Common Diabetes Rx May Affect Thyroid Function

Metformin lowered thyroid stimulating hormone levels in patients with type 2 diabetes

(RxWiki News) People who have type 2 diabetes and a sluggish thyroid may need to watch the medication they take to control their blood sugar.

Metformin, used to treat people with type 2 diabetes, may reduce thyroid-stimulating hormone (TSH) levels in people with diabetes already treated for poor thyroid function, a new study found.

"Speak to your endocrinologist about your thyroid hormone levels."

Laurent Azoulay, PhD, and colleagues at McGill University in Montreal authored the study.

Past small studies suggested that metformin might reduce TSH levels. The study authors wanted to find out whether this was true and note how much of a decrease in thyroid function the medication might cause.

TSH is produced in the pituitary gland and stimulates the thyroid. Without enough of it, the thyroid won't release its own hormones.

The thyroid is a gland that makes other hormones that regulate body functions like metabolism. Without enough of these hormones, patients can become very tired, gain weight or have an irregular heart rate.

Metformin is a medicine prescribed to treat diabetes when patients don’t need insulin.

In type 2 diabetes, the body isn't able to process insulin in the right way. Type 1 diabetes, on the other hand, is marked by the body not making insulin at all. Insulin is a hormone that controls blood sugar.

The study authors found that type 2 diabetes patients on metformin who were being treated for low thyroid hormone levels had a 55 percent higher risk for lower TSH levels with metformin — compared to patients who took sulfonylureas instead of metformin. Sulfonylureas are another class of medicines used to treat diabetes.

All the patients used oral medicines to control their diabetes. The authors looked at 5,689 people with hypothyroidism (low thyroid hormone levels) and 59,937 people who had normal amounts of thyroid hormones.

All the patients with low thyroid hormone levels were being treated for it — 762 took sulfonylureas, such as glyburide (sold as Micronase and Glynase PresTab), while 4,927 took metformin (sold as Glucophage and Riomet).

Those on sulfonylureas did not seem to have lower TSH levels.

“With respect to sulfonylureas, there is no clear indication from the scientific literature that these drugs may affect TSH levels,” Dr. Azoulay told dailyRx News. “That being said, since these drugs were the comparator group in our study, it can be concluded that metformin is associated with an increased risk which is above any with sulfonylureas, if indeed there is one.”

Those on metformin had the lowest TSH levels during the first 90 to 180 days of taking the medicine.

Since the rate of low thyroid hormones was fairly high among people on metformin, Dr. Azoulay said that “this may be a bigger problem than previously thought, and thus additional studies are needed to assess the clinical impact of these findings.”

The study was published Sept. 22 in CMAJ.

The authors did not disclose any funding sources or conflicts of interest.

Review Date: 
September 22, 2014