THYROIDITIS The most common cause of hypothyroidism in the United States is chronic autoimmune thyroiditis (Hashimoto's thyroiditis), a condition characterized by inflammation and damage to the thyroid tissue. Damage occurs because immune cells which usually fight off infection and colds, attack the body's own thyroid tissue instead.
AUTOIMMUNE DISEASES If you have other autoimmune diseases, you may also be at risk for Hashimoto's thyroiditis. Some common examples of these autoimmune diseases include rheumatoid arthritis (the immune cells attack the joints) and diabetes (the immune cells attack the pancreas which produces insulin). In fact, about 10% of patients with Type I or juvenile diabetes mellitus develop chronic thyroiditis during their life. In diabetic patients with thyroid disease, the amount of insulin they need every day may change. It is therefore important for diabetes patients to be checked for thyroid disease by routine thyroid function blood tests on a regular basis. Still other examples of autoimmune diseases include vitiligo (a skin condition in which patches of skin lose their color and become white), Addison's disease (a disorder of the adrenal glands), and pernicious anemia (a special form of low blood count).
LITHIUM MEDICATION This drug is mainly used for psychiatric disorders (manic depression) and can cause thyroid enlargement and an underproduction of thyroid hormone, resulting in an underactive thyroid.
PREGNANCY Thyroiditis is especially common in the post partum period (it is also sometimes confused with postpartum depression) and may require thyroid hormone adjustments. It is also important to note that hypothyroidism (and hyperthyroidism) can sometimes cause unexplained infertility or miscarriages particularly early in pregnancy. Therefore, thyroid function tests are often a routine part of evaluating a patient who may have problems with pregnancy. In addition, during pregnancy, the requirements for thyroid hormone may increase, and should be monitored closely in the first and second trimester. Although some forms of thyroid disease may actually improve during pregnancy, pregnancy is also often associated with the development of benign thyroid disease.
AGING As we age, hypothyroidism becomes increasingly common, particularly in women (10% of all women over the age of 50 show signs of a failing thyroid).
SURGERY Whenever the thyroid gland is removed completely by surgery because of a thyroid disorder, hypothyroidism results and thyroid hormone medication must be prescribed to make up for the hormone that body can no longer make itself. If only half of the thyroid is removed (and the other half that is not removed is normal), still enough thyroid hormone is produced from the remaining half of the thyroid in order to function normally.
RADIOACTIVE IODINE Another common cause of hypothyroidism is the use of radioactive iodine. Some thyroid diseases such as Graves' Disease is treated with radioactive iodine. The radioactive iodine destroys the overactive thyroid cells, thus eliminating the source of the excess thyroid hormone or hyperthyroidism. This destruction sometimes results in hypothyroidism. This type of hypothyroidism may be difficult to detect immediately, because there may be just a small amount of thyroid tissue that is not destroyed right away. This small piece of thyroid may produce enough thyroid hormone for the body for a little while. However, if this piece of thyroid burns out or gives up, hypothyroidism may result.
Because this hypothyroidism can occur anywhere from months to years after treatment with radioactive iodine, patients may not immediately recognize the subtle symptoms of fatigue, weight gain and difficulty concentrating. By checking these blood tests once a year, hypothyroidism may be discovered in it's earliest stages before symptoms like fatigue and weight gain develop.
SECONDARY HYPOTHYROIDISM Finally, there are some rare causes of hypothyroidism related to brain diseases, also called secondary hypothyroidism. Disorders of the pituitary gland or hypothalamus portions of the brain may cause thyroid hormone deficiency in addition to other hormonal imbalances. This type of rare hypothyroidism can also be treated with thyroid hormone medication.