An Overview of Influence of Thyroid Hormones on Cardiovascular Function

It has been very clearly established that thyroid hormones have a specific influence on cardio-vascular system. Hyper- or hypothyroidism induces cardiovascular disorders, atrial and ventricular arrhythmias, atherosclerotic vascular diseases, dyslipidemia and heart failure. Heart failure results from the impairment of ability of the ventricle to fill with or eject blood. Thyroid hormones contribute to cardiovascular events by directly acting on the cardiomyocyte via genomic and nongenomic pathways. Thyroid hormones play a critical role in maintaining cardiac homeostasis. Fluctuations in thyroid hormones due to hyper- or hypothyroidism has an adverse effect on the heart. Thyroid hormones have a direct influence on myocardial contractility, systolic and diastolic blood pressure, heart rate, heart mass, ejection fraction and heart output. Hyperthyroidism stimulates high cardiac output with low systemic vascular resistance and is associated with faster heart rate and increased left ventricular systolic and diastolic function and supraventricular atrial fibrillation. Hypothyroidism has an opposite effect on the heart rate and myocardial contractility. Hypothyroidism reduces cardiac repolarizing of K⁺ currents such as transient outward potassium current and increase in L-type calcium current. Alterations in Thyroid Stimulating Hormone level bring about cardiac electrical disturbances. Subclinical hypothyroidism is associated with increased heart rate, atrial arrhythmias, LV mass and impaired ventricular relaxation, and increased risk of mortality due to cardio-vascular impairment. In this overview, with the available information and publications, the physiological and patho-physiological relationships between thyroid hormones and the cardiovascular system are discussed.