Metformin treatment associated with low thyroid hormone

by Bruce Sylvester – Researchers report that metformin therapy, a treatment for type 2 diabetes, could induce a lowering thyroid-stimulating hormone (TSH) in patients with underactive thyroids (hypothyroidism). The findings were published on Sept 22, 2014 in CMAJ (Canadian Medical Association Journal).

As background, the authors noted that low levels of TSH can lead to adverse cardiovascular conditions and fractures.

In their retrospective study, the investigators reviewed data on 74,300 subjects who received metformin and sulfonylurea over a 25-year study period.

They found that 5,689 had been treated for hypothyroidism, and 59,937 maintained normal thyroid function.

Among the subjects with hypothyroidism, there were 495 episodes of low thyroid-stimulating hormone (119.7 per 1000) per year compared with 322 in the normal group (4.5 per 1000). Therefore, among patients with identified hypothyroidism, metformin monotherapy correlated to a 55% increased risk of low TSH levels compared with sulfonylurea monotherapy.

Metformin therapy did not appear to have any adverse effect on subjects 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,” says Laurent Azoulay, Ph.D, 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,” he added.