Sympathovagal imbalance in hyperthyroidism

We assessed sympathovagal balance in thyrotoxicosis. Fourteen patients with Graves' hyperthyroidism were studied before and after 7 days of treatment with propranolol (40 mg 3 times a day) and in the euthyroid state. Data wer e compared with those obtained in a group of age-, sex-, and weight-matched controls. Autonomic inputs to the heart were assessed by power spectral an alysis of heart rate variability. Systemic exposure to sympathetic neurohor mones was estimated on the basis of 24-h urinary catecholamine excretion. T he spectral power in the high-frequency domain was considerably reduced in hyperthyroid patients, indicating diminished vagal inputs to the heart. Inc reased heart rate and mid-frequency/high-frequency power ratio in the prese nce of reduced total spectral power and increased urinary catecholamine exc retion strongly suggest enhanced sympathetic inputs in thyrotoxicosis. All abnormal features of autonomic balance were completely restored to normal i n the euthyroid state. beta -Adrenoceptor antagonism reduced heart rate in hyperthyroid patients but did not significantly affect heart rate variabili ty or catecholamine excretion. This is in keeping with the concept of a joi nt disruption of sympathetic and vagal inputs to the heart underlying chang es in heart rate variability. Thus thyrotoxicosis is characterized by profo und sympathovagal imbalance, brought about by increased sympathetic activit y in the presence of diminished vagal tone.