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Hypothyroidism

Description

An in-depth report on the causes, diagnosis, treatment, and prevention of hypothyroidism.

Alternative Names

Autoimmune thyroiditis; Hashimoto's thyroiditis

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. In one study, nearly 6% of women over 60 had hypothyroidism, and some experts estimate that as many as 20% of women in this age group have a subclinical condition. 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 between 4 and 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 elderly are most susceptible, but hypothyroidism can affect people of all ages. For example, one out of every 3,000 to 4,000 infants is born with congenital hypothyroidism. Female infants are at higher risk than males.

Ethnicity. African Americans may be less likely to have thyroid disease than Caucasians and Asians.

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 to 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. Chemicals in cigarette smoke called thiocyanates appear to have especially harmful effects on the thyroid. 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.
• Breast cancer. There may be a link between breast cancer and thyroid levels, but the evidence is unclear. Some studies have indicated that women with breast cancer may be more susceptible to hypothyroidism. Other studies suggest the opposite. Several studies indicate that hypothyroidism itself may protect against breast cancer. In addition, when women with hypothyroidism develop breast cancer, it is often a less aggressive and more easily treated form.
• 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 cases, 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's syndrome. As many as half of patients with Turner's syndrome have hypothyroidism, usually in the form of Hashimoto's thyroiditis. This inherited condition is one of the most common genetic diseases in women.
• Glaucoma. A 2004 study of male veterans suggested that hypothyroidism may be associated with increased risk for developing open-angle glaucoma.

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

Physical Factors Associated with Hypothyroidism Factors

Hypothyroidism is associated with premature gray hair and left-handedness.

• Review Date: 4/19/2006
• Reviewed By: Harvey Simon, M.D., Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital

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