Overtreating hypothyroidism patients may increase risk of stroke

According to a new study, for patients who take medication to treat hypothyroidism, being treated with too much medication can lead to an increased risk of Atrial fibrillation, a common heart rhythm disorder associated with stroke.

The findings were presented by researchers from the Intermountain Medical Center Heart Institute in Salt Lake City at the American Heart Association Scientific Session conference in Chicago.

"We know patients with hypothyroidism have a higher risk of atrial fibrillation, but we didn't consider increased risk within what's considered the normal range of thyroid hormones. These findings show we might want to reconsider what we call normal." said lead researcher Jeffrey L. Anderson.

For the study, researchers surveyed the electronic medical records of 174,914 patients treated at Intermountain Healthcare facilities whose free thyroxine (fT4) levels were recorded and who were not on thyroid replacement medication. Researchers then took what's considered a normal range of fT4 levels, divided it into four quartiles, and then looked at those patients' records for a current or future diagnosis of Atrial fibrillation.

They found a 40 per cent increase in existing Atrial fibrillation for patients in the highest quartile of fT4 levels compared to patients in the lowest, and a 16 per cent increase in newly developing Atrial fibrillation during 3-years of follow up.

These findings suggest that the optimal healthy range of fT4 should be reconsidered and redefined, according to Dr. Anderson.

"Thyroid hormones are associated with losing weight and having more energy, which may lead to people being treated at the high end of the normal range. Are we harming people by putting them at a higher risk of atrial fibrillation, and therefore stroke?" said Dr. Anderson.

The study also showed that fT4 should be measured, along with thyroid-stimulating hormone (TSH), which is more commonly tested for in patients with irregular thyroid hormone levels but was not helpful within the normal range in refining risk.

"The next step for researchers is to conduct a randomized trial to see if targeting a lower versus a higher upper range of fT4 in patients receiving thyroid hormone replacement therapy leads to a lower risk of atrial fibrillation and stroke along with other possible heart-related issues, like atherosclerosis," Dr. Anderson said.