Dept of Physiology, D. Y. Patil Medical College, Kolhapur, Maharashtra, India
*Author for correspondence: Dr. Mrs. Sunita Milind Tiwale, Associate Professor, Dept of Physiology, D. Y. Patil Medical College, Kolhapur State-Maharashtra, India. E mail:- sunita.tiwale88@gmail.com, Contact no.:- 09561299237
Online published on 21 February, 2015.
Changes in the thyroid status has pronounced effects on cardiovascular system which reflect autonomic dysfunction. This may contribute to morbidity in thyroid patients. Exact nature of interaction between autonomic nervous activity & thyroid hormones is controversial. It is also unknown whether the extent of autonomic dysfunction is related with the severity of the thyroid disease. Hence the autonomic status in thyroid patients was assessed and compared with healthy age matched (18–45 yrs) controls.
Prospective case control.
Depending on clinical examination and serum levels of T3, T4 & TSH; three groups were categorized as hypothyroid, euthyroid and hyperthyroid. Autonomic functions based on heart rate responses to various maneuvers were evaluated in thyroid patients & compared with euthyroid (control) group. For the assessment of heart rate variability by thyroid status, Autonomic Function Tests (AFTs), like Resting heart rate, 30:15 ratio, Standing to lying ratio (SLR) tests were recorded on ECG in Lead II. Statistical analysis was done using student's t-test to compare between controls and thyroid patients.
AFTs like 30:15 ratio, SLR declined significantly both in hypothyroid and hyperthyroid patients as compared to euthyroid subjects (p-value< 0.01).
Hyperthyroidism leads to low excitability of cardiac vagal motor neurons, reducing parasympathetic activity and hence heart rate variability response is depressed. At the same time direct action of thyroid hormone on SA node and myocardium has additive effect to sympathetic system resulting in hyperadrenergic state. Hypothyroidism decreases direct action of thyroid hormones on the heart, which is compensated by increasing basal sympathetic tone and influencing resting heart rate. Also there is impaired response to Beta receptors, so there is over activity of Alpha receptors in hypothyroidism. This exhibits blunted sympatho-excitatory responses further reducing vagal tone and decreasing heart rate variability response.
Autonomic Function Tests (AFTs), Resting heart rate, Hypothyroidism, Hyperthyroidism, 30:15 ratio Standing to lying ratio (SLR)
