Subclinical hyperthyroidism as a risk factor for atrial fibrillation.
OBJECTIVE
Atrial fibrillation is a well-known manifestation of hyperthyroidism. We studied whether subclinical hyperthyroidism with low serum thyrotropin concentrations and free thyroid hormone concentrations within the normal range in clinically euthyroid persons is a risk factor for subsequent atrial fibrillation.


METHODS
We studied 23,638 persons. The subjects were classified according to their serum thyrotropin concentrations: group 1 comprised those with normal values of serum thyrotropin concentration (>0.4-5.0 mU/L) and free tri-iodothyronine and free thyroxine concentrations were within the normal range (22,300 subjects), group 2 comprised those with both low serum thyrotropin values (</=0.03 mU/L) and elevated free tri-iodothyronine and free thyroxine concentrations (725 subjects), and group 3 comprised those with low values of serum thyrotropin (<0.4 mU/L) and free triiodothyronine and free thyroxine concentrations were within the normal range (613 subjects).


RESULTS
Atrial fibrillation was present in 513 persons (2.3%) in group 1 with normal values for serum thyrotropin, 100 (13.8%) in group 2 with overt hyperthyroidism, and 78 (12.7%) in group 3 with subclinical hyperthyroidism. The prevalence of atrial fibrillation in patients with low serum thyrotropin concentrations (<0.4 mU/L) was 13.3% compared with 2.3% in patients with normal values for serum thyrotropin (P <.01). The relative risk of atrial fibrillation in subjects with low serum thyrotropin and normal free tri-iodothyronine and free thyroxine concentrations, compared with those with normal concentrations of serum thyrotropin, was 5.2 (95% CI 2.1-8.7, P <.01).


CONCLUSIONS
A low serum thyrotropin concentration is associated with a >5-fold higher likelihood for the presence of atrial fibrillation with no significant difference between subclinical and overt hyperthyroidism.
Auer
J
J
Division of Cardiology and Intensive Care, Department of Internal Medicine II, General Hospital Wels, Wels, Austria. johann.auer@khwels.at
Scheibner
P
P
Mische
T
T
Langsteger
W
W
Eber
O
O
Eber
B
B
D001281:Atrial Fibrillation / Q000097:blood / Q000453:epidemiology*; D015897:Comorbidity; D006801:Humans; D006980:Hyperthyroidism / Q000097:blood / Q000453:epidemiology*; D008875:Middle Aged; D012307:Risk Factors; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood
D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine