Metformin May Alter Thyroid Hormone in Diabetics Interview with:
Jean-Pascal Fournier, MD, PhD

Centre for Clinical Epidemiology, Lady Davis Institute, Jewish General Hospital,
Department of Epidemiology, Biostatistics and Occupational Health, McGill University, Montreal, Canada and

Laurent Azoulay, PhD
Centre for Clinical Epidemiology,
Lady Davis Institute, Jewish General Hospital, Montreal,
Department of Oncology, McGill University, Montreal, Quebec, Canada

Medical Research: What are the main findings of the study?

Response: Patients with type II diabetes and treated hypothyroidism showed a 55% increased risk for low levels of thyroid stimulating hormone (TSH) (below 0.4 mIU/L) when initiating metformin, compared with those initiating sulfonylurea (hazard ratio [HR] 1.55, 95% confidence interval [CI] 1.09–2.20). In contrast, this effect of metformin was not observed in euthyroid patients, with an adjusted HR for low TSH of 0.97(95% CI 0.69–1.36).

Medical Research: What was most surprising about the results?

Response: The study results confirmed the timing of this phenomenon suggested in previous small studies, with the highest risk being observed between 90 to 180 days after initiation (HR: 2.33; 95% CI: 1.01-5.36)

Medical Research: What should clinicians and patients take away from your report?

Response: Clinicians should be aware that low TSH could be observed in patients with treated hypothyroidism initiating metformin, especially between 90 to 180 days of use.

However, and given the uncertainty of the clinical consequences of this phenomenon, patients should not be unnecessarily worried, and should not stop their medications without the advice of a clinician.

Medical Research: What recommendations do you have for future research as a result of this study?

Response: Further research is needed to confirm these results, and assess the effect on other thyroid hormones (triioiothyronine and thyroxine). Furthermore, the clinical implications of this phenomenon should be investigated, especially regarding the potential cardiovascular consequences.


Jean-Pascal Fournier, Hui Yin, Oriana Hoi Yun Yu, and Laurent Azoulay

Metformin and low levels of thyroid-stimulating hormone in patients with type 2 diabetes mellitus CMAJ cmaj.140688; published ahead of print September 22, 2014, doi:10.1503/cmaj.140688

Last Updated on October 7, 2014 by Marie Benz MD FAAD