How Does Thyroid Disease Affect the Heart?

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Thyroid disorders can have a significant effect on the heart. They can worsen cardiac symptoms in people with heart disease and accelerate underlying cardiac problems. Thyroid disease can even produce brand new heart problems in people with otherwise healthy hearts.

One of the most important reasons to diagnose and treat thyroid disease is to prevent the cardiac conditions that can result from it.

how thyroid conditions affect the heart
Illustration by Emily Roberts, Verywell

The Thyroid Gland

By producing just the right amount of thyroid hormone, the thyroid gland helps to regulate your body's metabolism—most importantly, how much oxygen and energy your body uses—as well as your digestive function, muscle function, and skin tone.

The thyroid has at least some effect on every organ in the body, including the heart.

Thyroid disease affects the heart by producing either too little thyroid hormone (a condition called hypothyroidism) or too much thyroid hormone (called hyperthyroidism).

Hypothyroidism

Thyroid hormone is very important for normal cardiovascular function. When there is not enough thyroid hormone (hypothyroidism), neither the heart nor the blood vessels can function normally.

Hypothyroidism is often an extremely subtle condition. It typically has a very gradual onset, so its symptoms can sneak up on you. Furthermore, especially in older people, hypothyroidism often occurs without the typical constellation of "textbook" symptoms.

Hypothyroidism is treated with thyroid hormone medication. The adequate treatment of hypothyroidism is a bit tricky and even controversial.

Hypothyroidism is Common

If you have any symptoms of hypothyroidism that your doctor can't explain, you should ask them to measure your thyroid hormone levels—especially if you already have heart disease.

Cardiac Effects

In hypothyroidism, the reduced level of thyroid hormone causes the heart muscle to pump less vigorously and eventually become weakened.

In addition, the heart muscle cannot fully relax after each heartbeat. This failure to relax can produce diastolic dysfunction, a condition that can lead to heart failure.

Hypothyroidism also causes blood vessels to stiffen, which can cause hypertension.

Cardiac Symptoms

Cardiac symptoms can occur in anybody with hypothyroidism, but they are especially likely in people who already have underlying heart disease.

Common cardiac problems associated with hypothyroidism include:

  • Shortness of breath (dyspnea): Shortness of breath on exertion and poor exercise tolerance in hypothyroidism is usually due to weakness in the skeletal muscles. In people who also have heart disease, it may be due to worsening heart failure.
  • Slow heart rate (bradycardia): The heart rate is modulated by thyroid hormone. In people with hypothyroidism, the heart rate is typically 10 to 20 beats per minute slower than normal, especially in patients who also have heart disease.
  • Heart arrhythmias: Hypothyroidism may also worsen the tendency for premature beats (such as PVCs ) and may cause atrial fibrillation.
  • High diastolic blood pressure (diastolic hypertension): One might think that, because a lack of thyroid hormone slows down the metabolism, people with hypothyroidism might experience low blood pressure. Usually, the opposite is true—the arteries are stiffer in hypothyroidism, which causes the diastolic blood pressure to rise.
  • Heart failure (new or worsening): Hypothyroidism can cause heart failure for the first time in patients with relatively mild underlying heart disease.
  • Edema (swelling): Edema can occur as a result of worsening heart failure. In addition, hypothyroidism itself can produce a type of edema called myxedema, caused by an accumulation of abnormal proteins and other molecules in the interstitial fluid (fluid external to the body's cells).
  • Worsening of coronary artery disease (CAD): While the reduction in thyroid hormone can actually make angina (chest discomfort associated with CAD) less frequent in patients who have angina, the increase in LDL cholesterol (bad cholesterol) and in C-reactive protein seen with hypothyroidism may accelerate any underlying CAD.

Hyperthyroidism

Hyperthyroidism is caused by the overproduction of thyroid hormone. When there is too much thyroid hormone, the heart muscle must work harder. For a person with heart disease, this can take a toll.

Excess thyroid hormone increases the force of contraction of the heart muscle and increases the amount of oxygen demanded by the heart. It also increases the heart rate. As a result, the work of the heart is greatly increased.

As with hypothyroidism, hyperthyroidism may be present without producing the classic textbook symptoms. So anyone who has cardiac symptoms that cannot otherwise be readily explained should have their thyroid function measured.

Cardiac Symptoms

Cardiac symptoms can occur in anybody with hyperthyroidism but can be particularly dangerous in people with underlying heart disease.

Common symptoms include:

  • Fast heart rate (tachycardia) and palpitations: Undetected hyperthyroidism is a common cause of an increased heart rate at rest and with mild exertion. Hyperthyroidism should always be ruled out with blood tests before making the diagnosis of inappropriate sinus tachycardia.
  • Heart arrhythmias: Especially in patients with underlying heart disease, hyperthyroidism can also produce a host of other arrhythmias, such as PVCs, ventricular tachycardia, and especially atrial fibrillation. Indeed, it is important to rule out hyperthyroidism in anybody who has atrial fibrillation without a clear underlying cause.
Hyperthyroidism can cause increased heart rate at rest.
  • High systolic blood pressure (systolic hypertension): The forceful cardiac contraction increases the systolic blood pressure (the pressure within blood vessels during cardiac contraction).
  • Shortness of breath: Shortness of breath with exertion can be due to the skeletal muscle weakness associated with hyperthyroidism or to a worsening of heart failure.
  • Heart failure: Hyperthyroidism itself can produce heart failure, but only relatively rarely. On the other hand, if pre-existing heart disease is present, worsening of heart failure with hyperthyroidism is common and can be extremely difficult to treat.
  • Worsening angina: Patients with CAD often experience a worsening of symptoms with hyperthyroidism. These can include an increase in angina or even a heart attack.

Hyperthyroidism Treatments

The "best" way to treat hyperthyroidism is controversial. In the United States, most doctors immediately opt for ablating the overactive thyroid gland with radioactive iodine. They then give the patient thyroid hormone pills since the thyroid gland is no longer functional.

However, this can sometimes be a mistake since occasionally an overactive thyroid is a transient phase of Hashimoto's disease, in which case ablating the thyroid gland is unnecessary.

While using drugs to partially suppress the thyroid gland—such as Tapazole (methimazole) or PTU (propylthiouracil)—creates somewhat of a long-term management issue for doctors, many experts believe it may lead to ultimately happier patients.

A Word From Verywell

Thyroid disease can disrupt the normal functioning of several of the body’s organ systems. Among the most serious problems that can be caused by thyroid disease are problems affecting the heart. In fact, thyroid disease is a fairly common treatable cause of heart conditions.

Anyone who develops cardiac symptoms of almost any type should be sure that their doctor checks their thyroid function tests, and that any thyroid condition that may be found is adequately treated.

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  2. Udovcic M, Pena RH, Patham B, Tabatabai L, Kansara A. Hypothyroidism and the heart. Methodist Debakey Cardiovasc J. 2017;13(2):55-59. doi:10.14797/mdcj-13-2-55

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