Risk Factors

About 15 million Americans have unrecognized thyroid disease, mostly subclinical hypothyroidism. Less than 2% of the overall US population has full-blown hypothyroidism.

Women. Women have ten 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.

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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 it 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.