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Hypothyroidism - primary

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Definition:

Primary hypothyroidism is a condition of decreased hormone production by the thyroid gland.

Causes, incidence, and risk factors:

The thyroid gland is an important organ that regulates metabolism . It is located in the front of the neck just below the voice box (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 T3 and T4 is called primary hypothyroidism. Worldwide, the most common cause of primary hypothyroidism is deficiency of the element iodine. In the US, 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 used for treatment of hyperthyroidism (overactive thyroid), radiation exposure to the neck, special x-ray dyes, and certain drugs such as lithium. Approximately 5-10% of women develop hypothyroidism after pregnancy (often referred to as “postpartum thyroiditis"). Some cases of hypothyroidism may be caused by a lack of enzymes that convert T3 and T4 for use in the body. In other cases, the cause of hypothyroidism is unknown.

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

Primary hypothyroidism affects the whole body and may cause a variety of symptoms. The body's normal rate of functioning slows, causing mental and physical sluggishness. Symptoms may vary from mild to severe. The most severe form is called myxedema coma 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.

References:

AACE Thyroid Task Force. American Association of Clinical Endocrinologists Medical Guidelines for Clinical Practice for the Evaluation and Treatment Of Hyperthyroidism and Hypothyroidism. Endocr Pract . 2002;8 (6).

• Review Date: 5/12/2006
• Reviewed By: Robert Hurd, MD, Department of Biology, College of Arts and Sciences, Xavier University, Cincinnati, OH. Review provided by VeriMed Healthcare Network.

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