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Andrew D. Krahn, MD; George J. Klein, MD; Charles R. Kerr, MD; John Boone, MD; Robert Sheldon, MD, PhD; Martin Green, MD; Mario Talajic, MD; Xiaohua Wang, MSc; Stuart Connolly, MD

Arch Intern Med. 1996;156(19):2221-2224.

Abstract
Background
Patients with recent-onset atrial fibrillation often undergo routine thyroid function screening to rule out thyroid disease as a cause of atrial fibrillation.

Methods
Patients with recent (<3 months) onset of documented atrial fibrillation or flutter were enrolled in the Canadian Registry of Atrial Fibrillation from outpatient clinics, emergency departments, and hospital wards across Canada. Seven hundred twenty-six patients underwent baseline thyroid function screening and were assessed for presence of clinical thyroid disease. Serum thyrotropin level (TSH) was measured in 707 patients (97%), and thyroxine level (T₄) in 407 patients (56%).

Results
A TSH level less than 0.1 mU/L was present in 5 patients (0.7%). A TSH level less than normal but more than 0.1 mU/L was present in 34 patients (4.7%). No patient had definite hypothyroidism (TSH >20 mU/L), but 56 patients (7.7%) had an elevated TSH level that was less than 20 mU/L. During 1.7 years of follow-up, only 7 patients were found to have clinical hyperthyroidism, and 11 patients (1.5%) had hypothyroidism. Logistic regression analysis showed that palpitations (odds ratio, 4.9; 95% confidence interval, 1.7-14.0) and asymptomatic presentation (odds ratio, 5.5; 95% confidence interval, 1.9-16.2) were risk factors for low TSH level, and increasing age (odds ratio, 1.32 every 10 years; 95% confidence interval, 1.01-1.66) was a risk factor for high TSH level. The positive predictive value of palpitations and asymptomatic presentation for low TSH level were 9% and 8%, respectively.

Conclusions
An abnormal TSH level is common in patients with recent-onset atrial fibrillation. However, clinical thyroid disease is uncommon. Routine TSH screening of patients who have atrial fibrillation has a low yield and may be better applied to those patients at higher risk of having undiagnosed clinical thyroid disease.

Arch Intern Med. 1996;156:2221-2224

Author Affiliations
for the Canadian Registry of Atrial Fibrillation Investigators

From the University of Western Ontario, London (Drs Krahn and Klein); the University ofBritish Columbia, Vancouver (Drs Kerr and Boone and Ms Wang); the University of Calgary, Alberta (Dr Sheldon); the University of Ottawa, Ontario (Dr Green); the Montreal (Quebec) Heart Institute (Dr Talajic); McMaster University, Hamilton, Ontario (Dr Connolly). A list of the investigators for the Canadian Registry of Atrial Fibrillation appears at the end of the article.

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