Two hormones secreted by the butterfly-shaped thyroid gland in your neck—thyroxine (T4) and triiodothyronine (T3)—run your life. These hormones tell your body to speed up or slow down. They regulate how fast you burn calories, and how tired you feel.
They also regulate your heart rate.
When your body makes just the right amount of thyroid hormone, you’ll be unaware your thyroid is doing its job. But if it stops making enough hormone, you will gradually feel the effects throughout your body (see Box 1). One of these effects is that your heart rate will slow down.
If your thyroid makes too much hormone, you will experience the opposite effects (see Box 2). An overactive thyroid (hyperthyroidism) will cause your heart rate to speed up.
Underactive Thyroid Linked to Estrogen
An underactive thyroid (hypothyroidism) tends to affect women, because it is linked to estrogen levels. When estrogen fluctuates during pregnancy or perimenopause, women who are genetically predisposed can develop Hashimoto’s disease. In this form of thyroid disease, the immune system attacks the thyroid gland, interfering with hormone production.
Over the long term, untreated hypothyroidism can cause the heart muscle to weaken, sometimes to the point of heart failure. Levels of “bad” LDL cholesterol can also skyrocket, increasing the risk of heart attack and stroke.
Low thyroid levels also make the heart slow down—a condition doctors call bradycardia. “This usually doesn’t cause damage, but it does cause extreme fatigue,” says Cleveland Clinic endocrinologist Betul Hatipoglu, MD.
Revved Up By Too Much Hormone
The most common cause of hyperthyroidism is Graves’ disease, which is also an autoimmune disorder.
People with an overactive thyroid may develop atrial fibrillation (afib), a rapid, irregular heart rhythm. Afib causes the heart’s upper chambers to quiver, rather that pump vigorously. Blood rests in the heart, which can lead to blood clot formation. If a clot is ejected into the circulation, it can cause a stroke.
Treatment Lowers Heart Risk
An overactive or underactive thyroid is easy to treat. Treatment will lower the risk of a heart problem, and may eliminate it completely.
Overactive thyroids are removed or destroyed with radiation. The vital hormones are then replaced with supplements.
Underactive thyroids are also treated with T4 supplements. Sometimes, T3 must be added.
Pork thyroid tablets—which are FDA-controlled but not FDA-approved—contain both T4 and T3.
Thyroid supplements usually do the trick.
“On the right dose of thyroid replacement hormone, the heart problem always improves. If the issue was left untreated for a long time, however, it may not reverse,” says Dr. Hatipoglu.
Balance Is Key
When it comes to thyroid hormone, more is not better. Heart risk can jump, even when your levels are on the high side of normal. Maintaining an even level in the middle of the normal range is key.
Dr. Hatipoglu says today’s blood tests are more sensitive than in the past. Therefore, if you feel the need for more thyroid replacement hormone when a blood test says your levels are correct, ask your doctor to give you a thorough physical exam.
“We may need to make sure we’re not missing something. Some symptoms of thyroid disease may actually be caused by an entirely different disease,” says Dr. Hatipoglu.
Many symptoms of hypothyroidism mimic menopause, which can make successful treatment challenging. “I feel sorry for my patients when we diagnose low thyroid levels and provide thyroid replacement, and they still don’t feel right, because of menopause,” says Dr. Hatipoglu.