Hypothyroidism; primary

Definition:

A condition of decreased activity of the thyroid gland, an organ of the endocrine system, caused by disease of the 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, some thyroid disorders may result not only from defects in the gland itself, but also from disruption of the control system. However, the causes of primary hypothyroidism are conditions that affect the gland itself rather than its control mechanisms. These conditions may include diseases of the thyroid gland such as Hashimoto's disease, iodine therapy for hyperthyroidism, thyroid surgery, X-ray treatment of the neck, certain medications such as lithium, or the cause may be unknown.

Primary hypothyroidism 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 is called myxedema which is a medical emergency. Risk factors include being over 50 years old, being female, obesity, thyroid surgery, and X-ray or radiation treatments to the neck. The incidence is 9 out of 1000 people.