Subclinical hyperthyroidism was linked to atrial fibrillation but not to other clinical cardiovascular conditions or deaths in a new study.
“We report an independent association of subclinical hyperthyroidism with incident atrial fibrillation but not with other clinical cardiovascular conditions or mortality,” reported Dr. Anne R. Cappola from the University of Pennsylvania in Philadelphia and her colleagues (JAMA 2006;295:1033–41).
The study examined the link between unrecognized thyroid dysfunction and cardiovascular risk, including atrial fibrillation, coronary heart disease, cerebrovascular disease, and death (cardiovascular and all-cause) in a subgroup of 3,233 participants in the population-based, longitudinal Cardiovascular Health Study. The study subjects were community-dwelling older adults with a mean age of 73 years who were followed for an average duration of 12.5 years.
At baseline, 82% of the cohort were euthyroid, 15% had subclinical hypothyroidism (TSH above 4.5 mU/L and below 20 mU/L with a normal FT4 concentration), 1.6% had overt hypothyroidism (TSH of at least 20 mU/L or TSH of more than 4.5 mU/L with a low FT4), and 1.5% had subclinical hyperthyroidism (TSH of 0.10–0.44 mU/L or less than 0.10 mU/L with a normal FT4). Individuals with overt hyperthyroidism, or thyrotoxicosis (TSH below 0.10 mU/L with an elevated FT4 level) were excluded because of the small sample size.
Because the aim of the study was to detect previously unrecognized thyroid dysfunction, potential subjects were excluded if they were taking thyroid medication at baseline. However, once thyroid dysfunction was identified, medication use was included in the analysis because of its potential effect on subsequent cardiovascular risk.
The study found no differences in cardiovascular events at baseline between the euthyroid group and any of the three groups with thyroid dysfunction.
However, over the 12.5-year follow-up period, subclinical hyperthyroidism emerged as a risk factor for atrial fibrillation, but for no other clinical cardiovascular conditions. Subjects with subclinical hyperthyroidism had a greater incidence of atrial fibrillation than did euthyroid subjects (67 vs. 31 events per 1,000 person-years). After adjustment, this risk was nearly double (hazard ratio, 1.98). None of the other thyroid abnormalities were associated with increased CVD risk.
The findings do not support thyroid screening in older adults simply to prevent atrial fibrillation, since the estimated number needed to screen would be 2,500 to detect 1 case of atrial fibrillation, the authors noted.
However, the data do support the treatment of subclinical hyperthyroidism when it is detected.
The authors disagree with an earlier expert panel report that cited insufficient evidence to treat patients with TSH levels of 0.1 mU/L-0.45 mU/L and recommended treating only those patients with TSH levels below 0.1 mU/L (JAMA 2004;291:228–38).