Subclinical Hypothyroidism: An Overlooked Cause of Atrial Fibrillation?

The first patient, a 42-year-old female presented with frequent palpitations, corresponding to brief episodes of non-sustained, paroxysmal atrial fibrillation on 24hour Holter recordings. Hypertension was excluded, based on repeated blood pressure recordings and on ambulatory monitoring; echocardiography, exercise stress testing, as well as blood biochemistry were normal. However, mild subclinical hypothyroidism was diagnosed based on thyrotropin (thyroid-stimulating hormone, TSH) values of 5.69mIU/L, with normal triiodothyronine(T3) (1.53ng/ml, laboratory reference values: 0.56-1.56ng/ml) and free(T4) (1.12ng/ dl, laboratory reference values: 0.6-1.37ng/dl) values. She was treated with levothyroxin 50μg daily and her symptoms progressively resolved; during a 24-month follow-up, thyroid function tests remained normal and she had no further episodes of atrial fibrillation, either clinically or on repeated 24-hour Holter recordings.