Updated April 25, 2015.

The thyroid gland, a small gland located in the neck just below your Adam’s apple, is responsible for regulating many vital bodily functions. By producing just the right amount of thyroid hormone, the thyroid 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. In fact, the thyroid has at least some effect on every organ in the body - including the heart.

• Read all about the thyroid.

For people with almost any type of heart disease, disorders of the thyroid gland can worsen old cardiac symptoms or cause new ones, and can accelerate the underlying heart disorder. Thyroid disease can even cause cardiac problems in people with healthy hearts.

Disorders of the thyroid gland generally cause one of two types of problems: They either cause the thyroid to produce too little thyroid hormone (hypothyroidism), or to produce too much thyroid hormone (hyperthyroidism). Both types of thyroid disorders are common, and both can have a significant effect on the heart.

Hypothyroidism and the Heart

Thyroid hormone is very important for normal cardiovascular function, so when there is not enough thyroid hormone, neither the heart nor the blood vessels can function normally.

In hypothyroidism the heart muscle becomes weakened, and can no longer pump as vigorously as it should. In addition, the heart muscle cannot fully relax in between heart beats. 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 produce hypertension.

Cardiac symptoms of hypothyroidism

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

• Shortness of breath on exertion and poor exercise tolerance. Shortness of breath (also called dyspnea ) in hypothyroidism is usually due to weakness in the skeletal muscles; but in people who also have heart disease dyspnea may be due to worsening heart failure.
• Slow heart rate (bradycardia.) The heart rate is modulated by thyroid hormone, so with hypothyroidism the heart rate is typically 10 - 20 beats per minute slower than normal. Especially in patients who also have heart disease, however, hypothyroidism may worsen the tendency for premature beats (such as PVCs), and may cause atrial fibrillation.
• Diastolic hypertension . One might think that, because a lack of thyroid hormone slows down the metabolism, people with hypothyroidism might suffer from low blood pressure. Usually the opposite is true - the arteries are stiffer in hypothyroidism, which causes the diastolic blood pressure to rise.
• Worsening of heart failure, or the new onset of heart failure. Hypothyroidism can make well-controlled heart failure worsen, and can produce 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 can accelerate any underlying CAD.

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 that doctors usually expect. Hypothyroidism is also more frequent than many doctors realize. So if you have any of these symptoms and your doctor does not have a ready or convincing explanation for them, especially if you already have heart disease of any type, you should ask your doctor to measure thyroid hormone levels.

Treatment of hypothyroidism

Hypothyroidism is treated with thyroid hormone medication. The adequate treatment of hypothyroidism is a bit tricky, and even controversial. You can read about its treatment here.

There is a similar and less well-known thyroid condition, called subclinical hypythyroidism, that may also affect the heart. You can read about subclinical hypothyroidism here.

Hyperthyroidism and the Heart

Hyperthyroidism is caused by the overproduction of thyroid hormone. When there is too much thyroid hormone, the heart muscle is "whipped" like a horse, and for a person with heart disease it's like whipping a tired horse. 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 in hyperthyroidism.

Cardiac symptoms of hyperthyroidism

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. Occult (that is, unapparent) 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. 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 for no clear underlying cause.
• Systolic hypertension. The forceful cardiac contraction increases the systolic blood pressure (the pressure within blood vessels during cardiac contraction).
• Dyspnea with mild exertion. This can be due to the skeletal muscle weakness cause by hyperthyroidism, or to a worsening in 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 marked worsening in symptoms with hyperthyroidism. These can include an increase in chest pain (angina) or even a heart attack.

As with hypothyroidism, hyperthyroidism can be present - and often is - without the classic, textbook symptoms. So anyone who has any of these cardiac symptoms that cannot otherwise be readily explained should have their thyroid function measured.

Treating hyperthyroidism

The "best" way to treat hyperthyroidism is controversial. In the U.S., most doctors immediately opt for ablating the overactive thyroid gland with radioactive iodine, then giving 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.

Furthermore, using drugs to partially suppress the thyroid gland - in the U. S., Tapazole or PTU - creates somewhat more of a long-term management issue for doctors, but some experts believe it may lead to ultimately happier patients.

Read more about the treatment of hyperthyroidism.

<sub>Sources

Klein I, Ojamaa K. Thyroid hormone and the cardiovascular system. N Engl J Med 2001 Feb 15;344(7):501-9.</sub>