Thyroid and Heart Disease: What Happens When Both Conditions Coexist?

The connection between thyroid disease and heart disease is stronger than many people realise. Thyroid hormones help regulate heart rate, blood pressure, blood vessel function, and cholesterol metabolism. However, when the thyroid gets imbalanced, either by being overactive or underactive, it can have a direct impact on our cardiovascular system. For people who already have heart disease, not treating thyroid problems can lead to some serious complications, like arrhythmias, heart failure getting worse, high cholesterol, and even strokes. This makes it really important to understand this connection to take care of both our thyroid and heart health.

Managing thyroid disease alongside heart disease requires careful treatment, regular monitoring, and long-term lifestyle adjustments. Hyperthyroidism can overstimulate the heart and trigger irregular heart rhythms, while hypothyroidism can slow the heart and increase cholesterol levels, putting added strain on the cardiovascular system. In an exclusive interaction with us, our expert, Dr Bhat, shares that understanding how these two conditions interact is essential for reducing complications and protecting long-term heart health.

How Are the Thyroid and Heart Connected?

The thyroid gland produces hormones, Thyroxine (T4) and Triiodothyronine (T3), that help regulate how the body uses energy. These hormones directly affect the cardiovascular system by influencing:

• Heart rate
• Cardiac output
• Blood pressure
• Blood vessel resistance
• Cholesterol metabolism

When thyroid levels become imbalanced, the heart is the first organ to be affected, as per Harvard Health Publishing.

In hyperthyroidism:

When a person has excess thyroid hormone, it can make their body work faster than it should, which can cause the heart to beat really fast and work too hard.

In hypothyroidism:

On the other hand, low thyroid hormone slows the body’s processes, causing the heart to pump less efficiently and increasing cholesterol levels.

Dr Bhat adds, “Because of this close relationship, even mild thyroid dysfunction can have a noticeable impact on cardiovascular health.”

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How Hyperthyroidism Affects the Heart

The Mayo Clinic cites that hyperthyroidism occurs when the thyroid gland produces too much hormone. This overstimulates the cardiovascular system and increases the heart’s workload.

Some possible cardiac effects include:

• Rapid heartbeat (tachycardia)
• Heart palpitations
• Elevated systolic blood pressure
• Increased cardiac output
• Irregular heart rhythm

Thyroid hormones increase the heart's sensitivity to adrenaline-like hormones, a mechanism known as increased beta-adrenergic activity. This causes the heart to beat faster and increases the force of contraction.

At the same time, hyperthyroidism reduces systemic vascular resistance, meaning the blood vessels relax while the heart pumps harder, further increasing strain on the cardiovascular system, according to Archives of Medical Science (AMS), a peer-reviewed, open-access, bimonthly medical journal.

Hyperthyroidism and Atrial Fibrillation Risk

One of the most serious heart problems that can happen when you have an overactive thyroid is a condition called Atrial Fibrillation (AFib). Dr Bhat notes that this is when the heart beats too fast and in an irregular rhythm.

According to research published in Thyroid Research, atrial fibrillation develops in 10% to 15% of older adults with overt hyperthyroidism, significantly increasing the risk of blood clots and stroke.

Symptoms of AFib may include:

• Fluttering heartbeat
• Shortness of breath
• Dizziness
• Fatigue

“If AFib isn't treated, it can make the heart less efficient and increase the chance of having a stroke, because blood clots can form in the upper chambers of the heart,” highlights Dr Bhat.

How Hyperthyroidism Can Worsen Existing Heart Disease

In people with pre-existing heart conditions such as coronary artery disease, angina, heart failure, and/or hypertension, hyperthyroidism can worsen symptoms by increasing myocardial oxygen demand. In simple terms, the heart requires more oxygen because it is working harder.

Untreated hyperthyroidism can be dangerous in people with cardiovascular disease, because, as a result, this may trigger:

• Chest pain
• Shortness of breath
• Worsening angina
• Fluid retention
• Heart failure flare-ups

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How Hypothyroidism Affects the Heart

Hypothyroidism occurs when the thyroid does not produce enough hormones. This slows down metabolism, including the work of the heart and blood vessels. Low thyroid hormone levels reduce the heart's pumping strength and slow the relaxation of blood vessels, increasing resistance in the arteries.

Common cardiovascular effects include:

• Slower heart rate
• Increased diastolic blood pressure
• Reduced cardiac output
• Increased LDL cholesterol
• Stiffened blood vessels

Role of Hypothyroidism in Increasing Cholesterol Levels

One of the most important cardiovascular effects of hypothyroidism is its impact on cholesterol metabolism. Thyroid hormones help regulate how the liver processes cholesterol. Here’s what happens when thyroid hormone levels fall:

• Low-Density Lipoprotein (LDL or bad) cholesterol rises
• Triglycerides may increase
• Cholesterol clearance slows

In turn, this contributes to plaque buildup in the arteries, increasing the risk of:

• Atherosclerosis
• Coronary artery disease
• Heart attack

Link Between Hypothyroidism and Heart Failure

According to the Methodist DeBakey Cardiovascular Journal, severe or untreated hypothyroidism may weaken the heart muscle and reduce pumping efficiency. Possible complications include:

• Fluid retention
• Enlarged heart
• Reduced circulation
• Worsening heart failure

This can also lead to symptoms such as tiredness, swelling in legs, and trouble breathing.

“In patients who already have heart failure, hypothyroidism may worsen symptoms and reduce exercise tolerance,” says Dr Bhat.

What Happens If Thyroid Disease and Heart Disease Occur Together?

When thyroid and heart disease coexist, each condition can intensify the other.

