Metformin may increase risk of thyroid problems

Metformin has been linked to an increased risk of low TSH levels in patients with hypothyroidism, potentially exposing them to cardiovascular disease, research suggests.

Metformin: may increase risk of low TSH levels (Photo: SPL)
Metformin: may increase risk of low TSH levels (Photo: SPL)

Among patients with treated hypothyroidism, those taking metformin monotherapy had a 55% greater risk of low TSH levels compared with patients on a sulfonylurea.

The study, published in the Canadian Medical Association Journal, found patients with normal thyroid function who took the drug had no additional risk.

The researchers said it would be ‘imperative’ to assess the clinical consequences of the finding, given the relatively high incidence of low TSH levels in patients on metformin.

Their study looked at data from 5,689 patients with treated hypothyroidism and 59,937 with normal thyroid function, over a 25-year period.

Among those with hypothyroidism, there were 495 events of low TSH levels, equivalent to 119.7 per 1,000 person-years. This compared with 322 events in the normal group, or 4.5 per 1,000.

The risk among patients taking metformin peaked at 130% greater than those on a sulfonylurea at 90 to 180 days after treatment began.

Lower TSH levels can expose patients to the effects of subclinical hyperthyroidism, including cardiovascular conditions and fractures, the authors warned.

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