Subclinical hyperthyroidism as a risk factor for atrial fibrillation

Objectives 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 (