(HealthDay News) — Metformin may raise the risk of low levels of thyroid-stimulating hormone (TSH) among patients with hypothyroidism, according to a study published online September 22 in CMAJ, the journal of the Canadian Medical Association.

To examine the drug’s effect on TSH, the researchers examined data compiled on over 74,000 people who took metformin, along with sulfonylurea, over a 25-year period.

The researchers found that among those in the study with hypothyroidism, there were 495 incidences of low levels of thyroid-stimulating hormone per year, compared with 322 in the normal-thyroid group. Among patients treated for hypothyroidism, metformin was linked with a 55%  higher risk for low TSH levels, compared to those who were taking sulfonylurea for their diabetes.

“The results of this longitudinal study confirmed that the use of metformin was associated with an increased risk of low TSH levels in patients with treated hypothyroidism,” study author Laurent Azoulay, MD, from McGill University in Montreal, said in a journal news release. “Given the relatively high incidence of low TSH levels in patients taking metformin, it is imperative that future studies assess the clinical consequences of this effect.”

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