One of the most widely-prescribed medications for type 2 diabetes has been linked to lower levels of thyroid functioning, according to a study in the Canadian Medical Association Journal.

Taking metformin, used to lower blood glucose levels by reducing glucose production in the liver, may promote an increased risk of low thyroid-stimulating hormone levels -- which could in turn lead to harmful effects such as cardiovascular conditions and bone fractures, said a news release.

Sometimes metformin is administered for use in combination with insulin or other medications, but it is not intended for treating type 1 diabetes.

The study focused on patients previously disgnosed with underactive thyroids, or hypothyroidism.

Researchers looked at data generated on 74,300 patients who received metformin and sulfonylurea, another common diabetes drug, over a 25-year study period.

Out of the study subjects, 5,689 had been treated for hypothyroidism and 59,937 reported having normal thyroid function.

In the hypothyroidism group, there were 495 who experienced incidences of low thyroid-stimulating hormone annually, compared with 322 in the normal group.

In patients with treated hypothyroidism, metformin monotherapy -- in other words, injecting the drug as prescribed by physicians -- was associated with a 55 percent increased risk of low TSH levels, in comparison with the risk associated with sulfonylurea treatment.

Metformin therapy, the research said, did not appear to affect people with normal thyroid function.

"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," said study lead Dr. Laurent Azoulay of the Lady Davis Institute, Jewish General Hospital and the Department of Oncology, McGill University, Montréal, Quebec.

"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."