Many diabetes patients use Metformin for their condition. The drug, which is often prescribed to individuals diagnosed with type 2 diabetes, reduces blood sugar levels by lowering the production of glucose in the liver. Findings of a new study, however, suggest that the medication could have unwanted effects on patients with underactive thyroid.

For the new study published in the Canadian Medical Association Journal (CMAJ) on Sept. 22, Laurent Azoulay, from McGill University in Montréal, Canada, and colleagues looked at the data of over 74,000 patients who use either metformin or sulfonylurea, another drug commonly prescribed for diabetic patients, for about 25 years.

Of these patients, 5,689 had treated hypothyroidism, or underactive thyroid. The researchers found that this group had more incidences of low levels of thyroid-stimulating hormone (TSH) per year compared with the 59,937 patients who had normal thyroid function. Low levels of TSH could cause unwanted health consequences including broken bones and cardiovascular conditions.

Patients with hypothyroidism had 495 incidences of low TSH levels (119.7 per 1000) per year while those with normal thyroid had 322, or 4.5 per 1000. The researchers also found that in patients with treated hypothyroidism, use of metformin increased their risks for low TSH levels by 55 percent when compared with patients who had sulfonylurea treatment.

"Compared with sulfonylurea monotherapy, metformin monotherapy was associated with a 55 percent increased risk of low TSH levels in patients with treated hypothyroidism, with the highest risk in the 90-180 days after initiation," the researchers wrote. "No association was observed in euthyroid patients."

Azoulay said that given the relative prevalence of low TSH levels in the subjects taking metformin, which has been used by millions of Type 2 diabetes worldwide for over 50 years, the findings of the research warrant further studies.

Minisha Sood, an endocrinologist, from New York City's Lenox Hill Hospital, agrees that additional studies would help given the limitations of the newly published research such that.

"It is also unclear whether the low TSH levels associated with metformin in this study put patients at risk for developing other complications such as cardiovascular diseases," Sood said.

Figures from the U.S. Centers for Disease Control and Prevention (CDC) show that 8.3 percent of the American population has diabetes. Up to 95 percent of this has type 2 diabetes, which is associated with lifestyle practices, age, lack of exercise, and weight problems.

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