Hypothyroidism is a condition in which the thyroid gland fails to function adequately, resulting in reduced levels of thyroid hormone in the body. Cretinism is a type of hypothyroidism that occurs at birth and results in stunted physical growth and mental development. Severe hypothyroidism is called myxedema.

There are many causes of hypothyroidism. One common cause is Hashimoto’s thyroiditis, an autoimmune disease of the thyroid gland. Another common cause of hypothyroidism is medical treatment, such as surgery or radiation to the thyroid gland, to treat hyperthyroidism (over-activity of the thyroid gland). Some drugs, such as lithium and phenylbutazone, may also induce hypothyroidism. Extreme iodine deficiency, which is rare in the United States, is another possible cause. Failure of the pituitary gland or hypothalamus to stimulate the thyroid gland properly can cause a condition known as secondary hypothyroidism.

Some people with goiter (an enlargement of the thyroid gland) also have hypothyroidism. Goiter can be caused by an iodine deficiency, by eating foods that contain goitrogens (goiter-causing substances), or by other disorders that interfere with thyroid hormone production. In many cases the cause of goiter cannot be determined. While natural therapies may help to some extent, thyroid hormone replacement is necessary for most people with hypothyroidism.