– The short-term risk of developing heart failure in patients with newly identified hypothyroidism, be it overt or subclinical, is double that of euthyroid individuals, Caroline H. Noergaard, MD, reported at the American Heart Association scientific sessions.

Dr. Caroline H. Noergaard of Aalborg University in Denmark Bruce Jancin/MDedge News

Dr. Caroline H. Noergaard

“This is really important clinically. The association with heart failure has previously been shown in both overt and subclinical hyperthyroidism, but it’s actually new knowledge that hypothyroidism is associated with immediate risk of heart failure. And a lot of people have subclinical hypothyroidism,” said Dr. Noergaard, a PhD student in epidemiology at Aalborg (Denmark) University.

Also at the meeting, Jeffrey L. Anderson, MD, reported that free thyroxine levels within the normal reference range were associated in graded fashion with an increased prevalence and incidence of atrial fibrillation in a large Utah study, a finding that provides independent confirmation of an earlier report by investigators from the population-based Rotterdam Study.

“These findings validate those of the Rotterdam Study in a much larger dataset and may have important clinical implications, including a redefinition of the reference range and the target-free T4 levels for thyroxine replacement therapy,” observed Dr. Anderson, professor of internal medicine at the University of Utah, Salt Lake City, and a research cardiologist at the Intermountain Medical Center Heart Institute.

Hypothyroidism and heart failure

Dr. Noergaard presented a retrospective study of over 1 million Copenhagen-area adults (mean age, 50 years) with no history of heart failure, who had their first thyroid function test. She and her coinvestigators turned to comprehensive Danish national health care registries to determine how many of these individuals were diagnosed with new-onset heart failure within 90 days after their thyroid function test.

Risk of heart failure within 90 days of first thyroid function test

Subclinical hypothyroidism was defined by a thyroid-stimulating hormone level greater than 5 mIU/L and a free T4 of 9-22 pmol/L. Overt hypothyroidism required a TSH greater than 5 mIU/L with a free T4 less than 9 pmol/L.

Free T4 predicts atrial fibrillation risk

Dr. Anderson presented a retrospective analysis of 174,914 adult patients in the Intermountain Healthcare EMR database, none of whom were on thyroid replacement at entry. The patients, who were a mean age of 64 years and 65% women, were followed for an average of 6.3 years. Of these, 88.4% had a free T4 within the normal reference range of 0.75-1.5 ng/dL, 7.4% had a value below the cutoff for normal, and 4.2% had a free T4 above the reference range.

Upon dividing the patients within the normal range into quartiles based upon their free T4 level, he and his coinvestigators found that the baseline prevalence of atrial fibrillation was 8.7% in those in quartile 1, 9.3% in quartile 2, 10.5% in quartile 3, and 12.6% in quartile 4. In a multivariate analysis adjusted for potential confounders, the risk of prevalent atrial fibrillation was increased by 11% for patients in quartile 2, compared with those in the first quartile, by 22% in quartile 3, and by 40% in quartile 4.

The incidence of new-onset atrial fibrillation during 3 years of follow-up was 4.1% in patients in normal-range quartile 1, 4.3% in quartile 2, 4.5% in quartile 3, and 5.2% in the top normal-range quartile. The odds of developing atrial fibrillation were increased by 8% and 16% in quartiles 3 and 4, compared with quartile 1.

Serum TSH and free T3 levels showed no consistent relationship with atrial fibrillation.

The Utah findings confirm in a large U.S. population the earlier report from the Rotterdam Study (J Clin Endocrinol Metab. 2015 Oct;100(10):3718-24).

Dr. Noergaard and Dr. Anderson reported having no financial conflicts regarding their studies, which were carried out free of commercial support.