Hyperthyroidism is a disorder that occurs when the thyroid gland makes more thyroid hormone than the body needs. It is sometimes called thyrotoxicosis, the technical term for too much thyroid hormone in the blood. About 1 percent of the U.S. population has hyperthyroidism.1 Women are much more likely to develop hyperthyroidism than men.

What causes hyperthyroidism?

Hyperthyroidism has several causes, including

* Graves’ disease
* one or more thyroid nodules
* thyroiditis, or inflammation of the thyroid gland
* ingesting too much iodine
* overmedicating with synthetic thyroid hormone, which is used to treat underactive thyroid

Rarely, hyperthyroidism is caused by a pituitary adenoma, which is a noncancerous tumor of the pituitary gland. In this case, hyperthyroidism is due to too much TSH.

Graves’ Disease

Graves’ disease, also known as toxic diffuse goiter, is the most common cause of hyperthyroidism in the United States. Graves’ disease is an autoimmune disease, which means the body’s immune system acts against its own healthy cells and tissues. In Graves’ disease, the immune system makes an antibody called thyroid stimulating immunoglobulin (TSI), which mimics TSH and causes the thyroid to make too much thyroid hormone.

Thyroid Nodules

Thyroid nodules, also called adenomas, are lumps in the thyroid. Thyroid nodules are common and usually noncancerous. About 3 to 7 percent of the population has them. However, nodules may become overactive and produce too much hormone. A single overactive nodule is called a toxic adenoma.

When multiple nodules are overactive, the condition is called toxic multinodular goiter. Often found in older adults, toxic multinodular goiter can produce a large amount of excess thyroid hormone.

Thyroiditis

Several types of thyroiditis can cause hyperthyroidism. Thyroiditis does not cause the thyroid to produce excess hormone. Instead, it causes stored thyroid hormone to leak out of the inflamed gland and raise hormone levels in the blood.

Subacute thyroiditis. This condition involves painful inflammation and enlargement of the thyroid gland. Doctors aren’t sure what causes subacute thyroiditis, but it may be related to a viral or bacterial infection. The condition usually goes away on its own in a few months. Many people with subacute thyroiditis briefly develop hypothyroidism—when hormone levels are too low—after the hyperthyroidism resolves but before the thyroid gland is completely healed.

Postpartum thyroiditis. About 8 percent of women who have been pregnant develop postpartum thyroiditis within a few months of giving birth. Postpartum thyroiditis is believed to be an autoimmune condition and causes hyperthyroidism that usually lasts for 1 to 2 months. The condition is likely to recur with future pregnancies. As with subacute thyroiditis, women with postpartum thyroiditis often develop hypothyroidism before the thyroid gland is completely healed. In some women, the gland does not heal and hormone levels remain low. These women must take thyroid hormone replacement for the rest of their lives.

Silent thyroiditis. This type of thyroiditis is called “silent� because it is painless, as is postpartum thyroiditis, even though the thyroid may be enlarged. Like postpartum thyroiditis, silent thyroiditis is probably an autoimmune condition and sometimes develops into permanent hypothyroidism.

What are the symptoms of hyperthyroidism?

Hyperthyroidism has many symptoms that can vary from person to person. Some common symptoms of hyperthyroidism are

• nervousness or irritability
• fatigue or muscle weakness
• trouble sleeping
• heat intolerance
• hand tremors
• rapid and irregular heartbeat
• frequent bowel movements or diarrhea
• weight loss
• mood swings
• goiter, which is an enlarged thyroid that may cause your neck to look swollen

Who is likely to develop hyperthyroidism?

Women are five to 10 times more likely than men to develop hyperthyroidism. The American Thyroid Association recommends that adults, particularly women, have a blood test to detect thyroid problems every 5 years starting at age 35.

