Can 'Normal' T4 Levels Increase AF Risk?

Charles P. Vega, MD


December 11, 2018

Hello. I'm Dr Charles Vega, and I am a clinical professor of family medicine at the University of California at Irvine. Welcome to Medscape Morning Report, our 1-minute news story for primary care.

While it is known that hyperthyroidism is associated with a higher risk for atrial fibrillation (AF), could the risk also be increased for those within normal ranges for thyroid hormones? A study presented at the American Heart Association annual meeting suggests that the answer is yes.

The investigators conducted a retrospective analysis of data from almost 175,000 patients at the Intermountain Health System, who had records on free T4 levels but were not on thyroid replacement therapy at study entry. The mean age of this group was 63 years and about 65% were women. A sizeable majority, 88%, had T4 levels in the normal range. This large group was further stratified from low-normal to high-normal.

After adjustment, the relative risk for prevalent AF in the highest normal free T4 quartile group was double that of the lowest group. No similar associations were seen between the quartiles for free T3. Thyroid-stimulating hormone (TSH) results were mixed.

The findings could have particularly important implications in terms of treating even so-called subclinical hypothyroidism and the inclination to push levels into higher normal ranges. This is retrospective observational data which also carries the caveat that the free T4 levels considered high-normal would be considered overtly high in most practices.

Currently, TSH is generally used to adjust thyroid hormone treatment, but if these data are confirmed, it might mean that T4 levels should be routinely measured, too. For now, it should make us cautious about treating mild subclinical hypothyroidism, particularly among older adults at higher risk for AF.


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