Definition of Hypothyroidism:

Hypothyroidism is a condition in which the thyroid gland fails to produce enough thyroid hormone.

Causes, incidence, and risk factors:

The thyroid gland, located in the front of the neck just below the larynx, secretes hormones that control metabolism. These hormones are thyroxine (T4) and triiodothyronine (T3).

The secretion of T3 and T4 is controlled by the pituitary gland and the hypothalamus, which is part of the brain. Thyroid disorders may result not only from defects in the thyroid gland itself, but also from abnormalities of the pituitary or hypothalamus.

Hypothyroidism, or underactivity of the thyroid gland, may cause a variety of symptoms and may affect all body functions. The body's normal rate of functioning slows, causing mental and physical sluggishness. The symptoms may vary from mild to severe. The most severe form, called myxedema coma, is a medical emergency.

The most common cause of hypothyroidism is Hashimoto's thyroiditis, a disease of the thyroid gland where the body's immune system attacks the gland. Failure of the pituitary gland to secrete a hormone to stimulate the thyroid gland (secondary hypothyroidism) is a less common cause of hypothyroidism. Other causes include congenital (birth) defects, surgical removal of the thyroid gland, irradiation of the gland, or inflammatory conditions.

Risk factors include age over 50 years, female gender, obesity, thyroid surgery, and exposure of the neck to X-ray or radiation treatments.