The drug metformin, a commonly prescribed type 2 diabetes medication, may have potential to lower levels of thyroid-stimulating hormone (TSH). By doing so, the drug may raise the risk of heart disease, according to a Canadian study.

Metformin was found to lower the thyroid-stimulating hormone (TSH) levels in patients with underactive thyroids (hypothyroidism). Underactive thyroids directly affect the heart, resulting in slow heart rate and low blood pressure. In severe, prolonged hypothyroidism, the heart muscle fibers become weak and can lead to heart failure.

However, with previous studies suggesting metformin to have a relatively good heart-health safety record, patients taking the drug should not be unduly worried. It is used to lower blood glucose levels, and it works by reducing glucose production in the liver.

Debate over metformin

Metformin has long been a cornerstone of therapy for glycemic control in patients with type 2 diabetes.

The most common type of cardiovascular problem in the U.S. is coronary artery disease, which can cause heart attacks. Heart failure is a prevalent comorbidity in people with diabetes, affecting about 25 to 40 percent of all diabetic adults.

While some evidence points to metformin's increased risks of heart problems, other forms of diabetes research dismiss the link.

For the recent study, published in the Canadian Medical Association Journal, researchers analyzed data of 74,300 patients who received metformin and sulfonylurea, another common diabetes drug, over a 25-year period. During that time, about 5,689 were treated for hypothyroidism and 59,937 had normal thyroid function.

The results indicated that in the group with hypothyroidism, there were about 119.7 per 1000 cases of low thyroid-stimulating hormone per year, as opposed to 4.5 per 1000 in the normal group.

However, while low levels of TSH have been associated with heart disease, the study did not look for or show any links between metformin and heart disease.

"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," researcher Dr. Laurent Azoulay of McGill University in Montreal, Canada, said in a news release.

Other leading opinions

Experts agreed that the finding requires additional research.

Dr. Minisha Sood, an endocrinologist at Lenox Hill Hospital in New York City, added that the reason for metformin's effect on TSH is not currently clear. It is also not well understood whether the low TSH levels associated with metformin put patients at risk for developing other complications such as cardiovascular diseases.


To learn more about how metformin medication affects diabetes:

The Positive Effects of Metformin
Metformin: Side Effects and Precautions
Metformin versus Sulfonylurea