Cardiac Autonomic Modulation and Anti-Thyroid Peroxidase (TPO) Antibodies in Subclinical Hypothyroidism: Does a Correlation Exist?

Background: Heart rate variability (HRV) reflects the balance between the sympathetic and parasympathetic divisions of the autonomic nervous system. Anti-thyroid antibodies like anti-TPO and anti-Thyroglobulin have long been associated with thyroid dysfunction and abnormal thyroid profile testing. Subclinical hypothyroidism (SCHypo) is characterized by elevated thyroid-stimulating hormone (TSH) with normal thyroid hormones. We hypothesize that autonomic function may be deranged in anti-TPO positive sub-clinical hypothyroid cases, even before the onset of overt hypothyroidism. Objectives: To investigate the association between anti-Thyroid Peroxidase antibodies (anti-TPOAb) positive SCHypo and sympathovagal imbalance (SVI), if any. Methodology: The study was conducted on the age and body mass index (BMI) matched subclinical hypothyroid patients (n=52) and healthy controls (n=20). The cardiac autonomic activity was assessed by short-term HRV in the time (SDNN, RMSSD, pNN50) and frequency domains (LFms(2), HFms(2), LFnu, HFnu, TP, and LF/HF ratio). Nonlinear geometric measures (SD1, SD2, SD1/SD2, TINN, HRV triangular index) were also evaluated. Biochemical evaluation of serum thyroid profile and anti-TPOAb was done in all the subjects. Results: Decreased HRV was observed in the anti-TPOAb positive group when compared to the antibody-negative and control groups. Significant positive correlation of anti-TPOAb with TSH, LFnu, LF/HF and negative correlation with SDNN, RMSSD, pNN50, SD1, SD1/SD2, HFnu, and TP of HRV was observed. Conclusion: Anti-TPOAb positive SCHypo group exhibited modifications in HRV characterized by decreased parasympathetic modulation, as compared to controls. The findings were also suggestive of increased risk of autonomic dysfunction in TPOAb-positive patients, as compared to antibody negative. An increase in anti-TPO antibodies was significantly correlated with TSH and SVI in SCHypo patients.