The Effect of Metformin on Prolactin Concentration in Women with Hyperprolactinemia and Subclinical Hyperthyroidism.

INTRODUCTION

Because prolactin excess and hyperthyroidism are often complicated by hyperglycemia and impaired insulin sensitivity, many patients with both these disorders are treated with metformin. This drug inhibits secretory function of overactive anterior pituitary cells, including lactotrophs. The purpose of the current study was to investigate whether metformin action on prolactin oversecretion is impacted by coexisting hyperthyroidism.

METHODS

Our prospective, cohort study included two groups of women in reproductive age requiring metformin therapy with mild or moderate hyperprolactinemia. Patients with concomitant grade 1 subclinical hyperthyroidism (group A) and individuals without thyroid pathology (group B) were matched for age, insulin sensitivity, and total prolactin levels. Glucose homeostasis markers, thyroid-stimulating hormone (TSH), free thyroid hormones, total and monomeric prolactin, follicle-stimulating hormone (FSH), luteinizing hormone (LH), estradiol, adrenocorticotropic hormone, IGF-1, and peripheral markers of thyroid hormone action (ferritin and osteocalcin) were measured before and after 6-month metformin therapy.

RESULTS

At baseline, both groups differed in levels of TSH, free thyroid hormones, ferritin, and osteocalcin. Although metformin reduced glucose, improved insulin sensitivity and reduced total and monomeric prolactin in both groups, these effects were more pronounced in group A than group B. The impact on prolactin in group A correlated with concentrations of free thyroid hormones. Only in group A, did metformin slightly increase FSH and LH concentrations. In women with hyperthyroidism and without thyroid pathology, there were no statistical differences between baseline and follow-up levels of the remaining variables.

CONCLUSION

The study results suggest that hyperthyroidism potentiates the impact of metformin on secretory function of overactive lactotrophs in reproductive-age women.