Background: Abnormalities of cardiac autonomic function may contribute to excess vascular mortality reported in treated hyperthyroidism.
Aim: To evaluate cardiac autonomic function in hyperthyroidism before and after effective anti-thyroid therapy.
Methods: 405 consecutive unselected patients with hyperthyroidism were recruited with 405 age- and sex-matched euthyroid controls for matched comparisons; all had 24-hour Holter monitoring. Patients were reassessed when rendered euthyroid. Time domain parameters of heart rate variability (HRV) and heart rate turbulence (HRT) were determined from the Holter data.
Results: The median age (+IQR) of the hyperthyroid and control cohorts was 50.0 (37-65) with 320 females and 85 males in each group. The cohorts were matched with respect to past history and family history of vascular disease (88 vs 80 and 278 vs 280 respectively, p=ns). The median serum free thyroxine (T4) and tri-iodothyronine (T3) concentrations at presentation were 37.1pmol/L (IQR 27-52) and 11.3pmol/L (IQR 8-18) respectively; serum thyrotrophin (TSH) concentration was undetectable in all patients. Serum free T4, T3 and TSH were normal in all controls. Measures of overall autonomic control (SDNN and HRVi), and more specifically vagal modulation (pNN50 and RMSSD), were reduced at presentation compared with matched controls as well as measures of HRT (onset and slope). Two hundred and three patients were rendered euthyroid at a median follow up 21 weeks (IQR 15-42); all had serum free T4, T3 and TSH concentrations in the normal range. All HRV and HRT parameters returned to values comparable with matched controls (p=ns).
Conclusions: Overt hyperthyroidism is associated with cardiac autonomic dysfunction with reduced autonomic control and specifically reduced vagal modulation of heart rate; restoration of euthyroidism appears to correct this. Both HRV and HRT parameters convey prognostic significance in ischaemic heart disease. Long-term follow up of our cohort will address whether these parameters convey similar prognostic information in hyperthyroidism