If hyperthyroidism coexists with heart disease, the heart is pushed to work harder, increasing the risk of:

• Arrhythmias
• Angina
• Heart failure exacerbation
• Stroke

On the other hand, if hypothyroidism coexists with heart disease, the heart must work against rising cholesterol and increased vascular resistance, increasing the risk of:

• Coronary artery disease progression
• High blood pressure
• Reduced cardiac function

This can lead to a vicious cycle, where problems with your thyroid gland make your heart work harder, and heart disease makes it tougher to treat thyroid issues.

How Risky Is This Coexistence?

According to Dr Bhat, the severity of risk depends on:

• Age
• Thyroid hormone levels
• Type of heart disease
• Duration of untreated thyroid disease
• Overall cardiovascular health

The coexistence of thyroid disease and heart disease increases the risk of:

• Stroke
• Heart failure
• Arrhythmias
• Coronary artery disease progression
• Hospitalisation

“As people get older, they're more likely to have problems with their heart rhythm due to thyroid issues, which seem to happen more often with age,” he adds.

Possible Complications of Having Both Conditions

If thyroid and heart disease are not carefully managed, complications can become serious, leading to:

1. Atrial Fibrillation: Hyperthyroidism increases the risk of AFib, which raises stroke risk.

2. Worsening Heart Failure: Both thyroid hormone excess and deficiency can reduce cardiac efficiency.

3. Elevated Cholesterol: Hypothyroidism promotes atherosclerosis through abnormal lipid metabolism.

4. Blood Pressure Changes: Thyroid dysfunction can worsen hypertension.

5. Medication Challenges: Treating thyroid disease in people with heart disease requires slow, careful dose adjustments.

Explaining with an example, starting levothyroxine too quickly in hypothyroid patients with heart disease can trigger:

• Palpitations
• Chest pain
• Arrhythmias
• Angina

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Warning Signs That Need Immediate Medical Attention

If you have thyroid disease and heart disease, seek urgent medical care if you experience:

• Chest pain
• Severe shortness of breath
• Rapid or irregular heartbeat
• Fainting
• Sudden swelling in the legs
• Extreme dizziness

Dr Bhat emphasises that these symptoms could be a sign that your heart condition is getting worse.

Precautions If You Have Both Thyroid Disease and Heart Disease

Managing both conditions safely requires regular medical supervision. 

1. Monitor Thyroid Levels Regularly

Routine blood tests help track TSH, free T4, and T3 (if needed). This ensures treatment remains safe for the heart.

2. Adjust Medication Gradually

People with heart disease often require slow thyroid medication titration to avoid overloading the cardiovascular system.

3. Control Cholesterol and Blood Pressure

Managing blood pressure, LDL cholesterol, and blood sugar helps reduce cardiovascular complications.

4. Keep Cardiology and Endocrinology Follow-Up Appointments

Coordinated care between specialists improves outcomes and reduces treatment risks.

Lifestyle Changes That Support Both Thyroid and Heart Health

Lifestyle habits can significantly reduce complications. Here’s what Dr Bhat recommends:

1. Exercise Safely

Regular moderate activity improves:

• Circulation
• Cholesterol levels
• Blood pressure
• Weight control

Note: If you have hyperthyroidism that's not under control, it's best to avoid really intense exercise until your doctor says it's okay.

2. Maintain a Healthy Weight

Excess weight increases strain on the heart and worsens hormone imbalance.

3. Avoid Smoking

According to the National Heart, Lung and Blood Institute, smoking increases inflammation, worsens blood vessel health, and raises heart disease risk.

4. Reduce Stress

Stress can worsen both thyroid symptoms and cardiovascular strain. Some helpful strategies include:

• Meditation
• Yoga
• Breathing exercises
• Consistent sleep

Diet Modifications for Thyroid and Heart Disease

Diet plays an important role in reducing inflammation, managing cholesterol, and supporting thyroid function. Here’s what your diet should look like:

1. Prioritise Heart-Healthy Foods

Some foods support heart health and reduce inflammation. Therefore, Dr Bhatt suggests eating more:

• Fruits
• Vegetables
• Whole grains
• Legumes
• Nuts
• Fatty fish
• Limit Saturated Fats

On the other hand, some foods help lower LDL cholesterol. So, reduce intake of foods like:

• Fried foods
• Fatty meats
• Processed snacks

2. Reduce Sodium

Too much sodium may worsen:

• High blood pressure
• Fluid retention
• Heart failure symptoms

3. Include Selenium-Rich Foods

Selenium helps support thyroid hormone metabolism and antioxidant defence. Some sources include:

• Brazil nuts
• Eggs
• Tuna
• Sunflower seeds

4. Be Cautious With Iodine Supplements

Too much iodine may worsen some thyroid disorders, especially autoimmune thyroid disease. “Don't take iodine supplements unless your doctor tells you to,” warns Dr Bhat.

5. Include Omega-3 Fatty Acids

Omega-3s may support heart rhythm stability and reduce inflammation. Its sources include:

• Salmon
• Sardines
• Walnuts
• Chia seeds

Bottomline

The thyroid and heart are closely interconnected, and when thyroid and heart disease occur together, the risk of complications rises significantly. Hyperthyroidism can overstimulate the heart and trigger arrhythmias, while hypothyroidism can raise cholesterol and impair circulation. Together, these conditions increase the risk of atrial fibrillation, worsening heart failure, stroke, and coronary artery disease progression.

However, with early diagnosis, regular monitoring, carefully adjusted treatment, a heart-supportive lifestyle, and targeted dietary changes, many of these risks can be reduced. If you are managing both thyroid and heart disease, close follow-up with your healthcare team is essential to protect your long-term cardiovascular health.