Individuals with higher levels of thyroid hormone in the blood are more likely to develop irregular heartbeat, suggesting that the levels could help identify individuals at risk for the condition, researchers say.

Irregular heartbeat or atrial fibrillation occurs when the two upper chambers of the heart, called the atria, beat irregularly and faster than normal.Symptoms may include heart palpitations, dizziness, sweating, chest pain, anxiety, fatigue during exertion and fainting, and can cause stroke and heart failure, potentially associated with lifelong disability and even death.The findings showed that individuals with higher blood levels of thyroid hormone -- free thyroxine or FT4 -- had 45 per cent increased risk of suffering from irregular heartbeat, compared to people with the lowest levels.Even more modest increases in thyroid hormone were associated with an increased risk, the researchers noted in the journal Circulation.

"Our findings suggest that levels of the thyroid hormone, free thyroxine, circulating in the blood might be an additional risk factor for atrial fibrillation," said lead author Christine Baumgartner, a postdoctoral scholar at the University of California - San Francisco. "Free thyroxine hormone levels might help to identify individuals at higher risk," Baumgartner added.Elevated FT4 levels may also indicate hyperthyroidism, and other thyroid problems, such as thyroiditis or toxic multinodular goiter. For the study, the team analysed data from 11 studies from Europe, Australia, and the US, including 30,085 individuals, that measured thyroid function and the occurrence of irregular heartbeat. Blood levels of thyroid-stimulating hormone, which regulates the production of thyroid hormones and is primarily measured in clinical practice to assess thyroid function, however, were not associated with an increased risk of irregular heartbeat.