When your heart goes pitter-patter, it might not be love, and you may just want to have your doctor check your thyroid hormone levels. New research from a collaborative group of investigators led by scientists at the University Hospital of Bern in Switzerland has found that individuals with higher levels of thyroid hormone (free thyroxine, or FT4) circulating in the blood were more likely than individuals with lower levels to develop irregular heartbeat, or atrial fibrillation, even when the levels were within normal range. Findings from the new study were released in Circulation, in an article entitled “Thyroid Function Within the Normal Range, Subclinical Hypothyroidism and the Risk of Atrial Fibrillation.”
In the United States, irregular heartbeat (atrial fibrillation) affects between 2.7 to 6.1 million people and is estimated to affect up to 12.1 million people by 2030. It 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, but sometimes patients with atrial fibrillation have no symptoms at all.
While many individuals can often live with an irregular heartbeat, it can cause chronic fatigue and increase the risk of serious illnesses, such as stroke and heart failure, potentially associated with lifelong disability and even death. Fortunately, medication and other therapies are available to treat an irregular heartbeat and reduce the risk of the associated symptoms and complications.
“Our findings suggest that levels of the thyroid hormone FT4 circulating in the blood might be an additional risk factor for atrial fibrillation,” explained lead study investigator Christine Baumgartner, M.D., specialist in general unternal medicine at the University Hospital of Bern, Switzerland, and currently a postdoctoral scholar at University of California San Francisco. “FT4 hormone levels might help to identify individuals at higher risk.”
In response to a thyroid-stimulating hormone (TSH) released by the pituitary gland, the thyroid gland secretes thyroid hormones required to regulate energy metabolism. Patients with low levels of thyroid hormone, or hypothyroidism, may require medications containing thyroid hormone (thyroxine, or T4) to increase their hormonal levels. Sometimes intake of T4 sometimes can increase these levels too much.
Previous studies showed that the risk of irregular heartbeat is greater among individuals who produce too much thyroid hormone than among those with normal hormonal levels. What was unclear, however, was whether levels that were high but still within the normal range could also increase the risk of irregular heartbeat. To better understand this relationship, investigators looked at the occurrence of irregular heartbeat among individuals with thyroid hormone levels that were still within normal range. They found that individuals with higher blood levels of FT4 within the normal range at the beginning of the study were significantly more likely than those with lower levels to subsequently develop an irregular heartbeat.
The investigators analyzed data from 11 studies from Europe, Australia, and the United States that measured thyroid function and the occurrence of an irregular heartbeat. Overall, the studies included 30,085 individuals. Their average age was 69 years, and slightly more than half were women. On average, follow-up ranged from 1.3 to 17 years. The investigators obtained the studies by searching the MEDLINE and EMBASE medical databases through July 2016.
The research team found that when separated patients into four equal-sized groups, the group with the highest FT4 levels had a 45% increased risk of irregular heartbeat, compared to the group with the lowest levels. Even modest increases in thyroid hormone were associated with an increased risk. Among individuals with the second highest levels, the risk was 17% greater, and among those, with the third highest levels the risk was 25% greater, compared to those with the lowest levels. Elevated levels of TSH within the normal range, however, were not associated with an increased risk of atrial fibrillation.
“Patients who are treated with T4, one of the most frequently prescribed drugs in the United States, generally have higher circulating FT4 compared to untreated individuals,” Dr. Baumgartner concluded. “So, an important next step is to see whether our results also apply to these patients, in order to assess whether target FT4 thyroid hormone concentrations for thyroid-replacement therapy need to be modified.”
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