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The most common thyroid disorder is hypothyroidism, or an underactive thyroid. In hypothyroidism, the thyroid gland doesn’t produce enough thyroid hormone. The most common cause of this disorder worldwide is a diet deficient in iodine. In the U.S. in the early to mid-1900s, about half of all adults in the "goiter belt"—the Great Lakes and inland mountain areas—had a goiter, which is an enlargement of the thyroid gland, because of an iodine deficiency. Now, because of the addition of iodine to table salt, U.S. diets are not deficient in this trace element. Today, the most common cause of hypothyroidism in the U.S. is an autoimmune disease called Hashimoto's thyroiditis. The normal function of the body’s immune system is to protect against bacteria, viruses, and other foreign substances. For a person with Hashimoto’s thyroiditis, the body’s immune system attacks the thyroid cells as foreign bodies and destroys them. The thyroid gland is then unable to produce enough thyroid hormone to meet the body’s need. Other causes of hypothyroidism include surgical removal of the thyroid, radiation treatment, congenital hypothyroidism, medications, and conditions of the pituitary gland.

This condition is seven times more likely to occur in women than in men. Risk factors for hypothyroidism include being female; family history of thyroid disease; being 50 or older; use of some medications; having an autoimmune disease; and a history of radiation treatment to the neck.

More than half of those with hypothyroidism have one or more of the symptoms. Other possible symptoms include weakness, lethargy, decreased energy, dry skin, inability to concentrate, memory loss, constipation, and weight gain. Because the symptoms of hypothyroidism can be confused with symptoms of other, more common conditions, many people with the condition are unaware they have it.

Hypothyroidism can be detected by a thyroid stimulating hormone (TSH) test, a blood test that checks for the hormone made by the pituitary gland that stimulates the thyroid. If the levels of this hormone are high, it usually means that the thyroid gland is exhausted and that the pituitary gland is trying to push the thyroid's output by boosting the amount of TSH it sends to the thyroid. The test results can be affected by medications, so be sure to tell your health care provider about any medicines and herbs you are taking. The U.S. Preventive Services Task Force does not currently recommend routine screening for thyroid disease, but rather screening patients who have symptoms suggestive of thyroid problems or are high risk for thyroid disease.

When a woman with an underactive thyroid does become pregnant, her condition may be overlooked, because fatigue and weight gain are symptoms of both an underactive thyroid and pregnancy. An underactive thyroid can also cause changes to a woman's menstrual cycle, from irregular, heavy periods to a loss of periods.

People with untreated hypothyroidism also have high total and LDL ("bad") cholesterol levels.

If the cause of the hypothyroidism is the thyroid itself, the condition is treated with a daily dose of a synthetic hormone. Synthetic hormones come in tablet form and often are taken for life. Anyone taking a synthetic hormone must be monitored with periodic TSH tests, because the dose may need to be adjusted for age, weight, and disease conditions.