactin levels in the evening. The mean percentage of reduction in prolactin levels in the evening compared to the morning, in women with normal evening prolactin was significantly more than those who had high evening prolactin levels. Of women who had normal prolactin levels in the evening, 30.77% had normal mean levels of prolactin in the morning, as well. Conclusions: Prolactin levels of women were significantly higher in the morning than those in the evening. In 42% of the patients with borderline morning hyperprolactinemia (about twice the normal upper limit), evening prolactin levels return to normal. Therefore, single measurements may not be enough for the diagnosis of hyperprolactinemia and a criterion for treatment, particularly in the absence of galactorrhea in women with abnormal uterine bleeding.