The impact of oral hormonal contraception on metformin action on hypothalamic-pituitary-thyroid axis activity in women with diabetes and prediabetes: A pilot study.

Abstract

The impact of metformin on thyrotropin levels is sex-dependent. No previous study has assessed whether sex steroids determine metformin action on thyrotrope and thyroid cell functioning. The aim of the present study was to compare the effect of this agent on hypothalamic-pituitary-thyroid axis activity and thyroid function tests between women receiving oral contraceptive pills and women not using any contraception. The study population included 52 women with subclinical hypothyroidism and prediabetes or diabetes, 20 of whom had been using oral contraceptive pills for at least 12months before the beginning of the study. Over the entire study period (4months), all participants received oral metformin (1.7-3g daily). Circulating levels of glucose, insulin, thyrotropin, free thyroid hormones, oestradiol, gonadotropins and prolactin were measured, while the structure parameters of thyroid homeostasis and the degree of insulin sensitivity were calculated at the beginning and at the end of the study. The study groups differed in oestradiol, gonadotropins and prolactin levels and in structure parameter inference approach (SPINA)-GT. In both groups, metformin reduced glucose levels, homeostasis model assessment 1 of insulin resistance index (HOMA1-IR), thyrotropin levels and Jostel's thyrotropin index, as well as increased SPINA-GT. In women receiving oral contraceptive pills, the drug slightly decreased serum prolactin levels. The impact on metformin on HOMA1-IR, thyrotropin, prolactin, Jostel's thyrotropin index and SPINA-GT was more pronounced if women received oral contraception, as well as more pronounced in patients treated with higher doses of this agent. Treatment-induced changes in thyrotropin and Jostel's thyrotropin index correlated with their baseline values, baseline levels of oestradiol and gonadotropins, as well as with the degree of a reduction in HOMA1-IR. This study is the first one to have shown that oral oestrogens potentiate the effect of metformin on hypothalamic-pituitary-thyroid axis activity.