TSH-Lowering Effect of Metformin in Type 2 Diabetic Patients

Differences between euthyroid, untreated hypothyroid, and euthyroid on L-T4 therapy patients

  1. Carlo Cappelli, MD1,
  2. Mario Rotondi, MD2,
  3. Ilenia Pirola, MD1,
  4. Barbara Agosti, MD3,
  5. Elena Gandossi, MD1,
  6. Umberto Valentini, MD3,
  7. Elvira De Martino, MD1,
  8. Antonio Cimino, MD3,
  9. Luca Chiovato, MD2,
  10. Enrico Agabiti-Rosei, MD1 and
  11. Maurizio Castellano, MD1
  1. 1Internal Medicine and Endocrinology Unit, Department of Medical and Surgical Sciences, University of Brescia, Brescia, Italy;
  2. 2Unit of Internal Medicine and Endocrinology, Fondazione Salvatore Maugeri Istituto di Ricovero e Cura a Carattere Scientifico, Istituto Superiore Prevenzione e Sicurezza Lavoro Laboratory for Endocrine Disruptors, University of Pavia, Pavia, Italy;
  3. 3Diabetic Unit, Spedali Civili di Brescia, Brescia, Italy.
  1. Corresponding author: Carlo Cappelli, cappelli{at}med.unibs.it.


OBJECTIVE To assess the interplay between metformin treatment and thyroid function in type 2 diabetic patients.

RESEARCH DESIGN AND METHODS The acute and long-term effects of metformin on thyroid axis hormones were assessed in diabetic patients with primary hypothyroidism who were either untreated or treated with levothyroxine (L-T4), as well as in diabetic patients with normal thyroid function.

RESULTS No acute changes were found in 11 patients with treated hypothyroidism. After 1 year of metformin administration, a significant thyrotropin (TSH) decrease (P < 0.001) was observed in diabetic subjects with hypothyroidism who were either treated (n = 29; from 2.37 ± 1.17 to 1.41 ± 1.21 mIU/l) or untreated (n = 18; 4.5 ± 0.37 vs. 2.93 ± 1.48) with L-T4, but not in 54 euthyroid subjects. No significant change in free T4 (FT4) was observed in any group.

CONCLUSIONS Metformin administration influences TSH without change of FT4 in patients with type 2 diabetes and concomitant hypothyroidism. The need for reevaluation of thyroid function in these patients within 6–12 months after starting metformin is indicated.


  • The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

    • Received February 13, 2009.
    • Accepted May 30, 2009.
| Table of Contents

This Article

  1. Diabetes Care vol. 32 no. 9 1589-1590
  1. All Versions of this Article:
    1. dc09-0273v1
    2. 32/9/1589 most recent