How does hypothyroidism affect cardiovascular function?

Effects of Hypothyroidism on Cardiovascular Function

Hypothyroidism significantly impacts cardiovascular function by causing bradycardia, mild hypertension (often diastolic), narrowed pulse pressure, increased systemic vascular resistance, and impaired cardiac contractility, leading to potential cardiovascular complications. 1

Hemodynamic Changes in Hypothyroidism

• Bradycardia is one of the most common cardiovascular manifestations of hypothyroidism 1
• Systemic vascular resistance can increase by as much as 50% 1
• Diastolic relaxation and filling are significantly slowed 1
• Low cardiac output results from the combination of bradycardia, decreased ventricular filling, and decreased cardiac contractility 1
• Mild hypertension, particularly diastolic hypertension, is common due to increased peripheral resistance 1, 2

Structural and Functional Cardiac Changes

• Hypothyroidism promotes myocardial fibrosis by stimulating fibroblasts 1
• Chronic hypothyroidism leads to loss of coronary arterioles, impaired blood flow, and maladaptive changes in myocyte shape 1
• Left ventricular diastolic dysfunction is the most consistent cardiac abnormality in hypothyroidism, characterized by slowed myocardial relaxation and impaired ventricular filling 3
• Pericardial effusions and nonpitting edema (myxedema) can occur in severe, long-standing hypothyroidism 1
• Altered myocardial composition has been detected by ultrasonic myocardial textural analysis in patients with mild hypothyroidism 3

Vascular Effects

• Endothelial dysfunction occurs even in subclinical hypothyroidism 2, 3
• Increased arterial stiffness contributes to cardiovascular risk 2, 3
• The atherogenic lipid profile associated with hypothyroidism (particularly with TSH >10 mU/L) increases atherosclerosis risk 2
• Altered coagulation parameters further contribute to cardiovascular risk 2

Clinical Manifestations and Heart Failure Risk

• Despite significant hemodynamic changes, clinical heart failure is relatively rare in hypothyroidism because cardiac output is usually sufficient to meet the lowered systemic metabolic demands 1
• Dilated cardiomyopathy (DCM) is not a characteristic feature of hypothyroidism, but case reports exist of DCM improving with thyroid hormone replacement 1
• C-reactive protein and homocysteine levels are elevated, serving as additional cardiovascular risk factors 2

Subclinical Hypothyroidism and Cardiovascular Risk

• Even subclinical hypothyroidism is associated with impaired left ventricular diastolic function and subtle systolic dysfunction 4
• Enhanced risk for atherosclerosis and myocardial infarction exists in subclinical hypothyroidism 4
• Cardiovascular abnormalities are reversed by restoration of euthyroidism with appropriate levothyroxine therapy 4, 3

Treatment Considerations

• Levothyroxine replacement therapy reverses the negative cardiovascular effects of hypothyroidism 2, 3
• Careful titration of levothyroxine is essential, as overtreatment may cause cardiac arrhythmias, particularly in elderly patients and those with underlying cardiovascular disease 5
• Monitor patients with diabetes mellitus closely when initiating thyroid hormone therapy, as it may worsen glycemic control 5
• Thyroid function tests are recommended in all patients presenting with heart failure due to the potential contribution of thyroid dysfunction 1

Important Cautions

• Overtreatment with levothyroxine may precipitate angina or arrhythmias, particularly in elderly patients and those with cardiovascular disease 5
• Initiate levothyroxine therapy at lower doses in elderly patients and those with underlying cardiac disease 5
• Patients with adrenal insufficiency must receive glucocorticoid replacement before initiating thyroid hormone therapy to prevent acute adrenal crisis 5