Hypothyroidism is a condition that occurs when a patient's thyroid gland cannot produce enough thyroid hormone to keep the body's metabolism functioning normally. This lack of hormone causes the body processes to slow down, leading patients to experience symptoms such as fatigue, intolerance to cold, weight gain and dry skin.

About 10 percent of women and 3 percent of men have hypothyroidism, according to the American Association of Clinical Endocrinologists.

The thyroid gland sits just below the Adam's apple in the neck. It secretes hormones that regulate a person's metabolism, the physical and chemical processes necessary for the maintenance of life. Thyroid hormones help to:

• Control the rate at which the body uses fats and carbohydrates
  
• Maintain body temperature
  
• Influence heart rate
  
• Regulate the production of protein
  
• Regulate the amount of calcium in the blood

The thyroid gland is one link in the complex chain that regulates metabolism. This process begins in the brain, where a gland called the hypothalamus signals the pituitary gland to make and release a hormone called thyroid-stimulating hormone (TSH). TSH then causes the release of the thyroid's two major hormones: thyroxine (T4) and triiodothyronine (T3). When disorders affect the thyroid function, they may disturb the release of these thyroid hormones, which may affect many aspects of the metabolism and lead to major health problems.

Hypothyroidism can be associated with an enlarged thyroid gland (goiter), which can be felt or even seen as a swelling at the base of the neck. It results from constant stimulation of the thyroid by the pituitary in an effort to have the thyroid produce more hormones. An autoimmune condition known as Hashimoto's thyroiditis is among the most common causes of hypothyroidism and goiter.

Other health problems that can result from thyroid underactivity include:

• Heart disease. People with hypothyroidism are more vulnerable to developing high levels of low-density lipoprotein (LDL) cholesterol. This is the so-called "bad" cholesterol. Enlarged heart (cardiomegaly) and heart failure are other potential complications of underactive thyroid.
  
  
• Infertility and birth defects. Untreated hypothyroidism can complicate pregnancy. Children born to these mothers also are at increased risk of intellectual, neurological and developmental problems.
  
  
• Joint pain.
  
  
• Mental health problems. Depression and reduced mental functioning may be associated with hypothyroidism.
  
  
• Obesity.
  
  
• Dwarfism. Hypothyroidism results in growth hormone deficiency and in children can cause short stature.

According to recent research, hypothyroidism may also increase the risk of glaucoma, a leading cause of vision loss and blindness.

Left untreated, hypothyroidism can progress to a rare and severe form of the disease known as myxedema, which may include skin changes, swelling and lethargy, and eventually lead to coma. This is a potentially life-threatening condition.