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 (overt) clinical hypothyroidism.
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 can be 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, which causes postpartum subacute thyroiditis and can increase the risk of developing permanent hypothyroidism.
The risk for hypothyroidism is greatest after age 50 and increases with age. However, hypothyroidism can affect people of all ages.
Genetics plays a role in many cases of underactive and overactive thyroid. The genetics involved with hypothyroidism are complicated, however. 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 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. Many autoimmune disorders increase the 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, particularly during pregnancy.
- Gout. Hypothyroidism and gout often coexist and may have biologic mechanisms in common.
- Addison's disease.
- Myasthenia gravis.
- Polycystic ovarian syndrome.
- Eating disorders such as anorexia nervosa or bulimia nervosa. In these patients, reduced thyroid function may be an adaptation to malnutrition.
- Turner syndrome. Turner syndrome is a genetic disorder that affects women. As many as half of patients with Turner syndrome have hypothyroidism, usually in the form of Hashimoto's thyroiditis.
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|Click the icon to see an image of polycystic ovarian syndrome.|
Many drugs affect the thyroid, so anyone being treated for a chronic disease who takes thyroid medication or is at risk for a thyroid disorder should discuss with their doctor the impact these drugs may have on their thyroid.
Review Date: 05/03/2011
Reviewed By: Harvey Simon, MD, Editor-in-Chief, Associate Professor of Medicine, Harvard Medical School; Physician, Massachusetts General Hospital. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.