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Cardiac arrhythmias may often be of the thyroid origin. It is so-called thyroid-cardiac syndrome which may also be manifested by the circulatory failure and angina pectoris. The authors have been observed 54 cases of such arrhythmias; they have frequently been manifested by the paroxysmal tachycardia, extrasystolic beats, and paroxysmal or stable atrial fibrillation. Diagnosis of such arrhythmias may be difficult in case of the masked hyperthyroidism being its only clinical symptom. In order to establish a proper diagnosis the following tests of thyroid gland functioning have been carried out: TRH-TSH, scintigraphy and ultrasound examination of the thyroid gland. TRH-TSH test enables to detect disorders of hypophyseal-thyroidal regulation characteristic for both overt and masked hyperthyroidism. Scintigraphic examination reveals autonomic nodules of the thyroid glands being frequently a cause of cardiac arrhythmias. Ultrasound examination enables confirmation of the diagnosed autonomic thyroid nodules without TSH test. In the majority of cases of cardiac arrhythmias therapy with radioactive iodine isotope was introduced. Some patients with appropriate indications have been treated surgically after proper preparation. Pharmacological treatment in thyroid-cardiac syndrome produces transient and instable result.