Introduction
Hypothyroidism is a common hormonal disorder that occurs when the body does not have enough thyroid hormones, usually because the thyroid produces too little. In some cases, the hormones do not work properly in the tissues, leading to effects on many organ systems throughout the body. Hypothyroidism has widespread consequences throughout the body, affecting the nervous, circulatory, muscular, heart, and hormonal systems.1 The impact on the cardiovascular system is clearly apparent, manifesting as reduced cardiac output, limited pulse pressure, heightened systemic vascular resistance, and a lowered metabolic rate. Additionally, the central nervous system is significantly influenced, given the essential role of thyroid hormones in brain development and lifelong neurological function.2 Cardiomegaly or enlarged heart linked to hypothyroidism is a complex clinical condition driven by distinct physiological mechanisms. Hypothyroidism may trigger the onset of heart disease or aggravate pre-existing cardiac conditions, with affected individuals frequently exhibiting symptoms resembling heart failure, such as fatigue, shortness of breath, and difficulty breathing while lying down.3 Thyroid hormones play a critical role in sustaining cardiovascular health. When hyperthyroidism becomes severe, some patients may develop 'myxedema heart,' a condition involving a slow heart rate and fluid buildup around the heart.4
Pathophysiology of hypothyroidism and cardiovascular impact
Role of thyroid hormones (T3 and T4) in cardiovascular regulation
Thyroid hormones T3 and T4 help the heart and blood vessels work properly as they act through receptors of myocardium and vascularendothelium. In hypothyroidism or reduced thyroid function, this control is lost and disrupts cardiovascular homeostasis, allowing harmful processes to deprocesses that make heart disease progress faster and its effects more severe.5 Individuals with hypothyroidism often present with elevated total cholesterol, low‑density lipoproteins, and triglycerides, resulting in an atherogenic lipid profile.6 Overt hypothyroidism further increases the risk of atherosclerotic cardiovascular disease through both metabolic and hemodynamic alterations, including dyslipidemia, persistent low-grade inflammation, oxidative stress, and insulin resistance. Importantly, both overt and subclinical hypothyroidism are linked to an increased burden of cardiovascular risk factors and adverse clinical events.
Mechanisms by which hypothyroidism affects cardiac function7
- Impaired cardiac contractility
- Diastolic dysfunction or altered diastolic relaxation
- Reduced sarcoplasmic reticulum calcium ATPase (SERCA) activity
- Myofibrillar swelling, loss of striation, and interstitial fibrosis
- Increased vascular resistanceincreases, promotes hypertension, dyslipidemia, and endothelial dysfunction, contributing to atherosclerosis and heart failure development
Development of "myxedema heart" and its features
"Myxedema heart" is a condition linked to hypothyroidism where the heart becomes enlarged and weak, the heartbeat slows, ECG readings show low voltage and abnormal T-waves, and fluid builds up around the heart.4 From a pathological perspective, myxedema heart may exhibit both reversible and permanent myocardial alterations. Endomyocardial biopsy findings reveal vacuolated degeneration that can improve with appropriate treatment, alongside persistent fibrosis, suggesting that prolonged, untreated hypothyroidism can lead to irreversible cardiac injury.8 Echocardiographic studies also reveal that cardiac enlargement in these cases is primarily due to fluid accumulation in the pericardial sac, not myocardial infiltration. Importantly, this fluid buildup reliably resolves with appropriate thyroid hormone replacement therapy.4
Progression of heart enlargement in untreated hypothyroidism
Studies show that untreated hypothyroidism can cause the heart walls to thicken, especially in older people, with scans often detecting changes in the septum and the back wall of the left ventricle. Prolonged, untreated hypothyroidism can lead to serious cardiac complications such as dilated cardiomyopathy and heart failure, as illustrated by a case involving a 55-year-old man who experienced significant heart failure and abnormal enlargement of the left ventricle. Even mild thyroid dysfunction, like subclinical hypothyroidism, can affect heart tructure by changing the shape and integrity of heart muscle cells, disrupting both contraction and relaxation phases, and contributing to conditions like high blood pressure and coronary artery disease that may progress to heart failure. Fortunately, thyrhormone therapy can help reverse some of these effects, such as reducing wall thickness and enhancing cardiac performance, although it may not alter the size of the heart chambers.9
Impact of hypothyroidism management on cardiac outcomes
Thyroid hormone replacement therapy (THRT)10
Thyroid hormone replacement therapy (THRT) helps the heart and blood vessels work properly by influencing how the body’s genes function and by acting directly on heart cells. The main purpose is to bring the heart’s pumping ability and blood flow back to normal.
