Serum total thyroxine, triiodothyronine, and thyrotropin response to thyrotropin-releasing hormone (TRH-TSH test) were measured in 126 consecutive patients admitted with atrial fibrillation: 33 patients with an acute arterial limb embolism (Group I), 31 patients with an acute embolic stroke (Group II), and 62 patients without any arterial occlusion (Group III). A blunted TRH-TSH test, suggestive of thyrotoxicosis, was found in 5 patients in Group I, 8 patients in Group II, and 2 patients in Group III. The diagnosis of hyperthyroidism was confirmed in 8 patients (by repeated TRH-TSH test and scintigraphy): 4 patients in Group I (12.1%) and 4 patients in Group II (12.9%). All of them had a nonvalvular atrial fibrillation. Thyrotoxicosis should not be recognized in 6 of them if TRH-TSH test was not performed, because peripheral hormone levels were normal. Five of these 8 patients with thyrotoxicosis had reversion to sinus rhythm after treatment with carbimazole, either spontaneously or after cardioversion. This outcome prevented prolongation of anticoagulant therapy for an indefinite time.