Higher free thyroxine levels within normal range confer elevated AF risk

CHICAGO — Patients with elevated free thyroxine levels — still within what is considered the normal range — may be at greater risk for atrial fibrillation than those with lower levels, according to findings presented at the American Heart Association Scientific Sessions.

“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,” Jeffrey L. Anderson, MD, Distinguished Clinical and Research Physician at the Intermountain Medical Center Heart Institute, said in a press release. “These findings show we might want to reconsider what we call normal.”

To validate previous findings that suggested higher levels of free thyroxine within the defined reference range were associated with increased prevalence and incidence of AF, researchers used data related to 174,914 patients from the Intermountain Healthcare electronic medical record database warehouse (mean age, 64 years; 65% women). Researchers considered all patients’ free thyroxine levels and divided the reference range of 0.75 ng/dL to 1.5 ng/dL into quartiles.

Multivariable regression was used to determine the associations between free thyroxine levels and the prevalence and incidence of AF.

Patients were followed for a median of 6.3 years.

Of the total cohort, 7.4% had free thyroxine levels below the reference range, 88.4% had levels within the reference range and 4.2% had levels above the reference range.

Researchers found that the adjusted OR for prevalent AF was doubled for those with elevated free thyroxine levels in the normal range compared with those with lower levels in the first quartile of the normal range (aOR = 1.99; P < .0001). Within this referent range, a gradient of risk was observed.

In addition, patients with higher free thyroxine levels were at greater risk for incident AF, although this was a less marked pattern (adjusted HR for high vs. normal = 1.11; P = .01), according to the researchers.

No association was found between either AF prevalence or incidence and free triiodothyronine levels within the reference range and the relationship between thyroid-stimulating hormone levels and AF prevalence and incidence could not be determined.

“The next step for researchers is to conduct a randomized trial to see if targeting a lower vs. a higher upper range of [free thyroxine levels] 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,” Anderson said in the release. – by Melissa J. Webb

Reference:

Anderson JL, et al. Poster Sa1208. Presented at: American Heart Association Scientific Sessions; Nov. 10-12, 2018; Chicago.

Disclosures: The authors report no relevant financial disclosures.