October 23, 2017
2 min read

Elevated free thyroxine levels increase AF risk

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Patients with high free thyroxine levels had an increased risk for incident atrial fibrillation, according to a systematic review published in 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,” Christine Baumgartner, MD, specialist in general internal medicine at University Hospital of Bern in Switzerland, and a postdoctoral scholar at the University of California, San Francisco, said in a press release. “Free thyroxine hormone levels might help to identify individuals at high risk.”

Researchers reviewed data from 30,085 patients (median age, 69 years; 52% women) from 11 international studies that analyzed the link between thyroid-stimulating hormone and AF. Studies that included patients with overt thyroid dysfunction, pre-existing AF at baseline and those who took thyroid-altering medications were excluded, in addition to trials that only assessed postoperative AF.

Euthyroidism was defined as thyroid-stimulating hormone levels from 0.45 mIU/L to 4.49 mIU/L, and subclinical hypothyroidism was characterized as levels between 4.5 mIU/L and 19.9 mIU/L.

The outcome of interest was incident AF, which was determined by medical records, diagnostic codes, follow-up ECGs and self-report.

Patients were followed up for a median of 1.3 to 17 years. The analysis covered 278,955 person-years of follow-up.

Subclinical hypothyroidism was seen in 6.5% of patients. During follow-up, 8.6% of patients developed AF.

Baseline thyroid-stimulating hormone levels within the reference range were not linked to incident AF in patients with euthyroidism or subclinical hypothyroidism after adjusting for age and sex.

Patients with high free thyroxine levels at baseline had an increased risk for AF after adjusting for sex and age (HR of 4th vs. 1st quartiles = 1.45; 95% CI, 1.26-1.66; P for trend .001).

Adjustment for previous CVD did not change the results.

“The mechanism for the association between thyroid function and AF may be explained by the effects of thyroid hormones on the cardiovascular system,” Baumgartner and colleagues wrote. “Thyroid function in the high range leads to an increase in vascular resistance, cardiac contractility, heart rate and left ventricular mass. Thyroid hormone levels in the high range have been shown to be arrhythmogenic and increase the frequency of atrial premature beats, which in turn is a risk factor for AF. These effects may explain the observation of an increased risk of AF in subclinical hyperthyroidism, but not subclinical hypothyroidism.” – by Darlene Dobkowski

Disclosures: Baumgartner reports no relevant financial disclosures. Please see the study for all other authors’ relevant financial disclosures.