The active hormone T3 (Triiodothyronine) enters heart cells and changes gene activity. It increases proteins that make the heart contract faster and stronger. In addition, it reduces proteins that slow down heart function.
These changes improve:
- Heart pumping strength (contractility)
- Heart rate
- Both systolic and diastolic performance
THRT also relaxes blood vessels, which:
- Lowers blood pressure
- Reduces resistance to blood flow
Because T3 acts as both a vasodilator (widens blood vessels) and an inotropic agent (strengthens heart contractions), it’s useful for:
- People with heart failure
- Patients undergoing heart surgery with low T3 levels
Monitoring and dose adjustment11
Treating hypothyroidism requires close monitoring and slow dose adjustments, especially in patients with heart conditions. The standard treatment is Levothyroxine (T4), with the goal of reaching a TSH level between 1–2 mu/L in primary hypothyroidism.
- In elderly patients or those with coronary artery disease
- Start with a low dose
- Increase slowly to avoid chest pain (angina) or heart attack
- After changing the dose, check blood tests in six–eight weeks
- Avoid giving too much hormone
- Can cause heart problems
- Can lead to osteoarthritis (weak bones)
- High-risk heart patients may need hospital monitoring when starting treatment
- About 50% of patients are either under- or over-treated → shows why regular monitoring is essential
Reversal of cardiac changes12
Heart recovery is seen within months after starting thyroid hormone replacement. Studies support early and effective levothyroxine therapy to reverse heart problems caused by hypothyroidism, with cardiac function often returning to levels comparable to healthy individuals after treatment.
The outcomes of treatment include:
- Reduced thickening of the heart walls
- Disappearance of fluid around the heart (pericardial effusion)
- Improved pumping ability (cardiac output)
- Restored relaxation of the heart between beats (diastolic function)
- Improvements in ejection fraction
- Improvements in stroke volume
- Improvements in cardiac index
Clinical considerations and guidelines13
- People with higher TSH levels are more likely to have heart problems
- Because of this, doctors recommend doing routine heart checks like an ECG (a test of the heart’s rhythm) and an echocardiogram (an ultrasound of the heart) when someone is newly diagnosed with hypothyroidism
- These tests help find heart issues early so they can be treated before they become serious
- Management of hypothyroidism in special populations, particularly the elderly, requires careful attention due to atypical presentations, age‑related changes, comorbidities, and the risk of cardiac complications, making cautious dose adjustments and close monitoring essential
- Evidence also supports treatment consideration in subclinical cases with elevated TSH, while individualised approaches are needed in settings such as heart disease, emergency surgery, and polypharmacy
Evidence-driven comparison of outcomes in managed vs. unmanaged hypothyroidism14
- Treated patients (with THRT)
- Show sign significant improvement in both systolic and diastolic heart function within weeks, including higher ejection fraction and better ventricular performance
- Adequate therapy after percutaneous coronary interventions is linked to a lower risk of major adverse cardcardiovascular events and reduced sudden cardiac death
- In heart failure patients, thyroid hormone replacement enhances cardiac function, though higher baseline TSH may increase arrhythmia risk
- Untreated patients (without THRT)
- Face persistently impaired cardiac function with reduced pumping and relaxation capacity
- Have a substantially higher risk of adverse cardiovascular outcomes, including coronary heart disease, heart failure, and cardiac death
- May experience worsening structural and functional heart changes over time
Summary
Hypothyroidism is a serious condition that needs to be found and treated early to prevent long-term heart problems. It affects up to 12% of people worldwide and can cause issues like heart failure and heart disease.15 Hypothyroidism reduces heart strength and rate, raises vascular resistance, and encourages fluid accumulation, which together may lead to heart enlargement if not treated. Adequate thyroid hormone replacement is critical for protecting heart structure, improving function, and reducing cardiovascular risks, while untreated hypothyroidism leaves patients vulnerable to serious complications.14 Starting levothyroxine promptly brings thyroid levels back to normal and protects the heart from complications. Treating cardiac hypothyroidism involves slowly increasing thyroid hormone doses.
References
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- Zúñiga D, Balasubramanian S, Mehmood KT, Al-Baldawi S, Zúñiga Salazar G. Hypothyroidism and Cardiovascular Disease: A Review. Cureus [Internet]. 2024 Jan 18 [cited 2025 Sept 30]; Available from: https://www.cureus.com/articles/218316-hypothyroidism-and-cardiovascular-disease-a-review
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- Yang Y, Xue C, Zhao J, Zhang L, Wang Y, Ouyang M, et al. Changes of cardiac function: cardiac adaptation in patients with hypothyroidism assessed by cardiac magnetic resonance-a meta-analysis. Front Endocrinol. 2024 June 11;15:1334684.
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