Health A-Z

Medical Content Created by the Faculty of the Harvard Medical School


What Is It?

Hyperthyroidism is a condition in which your body makes too much thyroid hormone. It is also called overactive thyroid.

Thyroid hormones are made by the thyroid gland. The thyroid gland is located in the lower front of the neck.

Thyroid hormones regulate the body's energy. When levels of thyroid hormones are unusually high, the body burns energy faster and many vital functions speed up.

Hyperthyroidism is usually caused by the thyroid gland producing too much thyroid hormone. The three most common reasons for this are:

  • Graves' disease. Graves' disease is the most common cause of hyperthyroidism. It is an immune system disorder. The body produces antibodies that cause the thyroid to make and release too much thyroid hormone. If you have a relative with Graves' disease, you have an increased risk of developing hyperthyroidism.

  • Thyroid tumor. A noncancerous thyroid tumor may make and secrete increased amounts of thyroid hormones.

  • Toxic multinodular goiter. The thyroid gland is enlarged with many noncancerous thyroid tumors. They secrete increased amounts of thyroid hormone.

Rarely, the condition is caused by the pituitary gland making too much thyroid-stimulating hormone (TSH). This causes the thyroid gland to produce too much thyroid hormone.

Certain types of thyroid inflammation (thyroiditis) can cause short-term hyperthyroidism. This can occur after childbirth or after viral infections, for example.

In very rare situations, excess thyroid hormone can come from a source outside the thyroid. For example, abnormal tissue growth in the ovary can secrete thyroid hormone.

Symptoms of hyperthyroidism also can be caused by taking excessive amounts of thyroid supplements.

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From Health A-Z, Harvard Health Publications. Copyright 2007 by the President and Fellows of Harvard College. All rights reserved. Written permission is required to reproduce, in any manner, in whole or in part, the material contained herein. To make a reprint request, contact Harvard Health Publications. Used with permission of StayWell.

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