Different Effects of Metformin on the Hypothalamic-Pituitary-Thyroid Axis in Bromocriptine- and Cabergoline-treated Patients with Hashimoto’s Thyroiditis and Glucose Metabolism Abnormalities

Abstract

Metformin was found to reduce serum thyrotropin levels in patients with hypothyroidism. This effect was less pronounced if patients were additionally treated with bromocriptine. The study included 39 premenopausal women with autoimmune thyroiditis and thyrotropin levels exceeding 3.0 mU/L. All patients had been treated for at least 6 months with bromocriptine (5.0–7.5 mg daily) or cabergoline (0.5–1.0 mg weekly). Because of coexisting type 2 diabetes or impaired glucose tolerance, they were then given metformin (1.7–2.55 g daily). Glucose homeostasis markers, thyroid antibody titers, as well as serum levels of thyrotropin, total and free thyroid hormones and prolactin were determined before and after 6 months of metformin treatment. At baseline, cabergoline-treated patients were less insulin resistant as well as tended to have lower levels of prolactin than bromocriptine-treated patients. Although in both treatment groups, metformin decreased plasma levels of fasting and post-challenge plasma glucose and improved insulin receptor sensitivity, this effect was more prominent in patients receiving cabergoline. However, only in bromocriptine-treated patients, metformin decreased serum thyrotropin and this effect reached the level of significance in a subgroup of patients with subclinical hypothyroidism. Neither in cabergoline- nor in bromocriptine-treated patients, metformin affected thyroid hormone levels and thyroid antibody titers. Our results indicate that the effect of metformin on hypothalamic-pituitary-adrenal axis activity is partially determined by endogenous dopaminergic tone, thyrotrope activity and insulin sensitivity.

Key words

Hashimoto’s thyroiditis - dopamine agonists - metformin - hypothalamic-pituitary-thyroid axis

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