The thyroid gland is an important organ of the endocrine system, located in the front of the neck just below the voicebox. The thyroid 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 (structures in the brain).

Lowered levels of these thyroid hormones result in increased levels of pituitary and hypothalamic hormones. The reverse is also true -- when levels of the thyroid hormones rise, pituitary and hypothalamic hormones fall back. This helps keep levels appropriately balanced.

Since the thyroid gland is regulated by the pituitary gland and the hypothalamus, thyroid disorders may result not only from defects in the thyroid itself but also from the disruption of the control system in these other organs.

Thyroid disorders caused by overproduction of thyroid hormones are called hyperthyroidism, and underproduction of these hormones is known as hypothyroidism.

The cause of secondary hypothyroidism is failure of the pituitary gland to secrete thyroid stimulating hormone (TSH). This is usually caused by a tumor in the region of the pituitary. Rarely the cause is an infiltration of the pituitary by inflammatory cells from the immune system or foreign substances (such as iron in hemochromotosis). Hypothyroidism may cause a variety of symptoms and can affect all body functions.

The body's normal rate of functioning slows, causing mental and physical sluggishness. Symptoms vary from mild to severe. The most severe form is called myxedema, which is a medical emergency and can lead to coma and death.