Definition of Hypothyroidism:

Hypothyroidism is a condition in which the thyroid gland does not make enough thyroid hormone.

See also:

• Chronic thyroiditis (Hashimoto's disease)
• Subacute thyroiditis
• Silent thyroiditis
• Neonatal hypothyroidism

Causes, incidence, and risk factors:

The thyroid gland is located in the front of the neck just below the voice box (larynx). It releases hormones that control metabolism.

The most common cause of hypothyroidism is inflammation of the thyroid gland, which damages the gland's cells. Autoimmune or Hashimoto's thyroiditis, in which the immune system attacks the thyroid gland, is the most common example of this. Some women develop hypothyroidism after pregnancy (often referred to as "postpartum thyroiditis").

Other common causes of hypothyroidism include:

• Congenital (birth) defects
• Radiation treatments to the neck to treat different cancers, which may also damage the thyroid gland
• Radioactive iodine used to treat an overactive thyroid (hyperthyroidism)
• Surgical removal of part or all of the thyroid gland, done to treat other thyroid problems
• Viral thyroiditis, which may cause hyperthyroidism and is often followed by temporary or permanent hypothyroidism

Certain drugs can cause hypothyroidism, including:

• Amiodarone
• Drugs used for hyperthyroidism (overactive thyroid), such as propylthiouracil (PTU) and methimazole
• Lithium
• Radiation to the brain
• Sheehan syndrome, a condition that may occur in a woman who bleeds severely during pregnancy or childbirth and causes destruction of the pituitary gland

Risk factors include:

• Age over 50 years
• Being female