The heart can ail when the thyroid fails
By: Harvard Heart Letter (Friday, 01-11-2002)


Problems with the thyroid gland can lead to abnormal heart rhythms, high blood pressure, chest pain, and more.
Like the accelerator pedal on a car, the thyroid gland controls the pace at which your body runs. The heart, in particular, is exquisitely sensitive to the thyroid's signals. That means a malfunctioning thyroid can cause problems that masquerade as heart disease or make existing heart disease worse.

In fact, thyroid troubles are an often overlooked cause of heart and circulatory problems. These range from fast or disorganized rhythms to high cholesterol, high blood pressure, and chest pain. In such cases, fixing the thyroid problem can correct the heart symptoms.

Thyroid hormone influences:
The volume of blood circulating through the heart and blood vessels
The force of heart muscle contractions
The relaxation of heart muscle between beats
The heart rate
The flexibility and elasticity of small arteries
The ability of blood and other fluids to leave and enter the smallest blood vessels (capillaries)
Adapted from Seely and Williams. The heart in endocrine disorders. In: Braunwald et al. Heart disease, 6th edition


About 13 million Americans have a thyroid problem. Only half know it. The two main thyroid disorders are hyperthyroidism (overactive thyroid) and hypothyroidism (underactive thyroid).

Running smooth
The thyroid influences almost every cell, tissue, and organ in your body. From its perch on the front of your windpipe, this butterfly-shaped gland controls how quickly cells turn food into energy; regulates body temperature; and affects body weight, muscle strength, energy level, fertility, and more.

The thyroid issues its instructions by making thyroid hormone and releasing it into the bloodstream. The gland produces this vital chemical in two forms: thyroxine (called T4) and triiodothyronine (T3). Both slip easily into cells and latch onto special receptors. Cells convert T4 to T3, which directly activates a variety of genes.

Revved up
An overactive thyroid keeps churning out thyroid hormone even when the body has enough. This overabundance speeds up many of the body's processes. Common symptoms and signs include increased sweating or feeling warm, trouble sleeping, being short of breath, anxiety and nervousness, trembling hands, and losing weight in spite of a good appetite.

Too much thyroid hormone makes the heart work faster and harder. The cardiovascular consequences include:

Abnormal heart rhythms: Several rhythm disturbances can result from thyroid overstimulation. The most common are tachycardia, an abnormally fast heart rhythm that can top 100 beats per minute, and atrial fibrillation, a disorganized rhythm in the heart's upper chambers. A related symptom is palpitations, a sudden awareness of your heart's contractions.
High blood pressure: High systolic pressure (the top number of a blood pressure reading) is common among people with hyperthyroidism, especially older people.
Chest pain: An excess of thyroid hormone makes the heart beat more forcefully and pump more blood. That increases the heart muscle's need for oxygen. If clogged coronary arteries can't carry the extra blood the heart demands, the result is chest pain.
Heart failure: By forcing the body to work at a faster pace, an overactive thyroid can overtax the heart and cause heart failure, a condition in which the heart can't pump enough blood to meet the body's needs.
Out of gas
An underactive thyroid slows the body's activities. Common symptoms and signs of an underactive thyroid include fatigue, weakness, constantly feeling cold, weight gain or difficulty losing weight, dry or rough skin, hair loss, constipation, depression, and loss of interest in sex.

These symptoms come on slowly, and they look a lot like changes that normally happen with aging or menopause. That's why an underactive thyroid often goes undiagnosed. In turn, the related heart and circulation problems may be chalked up to something else. These include:

Slow heart rate: Low levels of thyroid hormone can slow the heart rate and may even cause bradycardia (bray-dee-CAR-dee-uh), an unusually slow rate of fewer than 50 to 60 beats per minute.
High blood pressure: Low levels of thyroid hormone make arteries less elastic. This can increase the pressure needed to circulate blood around the body.
Atherosclerosis: An underactive thyroid can cause cholesterol levels to jump. It can also boost blood pressure and levels of homocysteine. Alone or together, these changes can lead to narrowed, hardened arteries, a precursor to heart disease and stroke.
Diminished pumping ability: The combination of reduced blood volume, weaker muscular contractions, and a slower heart rate means the heart can't pump as well as it should. This can reduce blood flow to the skin, kidneys, brain, and other vital tissues.
Vice versa
The thyroid-heart disease connection isn't a one-way street. A heart attack can throw off the function of the thyroid gland, or at least lower the levels of thyroid hormone. So can heart failure.

Several drugs used to treat cardiovascular problems also affect the thyroid. The most common of these is amiodarone (Cordarone), which helps control abnormal heart rhythms. In some people, this drug leads to hypothyroidism. In others, it causes hyperthyroidism.

Need a tune-up?
If you have cardiovascular disease — a heart rhythm problem, high blood pressure, high cholesterol levels, atherosclerosis, or chest pain — a thyroid check is in order. For that matter, medical organizations such as the American Thyroid Association and the American College of Physicians recommend that all adults, especially older women, get regular checks for thyroid disease.

A thyroid check is quite simple. Your doctor will feel the gland to see if it is enlarged or has firm bumps, called nodules. You will also have a blood test for thyroid and thyroid-stimulating hormones.

If your thyroid isn't working as it should, slowing it down or taking thyroid hormone may help your "heart disease" vanish or improve the heart problems you've been having.

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