Correlation of Cardiovascular Autonomic Functions with Thyroid Dys-Function in Adults by Cold Pressor Test: A Prospective Case-Control Study

Abstract

The objective of the chapter is to evaluate the cardiac autonomic status in adult hyperthyroid and hypothyroid patients by Cold Pressor Test (CPT) and to study the correlation between thyroid status and the autonomic functions regulating cardiovascular (CV) system. 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. Prospective case–control study done 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. In hypothyroidism, basal sympathetic tone is increased to compensate for thyroid hormone deficiency, and this reduces reflex activation of sympathetic system during stress, which exhibits blunted sympatho - excitatory responses leading to aortic stiffness and resistance. This prevents fall in systolic pressure and further results in hypofunctioning of parasympathetic tone. Hence, rise in systolic BP after CPT declined but statistically insignificant (P > 0.05). In hyperthyroidism, the rise in systolic BP after CPT is significantly decreased (P < 0.01) which suggests unbalanced sympathovagal tone, i.e., 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, rise in diastolic BP after CPT is insignificant (P > 0.05) which suggests direct action of thyroid hormone on vascular smooth muscles causing vasodilatation. In addition there is reduced vagal & sympathetic activity. Thus, the study revealed thyroid dysfunction leads to sympathovagal imbalance affecting cardiac autonomic function.