Abstract
Background: Changes, in thyroid status, have pronounced effects on cardiovascular system reflecting autonomic dysfunction and increased morbidity. Exact interaction between autonomic nervous activity and thyroid hormones is controversial.

Aims and Objective: The cold pressor test (CPT) was used to evaluate cardiac autonomic function in thyroid patients and compared with age and sex-matched healthy controls (18-45 years).

Materials and Methods: This was a prospective case–control study. By clinical examination and serum levels of (T3, T4 and TSH) three groups hypothyroid, hyperthyroid and control group were categorized and blood pressure (BP) responses to CPT were evaluated in these groups. Statistical analysis was performed using student’s t-test.

Results: In hyperthyroid group, rise in systolic BP after CPT is not as that of control group. However, the difference in mean rise is statistically highly significant. While in hypothyroid group rise in diastolic BP is statistically highly significant as compared to that of control group.

Conclusion: In hypothyroidism, lack of direct action of thyroid hormones is compensated by increased basal sympathetic tone which exhibits blunted sympatho - excitatory responses leading to aortic stiffness and resistance. This prevents fall in systolic pressure and further reduces vagal tone. Hence, rise in systolic BP after CPT declined but insignificant (P > 0.05). In hyperthyroidism, significant decrease (P < 0.01) in the rise in systolic BP after CPT suggests reduced parasympathetic and sympathetic activity. In hypothyroidism, a significant rise in diastolic BP after CPT (P < 0.01) suggests impaired response to beta receptors, over activity of alpha receptors and endothelial dysfunction resulting in increased peripheral resistance. In hyperthyroidism, insignificant rise (P > 0.05) in diastolic BP after CPT suggests direct action of thyroid hormone on vascular smooth muscles causing vasodilatation and reduced vagal tone decreasing sympathetic activity. Thus, the study revealed thyroid dysfunction leads to sympathovagal imbalance affecting cardiac autonomic function.

Key words: Autonomic Function; Hypothyroidism; Hyperthyroidism; Cold Pressor Test