Health A-Z

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What Is It?

Hypothyroidism means your thyroid gland cannot produce the normal amount of thyroid hormone. Your thyroid gland is under active. 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 abnormally low, the body burns energy more slowly, and vital functions, such as heartbeat and temperature regulation, slow down. Causes of hypothyroidism include:

  • Thyroid surgery or radioiodine ablation treatments to treat thyroid cancer or hyperthyroidism (abnormally high levels of thyroid hormones)

  • An autoimmune disorder, in which the body's own immune system attacks the thyroid

  • An inborn (congenital) thyroid defect

Short-term hypothyroidism can be caused by certain types of thyroid inflammation or thyroid infections with a virus. In less than 5% of cases, hypothyroidism is caused by a problem with the hypothalamus or a pituitary gland rather than the thyroid gland. The hypothalamus is a brain structure that normally signals the pituitary gland to make thyroid stimulating hormone (TSH), which causes the thyroid to make thyroid hormones.

Some medical problems can affect either the hypothalamus or the pituitary gland, and interrupt the chain of signals from the brain to the thyroid. If this happens, the thyroid gland doesn't get the message to make thyroid hormones, even though it is able to function perfectly. This is called secondary hypothyroidism, which usually occurs when the pituitary gland is damaged by a tumor, infection, sarcoidosis, or cancer that has spread from somewhere else in the body. Less often, hypothyroidism is the result of an injury to the hypothalamus.

Hypothyroidism is more common in women than men. Babies can be born with hypothyroidism.

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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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