Causes and Risks

The thyroid gland is an important organ that regulates body metabolism. It is located in the front of the neck just below the larynx. The thyroid gland secretes two forms of thyroid hormone thyroxine (T4) and triiodothyronine (T3). The secretion of T3 and T4 by the thyroid is controlled by a feedback system involving the pituitary gland, a small organ at the base of the brain, and the hypothalamus, a structure in the brain.

Hypothyroidism caused by the inability of the thyroid gland to make hormone is called primary hypothyroidism.Worldwide, the most common cause of primary hypothyroidism is deficiency of the element iodine. In the United States, the most common cause is destruction of the thyroid gland by the immune system, a condition called Hashimoto's thyroiditis.

Other causes of primary hypothyroidism include surgical removal of part or all of the thyroid gland, radioactive iodine for treatment of hyperthyroidism (over active thyroid), radiation exposure to the neck, X-ray contrast agents, and certain drugs such as lithium. In some cases the cause of hypothyroidism is unknown.

Since the thyroid gland is regulated by the pituitary gland and hypothalamus, disease of these organs can cause the thyroid gland to produce too little thyroid hormone. This condition is called secondary hypothyroidism.

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 tosevere. The most severe form is called myxedema and is a medical emergency. Risk factors for hypothyroidism include age (older than age 50), female gender, obesity, thyroid surgery, and X-ray or radiation treatments to the neck. The incidence is about 9 out of 1000 people.

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