Risk Factors:

About 15 million Americans have unrecognized thyroid disease, mostly subclinical hypothyroidism (mildly underactive thyroid). Less than 2% of the U.S. population has full-blown hypothyroidism.

Women. Women have 10 times the risk of hypothyroidism as men, with the difference being significant after age 34. Because the symptoms of hypothyroidism and menopause are so similar, hypothyroidism may easily be missed.

Pregnancy is a major factor in the higher risk in women. It affects the thyroid in a number of ways and poses a high risk for hypothyroidism, both during pregnancy and afterward. For one, iodine requirements are high in both the mother and the fetus. Changes in reproductive hormones also cause changes in thyroid hormone levels. In addition, some women develop antibodies to their own thyroid during pregnancy, causing a condition known as postpartum autoimmune, or subacute, thyroiditis. This occurs in up to 10% of pregnant women and tends to develop 4 - 12 months after delivery. It is a limited condition and nearly always clears up. However, it does pose a risk for the development of permanent hypothyroidism later on.

Age. The risk for hypothyroidism is greatest after age 50 and increases with age. However, hypothyroidism can affect people of all ages.

Genetic Defects

Genetics plays a role in many cases of underactive and overactive thyroid. The genetics involved with hypothyroidism are complicated, however. Certain genetic features, for example, appear to play a role in Hashimoto's thyroiditis and postpartum thyroiditis in Caucasians, but others affect different ethnic groups. Thyroid disease will often skip generations. For example, someone with an underactive thyroid may have healthy parents but have grandparents who had thyroid troubles. Some people inherit a tendency for thyroid problems but never become ill, while others become very sick.

Smoking

Smoking significantly increases risk for thyroid disease, particularly autoimmune Hashimoto's thyroiditis and postpartum thyroiditis. Smoking also increases the negative effects of hypothyroidism, notably on the arteries and heart.

Medical Conditions Associated with Hypothyroidism

People with certain medical conditions have a higher risk for hypothyroidism. These conditions include:

• Autoimmune diseases. People with many autoimmune diseases have a higher risk for hypothyroidism. Type 1 (insulin-dependent) diabetes poses a higher risk and is a special problem since hypothyroidism can affect insulin requirements. Women with other autoimmune diseases, including systemic lupus erythematosus, pernicious anemia, and rheumatoid arthritis, are also at higher risk for hypothyroidism. Pregnant women with autoimmune conditions have a 25% risk for hypothyroidism during gestation.
• Gout. Hypothyroidism and gout often coexist and may have biologic mechanisms in common.
• Addison's disease.
• Myasthenia gravis.
• Polycystic ovarian syndrome.
• Anorexia or bulimia. People with eating disorders are at risk for hypothyroidism. In these patients, however, reduced thyroid function may be an adaptation to malnutrition and, therefore, some experts think that only the eating disorder should be treated, not hypothyroidism.
• Turner syndrome. As many as half of patients with Turner syndrome have hypothyroidism, usually in the form of Hashimoto's thyroiditis. This inherited condition is one of the most common genetic diseases in women.

Many drugs affect the thyroid, so anyone being treated for a chronic disease, taking thyroid medication, and those at risk for a thyroid disorder should discuss the impact these drugs may have on their thyroid.