A new research in the American Heart Association’s journal Circulation associates higher levels of thyroid hormone (free thyroxine, FT4) with Irregular Heartbeat or Atrial Fibrillation, even when the levels are within the normal range.

Nearly 2.7 to 6.1 million people in the United States suffer from atrial fibrillation and the number is estimated to reach 12.1 million by 2030. It occurs when the two upper chambers of the heart, called the atria, beat irregularly and faster than normal. The patient may be symptomless or may complain of palpitations, dizziness, sweating, chest pain, anxiety, fatigue during exertion and fainting. Left untreated, the condition can increase the risk of stroke and heart failure.

Previous studies had established the Increased Risk of irregular heartbeat in individuals who produce too much thyroid hormone (FT4). However, it was unclear whether levels that were high but still within the normal range could also increase this risk.

To understand this relationship, researchers investigated 30,085 individuals from Europe, Australia, and United States.

The participants were separated into four equal-sized groups. The group with the highest FT4 levels had a 45 percent increased risk of irregular heartbeat compared to the group with the lowest levels. However, high level of thyroid-stimulating hormone (TSH) within the normal range was not associated with an increased risk of atrial fibrillation.

“Our findings suggest that the level of free thyroxine circulating in the blood might be an additional risk factor for atrial fibrillation. Patients with thyroid disorders are generally prescribed thyroxine and have higher circulating free thyroxine levels as compared to the untreated individuals. So, our next step is to see whether the results apply to these patients as well and access if the current thyroid-replacement therapy needs any modification,” said study lead author Christine Baumgartner.