Hypothyroidism

Alternative names:

adult hypothyroidism; myxedema

Definition:

A condition of decreased activity of the thyroid gland. See also hypothyroidism; primary and hypothyroidism; secondary.

Causes, incidence, and risk factors:

The thyroid gland is located in the front of the neck just below the larynx. The thyroid gland secretes several hormones which control body metabolism and regulate calcium balance. These hormones are thyroxine (T4), triiodothyronine (T3), and calcitonin.

The secretion of T3 and T4 by the thyroid is controlled by an endocrine feedback system involving the pituitary gland and the hypothalamus in the brain. Calcitonin is regulated by the amount of calcium in the blood and acts in conjunction with the parathyroid gland to control calcium levels. Since the thyroid gland is regulated by the pituitary gland and the hypothalamus, thyroid disorders may result not only from defects in the gland itself, but also from disruption of the control system. Thyroid disorders caused by overproduction of thyroid hormones are called hyperthyroidism, and underproduction of these hormones is known as hypothyroidism.

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, with the most severe form called myxedema, which is a medical emergency.

The causes may be due to disease of the thyroid gland (primary hypothyroidism) such as Hashimoto's disease where the body's immune system attacks the thyroid gland, or from failure of the pituitary gland to secrete a hormone to stimulate the thyroid gland (secondary hypothyroidism). Other causes are congenital defects, surgical removal of the thyroid gland, irradiation of the gland, or inflammatory conditions. Risk factors include being over 50 years old, being female, obesity, thyroid surgery, and x-ray or radiation treatments. The incidence is 2 out of 100 people.