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M. D. Gammage, MD, FRCP; J. V. Parle, MD, FRCGP; R. L. Holder, BSc; L. M. Roberts, PhD; F. D. R. Hobbs, FRCP, FRCGP; S. Wilson, PhD; M. C. Sheppard, PhD, FRCP; J. A. Franklyn, MD, PhD

Arch Intern Med. 2007;167(9):928-934.

Background  Previous studies have suggested that minor changes in thyroid function are associated with risk of atrial fibrillation (AF). Our objective was to determine the relationship between thyroid function and presence of atrial fibrillation (AF) in older subjects.

Methods  A population-based study of 5860 subjects 65 years and older, which excluded those being treated for thyroid dysfunction and those with previous hyperthyroidism. Main outcome measures included tests of thyroid function (serum free thyroxine [T₄] and thyrotropin [TSH]) and the presence of AF on resting electrocardiogram.

Results  Fourteen subjects (0.2%) had previously undiagnosed overt hyperthyroidism and 126 (2.2%), subclinical hyperthyroidism; 5519 (94.4%) were euthyroid; and 167 (2.9%) had subclinical hypothyroidism and 23 (0.4%), overt hypothyroidism. The prevalence of AF in the whole cohort was 6.6% in men and 3.1% in women (odds ratio, 2.23; P<.001). After adjusting for sex, logistic regression showed a higher prevalence of AF in those with subclinical hyperthyroidism compared with euthyroid subjects (9.5% vs 4.7%; adjusted odds ratio, 2.27; P = .01). Median serum free T₄ concentration was higher in those with AF than in those without (1.14 ng/dL; interquartile range [IQR], 1.05-1.27 ng/dL [14.7 pmol/L; IQR, 13.5-16.4 pmol/L] vs 1.10 ng/dL; IQR, 1.00-1.22 ng/dL [14.2 pmol/L; IQR, 12.9-15.7 pmol/L]; P<.001), and higher in those with AF when analysis was limited to euthyroid subjects (1.13 ng/dL; IQR, 1.05-1.26 ng/dL [14.6 pmol/L; IQR, 13.5-16.2 pmol/L] vs 1.10 ng/dL; IQR, 1.01-1.21 ng/dL [14.2 pmol/L; IQR, 13.0-15.6 pmol/L]; P = .001). Logistic regression showed serum free T₄ concentration, increasing category of age, and male sex all to be independently associated with AF. Similar independent associations were observed when analysis was confined to euthyroid subjects with normal TSH values.

Conclusions  The biochemical finding of subclinical hyperthyroidism is associated with AF on resting electrocardiogram. Even in euthyroid subjects with normal serum TSH levels, serum free T₄ concentration is independently associated with AF.

Author Affiliations: Departments of Cardiovascular Medicine (Dr Gammage) and Medicine (Drs Sheppard and Franklyn), Division of Medical Sciences and Department of General Practice (Drs Parle, Roberts, Hobbs, and Wilson and Mr Holder), Division of Primary Care, Public, and Occupational Health, University of Birmingham, Birmingham, England.

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