Hashimoto′s thyroiditis (chronic autoimmune thyroiditis): This is the most common cause of hypothyroidism in iodine-sufficient regions of the world and is discussed in more detail later in this chapter.

Iatrogenic disease: Causes include thyroidectomy, radioiodine treatment, or external radiotherapy.

Iodine deficiency or iodine excess: Both can cause hypothyroidism. The most common cause of hypothyroidism worldwide is iodine deficiency.

Drugs: Medications used to treat hyperthyroidism (such as methimazole or propylthiouracil [PTU]) can cause hypothyroidism. Other drugs that can cause hypothyroidism include amiodarone, lithium carbonate, interleukin-2, and interferon-α.

Transient hypothyroidism: This can occur during the course of several types of thyroiditis, followed by recovery of thyroid function. Some patients who undergo subtotal thyroidectomy become hypothyroid after 4 to 8 weeks but recover several weeks or months later. This condition may also follow radioactive iodine (RAI) treatment for Graves’ disease (see Chapter 7.4), when glands are not completely ablated by radioiodine and there is some remaining thyroid tissue.

Hypothyroidism in infants and children: The most common causes of congenital hypothyroidism are agenesis and dysgenesis of the thyroid, but a few cases are delivered by mothers who were receiving an antithyroid drug for hyperthyroidism. Among children who become hypothyroid later, the most common cause is chronic autoimmune thyroiditis.

Most often the cause is a pituitary tumor (macroadenomas), pituitary surgery, or irradiation.

Less common causes include head injury, postpartum pituitary necrosis (Sheehan′s syndrome), pituitary apoplexy (bleeding in a pituitary tumor), hypophysitis, nonpituitary tumors such as craniopharyngiomas, and infiltrative diseases.