What Causes Hypothyroidism?

The vast majority of cases of hypothyroidism (roughly 95% or more) are caused by a problem within the thyroid gland. A much smaller number of cases (roughly 5% or less) are caused by a problem in the brain or pituitary gland, a small gland located beneath the brain.

The causes of hypothyroidism include:

• A condition called Hashimoto's thyroiditis

• Treatment of hyperthyroidism

• Treatment of thyroid tumors

• Radiation

• Interference from drugs

• Subacute thyroiditis

• Postpartum thyroiditis

• Problems in the pituitary gland

Hashimoto's Thyroiditis

Hashimoto's thyroiditis is a condition resulting from a problem in the body's immune system.

It is the most common cause of hypothyroidism.

The immune system is a complex network that normally defends the body against "invading" organisms and other foreign substances. When the immune system detects a foreign substance in the body, it responds to this by producing antibodies against the invaders. These antibodies will recognize and attack this foreign substance when they next encounter

In Hashimoto's thyroiditis the immune system mistakenly directs an immune "attack" against its own healthy cells. Antibodies are manufactured and misdirected against the thyroid gland.

Any condition in which the body's immune system attacks its own cells is called an auto-immune disease.

Inflammation associated with the abnormal immune response causes continuing loss of thyroid cells. Eventually, this loss in function (which may take months or even years) reaches the level where there is insufficient thyroid hormone to minimally support body function, and the person develops the symptoms of hypothyroidism.

Hashimoto's thyroiditis is more common in women than in men (roughly 7 to 1), and it typically develops in women over age 40 years. It may be much more common than previously thought in older adults. Studies have indicated that up to 15 percent of women over age 60 years have evidence of mild hypothyroidism due to Hashimoto's thyroiditis.

Because genetic factors can make someone vulnerable to developing autoimmune disease, it is not uncommon for persons with Hashimoto's thyroiditis to have a history of other autoimmune disorders, including Graves' disease (another thyroid disorder that causes an overactive thyroid gland), juvenile diabetes mellitus, pernicious anemia, or the skin disorder vitiligo.

In addition, the same genetic vulnerability toward disease means that more than one member of a family may develop Hashimoto's thyroiditis or another autoimmune disorder. If you are being evaluated for possible hypothyroidism, it is valuable to give your doctor any information you have on family or personal history of autoimmune disease, especially thyroid disease (namely, Hashimoto's thyroiditis or Graves' disease).

In the early stages of Hashimoto's disease, inflammation causes the thyroid to become enlarged and rubbery, and this may be enough to be noticeable. Indeed, diagnostic blood testing may be done because a doctor notices an enlarged thyroid during a physical examination.

Blood testing typically shows low levels of the thyroid hormones T3 and T4 in the blood, high levels of the hormone TSH that stimulates the thyroid to produce thyroid hormone, and the presence of antibodies directed against portions of the thyroid gland.

Treatment Of Hyperthyroidism

The treatment of hyperthyroidism - the condition in which the thyroid gland is "overactive" because there is too much thyroid hormone in the blood - can result in hypothyroidism. This can happen if:

• The hyperthyroidism was treated with radioactive iodine. Nearly 50 percent of individuals who receive radioactive iodine for the treatment of hyperthyroidism from Graves' disease will develop hypothyroidism within a year, and 65 percent will develop it within five years.

• Most of the thyroid gland was removed surgically in order to treat the hyperthyroidism.

Treatment Of Thyroid Tumors

Surgery for a noncancerous (benign) or cancerous (malignant) thyroid tumor is another common cause of hypothyroidism.

• If the tumor is noncancerous, usually only a portion of the thyroid is removed, and hypothyroidism does not usually occur, unless most of the gland was removed.

• If the tumor is cancerous, usually the entire thyroid is removed.

In either case, hypothyroidism may develop at the time of treatment or a number of years afterward.

Radiation Treatment For Head And Neck Cancers

Exposure to high doses of radiation for the treatment of head and neck cancers also can cause hypothyroidism. Hypothyroidism may occur in up to 65% of such individuals within 10 years of the radiation treatment.

Interference From Drugs

Some drugs given for nonthyroid conditions have the side effect of inhibiting production of thyroid hormone within the thyroid gland. If these drugs are taken in large dosages or for a long time, hypothyroidism may result.

• Use of lithium as a treatment for certain psychiatric conditions is an example of drug-induced hypothyroidism. Almost half of all people who take lithium may develop an enlarged thyroid, although less than a quarter of them will actually develop symptoms of hypothyroidism.

• Another example is use of the iodine-containing drug amiodarone (Cordarone) for heart arrhythmias.

• In addition, drugs used in treatment of hyperthyroidism (propylthiouracil, for instance) can also impair thyroid function and cause hypothyroidism.

Subacute Thyroiditis

Subacute thyroiditis is an inflammatory thyroid condition of unclear origin. It usually causes only temporary and mild hypothyroidism - that is, it produces a relatively less serious form of hypothyroidism that goes away on its own.

Subacute thyroiditis is an uncommon disorder that can occur in men or women of any age. The characteristic inflammation in the thyroid often follows an upper respiratory infection, and patients often come to a doctor's attention because of noticeable and painful enlargement of the thyroid gland.

Postpartum Thyroiditis

Postpartum thyroiditis means inflammation of the thyroid following pregnancy, (from post, meaning "after," and partum, meaning "birth"). It occurs in the first six months after delivery in about 5% to 9% of women. Typical symptoms relate to an initial phase of hyperthyroidism (with symptoms of rapid heart rate, increased sweating, nervousness) and a later phase of hypothyroidism (with fatigue, dry skin, feeling cold, depression).

Studies have shown that 80% to 90% or more of affected women had detectable levels before or during their pregnancies of antibodies directed against the thyroid gland.

• In up to 5% of women, this can cause inflammation of the thyroid up to 12 months after the delivery.

• Roughly 20% to 33% of the women who develop postpartum thyroiditis will have permanent hypothyroidism.

• Almost one-third of women with hypothyroidism due to postpartum thyroiditis develop mild to moderate depression after pregnancy.

Problems In The Pituitary Gland

Hypothyroidism can also develop due to a problem in other parts of the thyroid hormone production line (namely the brain or pituitary gland). These cases of hypothyroidism, which do not originate in the thyroid gland, are much less common (perhaps 5%), and almost all of these cases are due to failure of the pituitary gland to produce or release the hormone TSH, which is responsible for stimulating the production of thyroid hormone in the thyroid gland. Diagnostic blood testing shows low levels of T3 and T4 thyroid hormones, low or absent TSH, and absence of the antithyroid antibodies characteristic of Hashimoto's thyroiditis, the commonest cause of hypothyroidism.