The Association between Vitamin D Status and the Impact of Metformin on Hypothalamic–Pituitary–Thyroid Axis Activity in Women with Subclinical Hypothyroidism

Metformin inhibits enhanced secretion of anterior pituitary hormones. Its impact on prolactin and gonadotropin concentrations is absent in individuals with hypovitaminosis D. The aim of this prospective cohort study was to investigate whether vitamin D status determines the effect of metformin on hypothalamic-pituitary-thyroid axis activity in levothyroxine-naïve women. The study included three groups of women of reproductive age with subclinical non-autoimmune hypothyroidism, which were matched for age, thyroid-stimulating hormone (TSH) concentration, and insulin sensitivity: untreated women with vitamin D deficiency/insufficiency (group A), women effectively supplemented with exogenous calciferol (group B), and untreated women with normal 25-hydroxyvitamin D concentrations (25OHD) (group C). Owing to concomitant type 2 diabetes or prediabetes, all subjects were treated with metformin. Concentrations of 25OHD, TSH, total and free thyroid hormones, glucose, insulin, glycated hemoglobin (HbA_1c), prolactin, and peripheral markers of thyroid hormone action were assayed before metformin treatment and six months later. Based on hormone concentration, structure parameters of thyroid homeostasis were calculated. Except for 25OHD concentrations, there were no between-group differences in baseline values of the measured variables. Metformin reduced glucose, the homeostatic model assessment 1 of insulin resistance ratio (HOMA1-IR), and HbA_1c in all study group, but these effects were less pronounced in group A than in the remaining groups. The reduction in TSH and Jostel's index was observed only in groups B and C, and its degree correlated with baseline TSH concentrations, baseline 25OHD concentrations, and the degree of improvement in HOMA1-IR. The drug did not affect circulating levels of 25OHD, free and total thyroid hormones, prolactin, other structure parameters of thyroid homeostasis, and markers of thyroid hormone action. The obtained results allow us to conclude that low vitamin D status in young women mitigates the impact of metformin on thyrotroph secretory function.