Talking about the causes of hyperthyroidism requires discussion of far more potentials—Graves’ disease, thyroiditis, goiter, and others—than the average condition. While there is a characteristic commonality among them in that they all cause your thyroid gland to produce too much thyroid hormone, how they do this varies. A few causes are technically avoidable, but most are not—and determining which one is at the root of your specific case requires testing.

Common Causes

The three most common causes of an overactive thyroid are Graves’ disease, toxic nodular or multinodular goiter, and thyroiditis. 

Graves’ Disease

Graves’ disease, an autoimmune disorder, is the most common cause of hyperthyroidism in the United States. In Graves’ disease, a person’s immune system produces antibodies that bind to thyroid cells, triggering them to overproduce thyroid hormone.

Toxic Nodular or Multinodular Goiter

Toxic nodular or multinodular goiter is characterized by one or more thyroid nodules or lumps that produce excess thyroid hormone. Sometimes these lumps are referred to as “hot nodules,” because they absorb radioiodine on a radioactive iodine uptake test.

Thyroiditis

Thyroiditis means “inflammation of the thyroid gland” and is a blanket term for several inflammatory thyroid disorders.

One common example of thyroiditis is postpartum thyroiditis, which occurs after a woman gives birth. Some women experience temporary hyperthyroidism, followed by temporary hypothyroidism, whereas other women experience only hyperthyroidism, and still others, only hypothyroidism. 

In addition to the postpartum period, thyroiditis may be caused by an infection (for example, a bacteria that damages thyroid cells), certain medications (for example, amiodarone, lithium, or interferon), trauma, radiation, or major stress.

Lastly, subacute thyroiditis (also called de Quervain’s thyroiditis) causes temporary hyperthyroidism followed by temporary (although sometimes permanent) hypothyroidism. A distinguishing feature of subacute thyroiditis is that a person will have a tender thyroid gland. 

Other Causes

While most cases of hyperthyroidism are due to the above, other causes may also be considered:

Medication-Induced Hyperthyroidism

Taking too much prescription thyroid hormone—whether by accident or by deliberate self-medication—can cause medication-induced hyperthyroidism. Some over-the-counter energy, diet, and glandular supplements also contain some active thyroid hormone, which can make you hyperthyroid. 

Iodine

Being exposed to or ingesting an excess amount of iodine (for example, taking iodine or supplements containing iodine) can trigger hyperthyroidism.

Temporary Hyperthyroidism in Hashimoto’s Disease

Normally people with Hashimoto’s thyroiditis are hypothyroid, as a person’s immune system cells destroy thyroid tissue. In rare cases, though, a person may be initially hyperthyroid, before becoming hypothyroid. This is sometimes called Hashitoxicosis.

Transient Hyperthyroidism of Hyperemesis Gravidarum

Hyperemesis gravidarum is a rare disorder of persistent nausea and vomiting and weight loss of 5% or more during early pregnancy.

Some women with hyperemesis gravidarum develop hyperthyroidism, although thyroid hormone levels are generally only minimally elevated.

Pituitary-Induced Hyperthyroidism

Your pituitary gland is called the “master” gland, as it produces hormones that trigger other glands, like your thyroid gland, to release other hormones.

There are two types of pituitary-induced hyperthyroidism, also called central hyperthyroidism. One type is caused by a tumor in the pituitary gland that overproduces thyroid-stimulating hormone, or TSH (called a pituitary adenoma).

Even rarer is a type involving overproduction of TSH by the pituitary gland due to mutations in the gene that codes for a thyroid hormone receptor.

Fetal-Neonatal Hyperthyroidism

Graves’ disease is the most common cause of hyperthyroidism in pregnancy, although it’s still uncommon, occurring in less than one percent of pregnant women.

Of these women with Graves’ disease (or a history of treated Graves’ disease), about 1 in 50,000 of their newborns develop fetal or neonatal hyperthyroidism, which is characterized by a variety of signs and symptoms. Some of them include frequent bowel movements, increased heart rate, low birth weight, small head circumference, and an enlarged thyroid gland (goiter). 

Genetics

Genetics plays a role in the development of Graves’ hyperthyroidism, as evidenced by the fact that Graves’ disease clusters in families.

That said, scientists have not fully teased out all these genetic associations, so genetic testing is not currently done on people suspected of having Graves’ disease.

This means that carrying certain genes (or having a family history significant for Graves’ disease or other autoimmune diseases) may make a person more likely to develop an overactive thyroid than someone without those genes.

Common Risk Factors

Being aware of the factors that can increase your risk of hyperthyroidism can help you have an informed discussion with your physician and, perhaps, heighten your awareness of any symptoms you may be experiencing:

  • Being female
  • Having a personal or family history of autoimmune disease (for example, rheumatoid arthritis, lupus or celiac disease)
  • Having a personal or family history of thyroid disease, including thyroid nodules
  • Being recently pregnant
  • Smoking
  • Taking iodine intake or an iodine-containing medication (for example, amiodarone)
  • Experiencing trauma to the thyroid gland
  • Being deficient in Vitamin D and selenium
  • Experiencing psychological stress (for example, divorce or loss of a partner)

Frequently Asked Questions

  • What causes Graves’ disease?
  • Doctors aren’t positive what triggers autoimmune disorders like Graves’ disease. It’s believed that a combination of genetics and outside factors such as a virus or stress disrupt normal immune system function. In this case, the immune system makes antibodies that tell the thyroid to make too much thyroid hormone.
  • What causes hyperthyroidism during pregnancy?
  • Pregnancy can trigger an autoimmune disorder such as Graves’ disease, the leading cause of hyperthyroidism. However, it’s also possible to have transient, or temporary, hyperthyroidism caused by high levels of hormones during early pregnancy, which is common with extreme morning sickness. In those instances, the symptoms of hyperthyroidism should stop after your first or second trimester.
  • How is my mean platelet volume related to hyperthyroidism?
  • Higher mean platelet volume (MPV) may indicate that you have hyperthyroidism. MPV is a measure of the average size of platelets, the cells in your blood that help prevent bleeding. However, MPV levels are associated with several conditions and are not used to diagnose hyperthyroidism. If the condition is related to your thyroid, MPV should return to normal once your hyperthyroidism is properly treated.

Doctors aren’t positive what triggers autoimmune disorders like Graves’ disease. It’s believed that a combination of genetics and outside factors such as a virus or stress disrupt normal immune system function. In this case, the immune system makes antibodies that tell the thyroid to make too much thyroid hormone.

Pregnancy can trigger an autoimmune disorder such as Graves’ disease, the leading cause of hyperthyroidism. However, it’s also possible to have transient, or temporary, hyperthyroidism caused by high levels of hormones during early pregnancy, which is common with extreme morning sickness. In those instances, the symptoms of hyperthyroidism should stop after your first or second trimester.

Higher mean platelet volume (MPV) may indicate that you have hyperthyroidism. MPV is a measure of the average size of platelets, the cells in your blood that help prevent bleeding. However, MPV levels are associated with several conditions and are not used to diagnose hyperthyroidism. If the condition is related to your thyroid, MPV should return to normal once your hyperthyroidism is properly treated.