From the Guidelines
Hypothyroidism causes decreased cardiac output primarily due to reduced contractility, bradycardia, increased peripheral vascular resistance, and diastolic dysfunction, as noted in a study published in Circulation in 2016 1. The pathophysiology of hypothyroidism-induced decreased cardiac output is multifactorial. Key factors include:
- Decreased contractility of the heart muscle, making each heartbeat less forceful
- Bradycardia, which reduces the number of times the heart pumps blood per minute
- Increased peripheral vascular resistance, making it harder for the heart to pump blood throughout the body
- Diastolic dysfunction, which impairs the filling of the heart These changes result in reduced stroke volume and heart rate, the two key components of cardiac output. According to the study published in Circulation 1, systemic vascular resistance can increase by as much as 50% in hypothyroidism, and diastolic relaxation and filling are slowed, further contributing to decreased cardiac output. Treatment with levothyroxine can reverse these cardiac manifestations by restoring normal thyroid hormone levels, as implied by the improvement in cardiomyopathy with thyroid hormonal replacement in case reports mentioned in the study 1.
From the Research
Mechanisms of Decreased Cardiac Output in Hypothyroidism
- Low thyroid hormone levels alter cardiac gene expression and increase systemic vascular resistance, resulting in a reduction of cardiac contractility and cardiac output 2.
- Hypothyroidism can cause impaired left ventricular diastolic function, characterized by slowed myocardial relaxation and impaired ventricular filling 3.
- The hypothyroid state can also lead to changes in myocardial time intervals in several left ventricular segments, as well as alterations in cardiac hemodynamics 3.
- Vascular function is impaired in patients with mild and subclinical hypothyroidism, with increased systemic vascular resistance and arterial stiffness, and impaired endothelial function 3.
Effects of Thyroid Hormone on the Cardiovascular System
- Thyroid hormone can increase myocardial inotropy and heart rate, and dilate peripheral arteries to increase cardiac output 4.
- An excessive deficiency of thyroid hormone can cause cardiovascular disease and aggravate many preexisting conditions 4.
- Thyroid hormones, specifically triiodothyronine (T3), have significant effects on the heart and cardiovascular system, and alterations in thyroid hormone metabolism may further impair cardiac and cardiovascular function 5.
Reversal of Cardiovascular Abnormalities with Treatment
- Restoring euthyroidism with levothyroxine (L-T4) therapy can reverse the negative effects of mild hypothyroidism on the cardiovascular system 3.
- L-T4 replacement should be considered in patients with mild hypothyroidism in the presence of associated cardiovascular risk factors to reverse these negative prognostic factors and improve the cardiovascular risk 3.