21 May 2007
Arch Intern Med 2007; 167: 928-934
MedWire News: Subclinical hyperthyroidism is associated with atrial fibrillation (AF) on resting electrocardiogram (ECG), report UK researchers.
Their study showed that serum free thyroxine (T4) levels were linked to AF even in euthyroid patients with normal thyrotropin (TSH) levels.
It is well recognized that overt hyperthyroidism is associated with AF, note M Gammage and colleagues from the University of Birmingham in the Archives of Internal Medicine.
But some studies based on biochemical findings have suggested that even minor changes in thyroid function, which become more likely with increasing age, are associated with AF risk. To investigate further, the researchers set out to determine the relationship between thyroid function and AF in a community-based cohort of elderly people.
The team studied 5860 individuals aged 65 years and older, having excluded anyone being treated for thyroid dysfunction or with previous hyperthyroidism. The participants underwent thyroid function tests and a resting 12-lead ECG.
Fourteen (0.2%) participants were found to have previously undiagnosed overt hyperthyroidism, defined as raised free T4 and free tri-iodothyronine (T3) levels, or raised T3 alone, with serum TSH <0.4 p="0.01)." color="#000099">After adjusting for gender, this translated into an odds ratio for AF associated with subclinical hyperthyroidism of 2.27. There was no difference in AF prevalence between euthyroid individuals and those in other thyroid function categories, however. Further analysis revealed that, for the whole cohort, the median serum free T4 concentration was higher in those with AF than in those without, at 1.14 ng/dl versus 1.10 ng/dl (p<0.001). color="#000099">In contrast, TSH levels were not associated with AF. "The finding of increased likelihood of AF in those with subclinical hyperthyroidism lends support to the small number of studies linking this biochemistry with AF risk," the researchers conclude. The findings add to the debate over screening at-risk populations, such as elderly individuals who are known to have a higher prevalence of undetected (and usually mild) thyroid dysfunction, the team adds.