Occult thyrotoxicosis in patients with chronic and paroxysmal isolated atrial fibrillation.

Thyroid function tests, including thyrotropin releasing hormone administration (TRH), were performed in 40 consecutive patients with isolated atrial fibrillation (IAF) (i.e., without any other evidence of cardiac disease). The arrhythmia was chronic in 5 and paroxysmal in 35 patients. Thyrotoxicosis could not be diagnosed either clinically or by abnormal serum levels of T4, T3, T3 BC, and thyroid stimulating hormone (TSH). Thyroid stimulating hormone response to TRH, which was normal in 35 patients, was absent in 5 (12.5%) who were considered to have occult thyrotoxicosis. One had chronic and the other 4 had paroxysmal IAF. The arrhythmia did not recur after antithyroid treatment in these four patients who were in sinus rhythm after a mean follow-up period of 21 months. Full exploration of the thyroid function therefore seems useful not only in patients with chronic IAF, but also in those affected by the paroxysmal form.