Gender

Numerous population studies over the years have reported that both hypothyroidism and hyperthyroidism are far more prevalent in women than in men. The American Association of Clinical Endocrinologists (AACE) reports that women may be as much as five to eight times more likely than men to suffer from thyroid disease. Researchers do not fully understand why women are more likely than men to have thyroid problems, but it may be linked to estrogen. Some evidence suggests that this hormone affects the immune system, particularly the white blood cells called B cells that produce antibodies. This may have implications for the development of autoimmune diseases, including thyroid disease. One explanation for the development of postpartum thyroiditis is that during pregnancy, high levels of circulating estrogen may suppress the function of both B cells and T cells, another type of cell involved in immune response. The rebound of B and T cell function that occurs after delivery appears to trigger the disease.

Evidence also points to estrogen's involvement in the development of Graves' disease. Since Graves' disease is uncommon before puberty, when estrogen levels are low, estrogen's effect on the immune system — rather than sex-linked genes — is believed to be responsible for the fact that more women develop Graves' disease.

Estrogen's impact on the immune system is also believed to play a role in the development of other autoimmune diseases; in general, women are at a higher risk for autoimmune disease. Graves' disease and Hashimoto's thyroiditis, both autoimmune conditions, are to blame for most cases of thyroid malfunction as opposed to nodular thyroid problems, particularly in women (see "Family health history" below). Moreover, changes in estrogen levels may contribute to the development of thyroid disease after menopause, when hormone levels drop.

Age

Hypothyroidism is common in people over age 60, and the risk of hypothyroidism increases steadily with age, especially among women. These increases may reflect an increase in antibodies that cause Hashimoto's thyroiditis.

One observational study that followed 599 people, ages 85 to 89, for four years found that those in this age group who have hypothyroidism do not experience the adverse effects of the condition and even have a prolonged life span. That study, published in the Journal of the American Medical Association in 2004, suggests that treating the "oldest old" may have limited benefit. Still, evidence for not treating the oldest old can only come from a well-designed, case-controlled study in which people are randomly assigned to two groups — one receiving treatment and the other taking a placebo.

It is important to see your doctor if you notice any of the symptoms of hypothyroidism common to older people (see "Signs and symptoms of hypothyroidism in an older person" below).

Ethnicity

If you are of African descent, you are less likely to develop an autoimmune thyroid disease such as Graves' disease, Hashimoto's thyroiditis, and postpartum thyroiditis than people of other ethnic groups. The NHANES III study found that concentrations of thyroid antibodies suggest the presence of autoimmune disease in 14.3% of whites, 10.9% of Mexican Americans, and only 5.3% of blacks.