Hypothyroidism; secondary

Definition:

A condition of decreased activity of the thyroid gland, an organ of the endocrine system caused by failure of the pituitary gland.

Causes, incidence, and risk factors:

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

The causes of secondary hypothyroidism are failure of the pituitary gland to secrete a hormone to stimulate the thyroid gland, or failure of the 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, with the most severe form called myxedema which is a medical emergency. Risk factors include being over 50 years old, being female, and having a history of pituitary or hypothalamic dysfunction. The incidence is 2 out of 1,000 people.