The most common thyroid disorder is hypothyroidism. This means an underactive thyroid. This is when the gland doesn’t make enough thyroid hormone. The most common cause used to be a diet with not enough iodine in it. In the U.S. in the early to mid-1900s, about half of all adults in the Great Lakes and inland mountain areas had a goiter. This area was called the "goiter belt.” A goiter is an enlargement of the thyroid gland. It happens when a person doesn't get enough iodine. Today iodine is added to table salt. Because of this, most U.S. diets don't lack this trace element. Today, the most common cause of hypothyroidism in the U.S. is Hashimoto's thyroiditis. This is a type of autoimmune disease. The normal job of your body’s immune system is to protect against bacteria, viruses, and other foreign items. If you have Hashimoto’s thyroiditis, your immune system attacks the thyroid cells. It treats them as foreign cells and destroys them. The thyroid gland is then not able to make enough thyroid hormone to meet the body’s need. Other causes of hypothyroidism include surgery to remove the gland or radiation treatment to the gland. Hypothyroidism can be something you are born with (congenital). Or it may be caused by medicines or conditions of the pituitary gland.

This condition is 8 times more likely to occur in people assigned female at birth. Other risk factors include having a family history of thyroid disease, being age 50 or older; or using certain medicines. You are also more at risk if you have an autoimmune disease or had radiation treatment to your neck.

More than half of those with hypothyroidism have 1 or more of the symptoms. Common symptoms include weakness, feeling low in energy (lethargy), joint and muscle pain, intolerance to cold, dry skin, dry and thinning hair, inability to concentrate, memory loss, heavy or irregular menstrual periods, slow heart rate, fertility problems, constipation, and weight gain.

Hypothyroidism can be detected by a thyroid stimulating hormone (TSH) test. This is a blood test that checks for the hormone made by the pituitary gland. TSH is a hormone that stimulates the thyroid. If the levels of this hormone are high, it often means that the thyroid gland is not making enough hormones. The pituitary gland is trying to cause the thyroid to make more. It does this by boosting the amount of TSH it sends to the thyroid. Your test results can be affected by medicines. Tell your healthcare provider about any medicines and herbs you are taking. Routine screening is not advised for thyroid disease. But if you have symptoms of thyroid problems or are at high risk for thyroid disease, ask about the TSH test.

Hypothyroidism can cause menstrual changes that can lower someone's chances of becoming pregnant. When someone with an underactive thyroid is pregnant, their condition may be overlooked. This is because severe tiredness (fatigue) and weight gain are symptoms of both an underactive thyroid and pregnancy. An underactive thyroid can also cause changes to person's menstrual cycle. It can cause irregular, heavy periods. Or it can cause periods to stop.

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. These are taken as a tablet by mouth. They often are taken for life. Anyone taking this medicine must be checked with regular TSH tests. This is because the dose may need to be changed for age, weight, and changes in health.