Certain factors can increase your chances of developing thyroid disorders. You may need more frequent testing if you

• have had a thyroid problem before, such as goiter or thyroid surgery
• have pernicious anemia; type 1 diabetes; or primary adrenal insufficiency, a hormonal disorder
• have a family history of thyroid disease
• eat large amounts of food containing iodine, such as kelp, or use iodine-containing medications such as amiodarone, a heart medication
• are older than 60 years
• have been pregnant or delivered a baby within the past 6 months

Getting tested routinely helps uncover thyroid problems—especially subclinical problems. Subclinical means that a person has no apparent symptoms. Some doctors treat subclinical hyperthyroidism immediately; others prefer to leave it untreated but monitor their patients for signs that the condition is worsening.

Hyperthyroidism in the Aging Population

Hyperthyroidism is more common in people over age 60 and is often caused by thyroid nodules. Hyperthyroidism in this age group is sometimes misdiagnosed as depression or dementia because older adults do not always display the more common symptoms of the disorder. For example, older adults may lose their appetites, whereas younger people tend to have increased appetites. Older adults with hyperthyroidism may seem disinterested or withdraw socially. They may have different physical symptoms as well.

Treating subclinical hyperthyroidism may be more important in older adults than in younger patients. For people over age 60, subclinical hyperthyroidism increases the chance of developing a rapid, irregular heartbeat, known as atrial fibrillation?a condition that may lead to heart failure or stroke. Untreated hyperthyroidism can also speed the bone-thinning disease osteoporosis, particularly in women, and increase the likelihood of bone fractures.

Pregnancy and Hyperthyroidism

If you have hyperthyroidism, you should discuss your condition with your doctor before becoming pregnant. Uncontrolled hyperthyroidism raises your chance of miscarriage, preterm delivery, and preeclampsia, a potentially serious complication that increases blood pressure.

Symptoms of hyperthyroidism may be difficult to assess during pregnancy. Normally, the thyroid gland gets bigger in healthy women when they become pregnant. That normal enlargement, combined with fatigue, makes a new thyroid problem easy to miss. A rapid heart rate and unexplained weight loss or failure to have normal pregnancy weight gain are signs that hyperthyroidism could be developing.

How is hyperthyroidism diagnosed?

Your doctor will begin by asking you about your symptoms and performing a thorough physical examination. Your doctor may then use several tests to confirm a diagnosis of hyperthyroidism and to find its cause.

Thyroid-Stimulating Hormone (TSH) Test

The ultrasensitive TSH test will probably be the first test your doctor performs. This test can detect even tiny amounts of TSH in the blood and is the most accurate measure of thyroid activity available. The TSH test is especially useful in detecting mild hyperthyroidism.

The TSH test is based on the way TSH and thyroid hormone work together. The pituitary gland boosts TSH production when the thyroid is not making enough thyroid hormone; the thyroid normally responds by making more hormone. Then, when the body has enough thyroid hormone circulating in the blood, TSH output drops. In people who produce too much thyroid hormone, the pituitary shuts down TSH production, leading to low or even undetectable TSH levels in the blood.

Generally, a TSH reading below normal indicates hyperthyroidism and a reading above normal means a person has hypothyroidism.

Other Tests

If your doctor confirms you have hyperthyroidism, additional tests may be needed to find the cause and determine the best treatment.

T₃ and T₄ test. This test will show the levels of T₃ and T₄ in your blood. If you have hyperthyroidism, the levels of one or both of these hormones in your blood will be higher than normal.

Thyroid-stimulating immunoglobulin (TSI) test. This test, also called a thyroid-stimulating antibody test, measures the level of TSI in your blood. Most people with Graves’ disease have this antibody, but people whose hyperthyroidism is caused by something else do not.

Radioactive iodine uptake test. The radioactive iodine uptake test measures the amount of iodine your thyroid collects from the bloodstream. Measuring the amount of iodine in your thyroid will help your doctor know what is causing your hyperthyroidism. For example, low levels of iodine uptake might be a sign of thyroiditis, whereas high levels could indicate Graves’ disease.

Thyroid scan. A thyroid scan shows how and where iodine is distributed in your thyroid. This information helps your doctor diagnose the cause of your hyperthyroidism by providing images of nodules and other possible thyroid irregularities.