Definition of Hypothyroidism - primary:

Primary hypothyroidism is a condition in which a defect in the thyroid gland leads to reduced production of thyroid hormone.

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 releases two forms of thyroid hormone – thyroxine (T4) and triiodothyronine (T3). The thyroid gland, along with the pituitary gland and hypothalamus in the brain, usually controls how much of these hormones are produced.

Primary hypothyroidism is when the thyroid cannot make the hormones T3 and T4 because of a problem with the gland itself. In the U.S., the most common cause is destruction of the thyroid gland by the immune system. This condition is called Hashimoto's thyroiditis. Not having enough iodine in the diet is a rare cause of hypothyroidism in the U.S.

Other causes of primary hypothyroidism include:

• Certain drugs such as lithium
• Radiation exposure to the neck
• Radioactive iodine used for treatment of hyperthyroidism (overactive thyroid)
• Special x-ray dyes
• Surgical removal of part or all of the thyroid gland

Some women develop hypothyroidism after pregnancy (often referred to as “postpartum thyroiditis"). In other cases, the cause of hypothyroidism is unknown.

Problems with the pituitary gland and hypothalamus may also cause the thyroid gland to produce too little thyroid hormone. This condition is called secondary hypothyroidism.

Risk factors for hypothyroidism include:

• Age (older than 50)
• Female gender
• Obesity
• Thyroid surgery
• X-ray or radiation treatments to the neck