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, a new study of more than 174,000 patients has found.

Hypothyroidism is an underactive thyroid gland. Hypothyroidism means that the thyroid gland can’t make enough thyroid hormone to keep the body running normally. People are hypothyroid if they have too little thyroid hormone in the blood. Common causes are autoimmune disease, such as Hashimoto’s thyroiditis,surgical removal of the thyroid, and radiation treatment.

The thyroid gland is a butterfly-shaped endocrine gland that is normally located in the lower front of the neck. The thyroid’s job is to make thyroid hormones, which are secreted into the blood and then carried to every tissue in the body. Thyroid hormone helps the body use energy, stay warm and keep the brain, heart, muscles, and other organs working as they should.

The findings were presented by researchers from the Intermountain Medical Center Heart Institute in Murray at the American Heart Association Scientific Session conferencelast monthin 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,” said lead researcher Jeffrey L. Anderson, MD, clinical and research cardiologist at the Intermountain Medical Center Heart Institute. “These findings show we might want to re-consider what we call normal.”

Atrial fibrillation (also called AFib) is a quivering or irregular heartbeat (arrhythmia) that can lead to blood clots, stroke, heart failure and other heart-related complications. At least 2.7 million Americans are living with AFib.

In the new 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, then looked at those patients’ records for a current or future diagnosis of atrial fibrillation.

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

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

“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,” said Dr. Anderson. “Are we harming people by putting them at a higher risk of atrial fibrillation, and therefore stroke?”

Prevalence and Impact of Thyroid Disease
More than 12 percent of the U.S. population will develop a thyroid condition during their lifetime.

·      An estimated 20 million Americans have some form of thyroid disease.

·      Up to 60 percent of those with thyroid disease are unaware of their condition.

·      Women are five to eight times more likely than men to have thyroid problems.

·      One woman in eight will develop a thyroid disorder during her lifetime.

·      Most thyroid cancers respond to treatment, although a small percentage can be very aggressive.

·      The causes of thyroid problems are largely unknown.

·      Undiagnosed thyroid disease may put patients at risk for certain serious conditions, such as cardiovascular diseases, osteoporosis and infertility.

·      Pregnant women with undiagnosed or inadequately treated hypothyroidism have an increased risk of miscarriage, preterm delivery, and severe developmental problems in their children.

·      Most thyroid diseases are life-long conditions that can be managed with medical attention.

The Intermountain Medical Center Heart Institute 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 link between fT4 and atrial fibrillation was recently recognized in the Rotterdam Study, a population-based cohort study first started in 1990. However, with any new and unexpected report requires replication and independent confirmation. The new study by Intermountain Medical Center Heart Institute researchers establishes the link between fT4 level and atrial fibrillation  in patients treated in the United States.

“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.

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