Hyperthyroidism: Causes

Part 2: Causes of Hyperthyroidism

There are several causes of hyperthyroidism. Most often, the entire gland is overproducing thyroid hormone. This is called Graves' disease. Less commonly, a single nodule is responsible for the excess hormone secretion. We call this a "hot" nodule.  Thyroiditis (inflammation of the thyroid) can also cause hyperthyroidism.

The most common underlying cause of hyperthyroidism is Graves' disease, a condition named for an Irish doctor who first described the condition. This condition can be summarized by noting that an enlarged thyroid (enlarged thyroids are called goiters) is producing way too much thyroid hormone. (Remember that only a small percentage of goiters produce too much thyroid hormone; the majority of thyroid goiters actually become large because they are not producing enough thyroid hormone.)

Graves' disease is classified as an autoimmune disease, a condition caused by the patient's own immune system turning against the patient's own thyroid gland. The hyperthyroidism of Graves' disease, therefore, is caused by antibodies that the patient's immune system makes.  The antibodies attach to specific activating sites on the thyroid gland, and that in turn causes the thyroid to make more hormone.

There are actually three distinct parts of Graves' disease:

  1. overactivity of the thyroid gland (hyperthyroidism)
  2. inflammation of the tissues around the eyes, causing swelling
  3. thickening of the skin over the lower legs (pretibial myxedema).

Most patients with Graves' disease, however, have no obvious eye involvement. Their eyes may feel irritated or they may look like they are staring. About one out of 20 people with Graves' disease will suffer more severe eye problems, which can include bulging of the eyes, severe inflammation, double vision, or blurred vision. If these serious problems are not recognized and treated, they can permanently damage the eyes and even cause blindness. Thyroid and eye involvement in Graves' disease generally run a parallel course, with eye problems resolving slowly after hyperthyroidism is controlled.

Characteristics of Graves' Disease

Graves Disease is too much hormone from a big thyroid

  • Graves' disease affects women much more often than men (about 8:1 ratio, thus 8 women get Graves' disease for every man that gets it).
  • Graves' disease is often called diffuse toxic goiter because the entire thyroid gland is enlarged, usually moderately enlarged, sometimes quite big.
  • Graves' disease is uncommon over the age of 50 (more common in the 30s and 40s)
  • Graves' disease tends to run in families (not known why)

Other Less Common Causes of Hyperthyroidism

Solitary HOT nodule in right thyroid lobe.Hyperthyroidism can also be caused by a single nodule within the thyroid instead of the entire thyroid. As outlined in detail on our nodules page, thyroid nodules usually represent benign (non-cancerous) lumps or tumors in the gland. These nodules sometimes produce excessive amounts of thyroid hormones. This condition is called "toxic nodular goiter." The picture on the right is an iodine scan (also simply called a thyroid scan) which shows a normal sized thyroid gland (shaped like a butterfly).

This scan is abnormal because a solitary "hot" nodule is located in the lobe on the left. This single nodule is comprised of thyroid cells which have lost their regulatory mechanism that dictates how much hormone to produce. Without this regulatory control, the cells in this nodule produce thyroid hormone at a dramatically increased rate causing the symptoms of hyperthyroidism. (As a point of reference, some nodules are "cold" since they don't produce any hormone at all. There is a picture of a cold nodule on the nodule page.)

Inflammation of the thyroid gland, called thyroiditis, can lead to the release of excess amounts of thyroid hormones that are normally stored in the gland.

In subacute thyroiditis, the painful inflammation of the gland is believed to be caused by a virus, and the hyperthyroidism lasts a few weeks.

A more common painless form of thyroiditis occurs in one out of 20 women, a few months after delivering a baby and is, therefore, known as postpartum thyroiditis.

Although hyperthyroidism caused by thyroiditis causes the typical symptoms listed on our introduction to hyperthyroidism page, they generally last only a few weeks until the thyroid hormone stored in the gland has been exhausted. For more about thyroiditis, see our article on this topic.

Hyperthyroidism can also occur in patients who take excessive doses of any of the available forms of thyroid hormone. This is a particular problem in patients who take forms of thyroid medication that contains T3, which is normally produced in relatively small amounts by the human thyroid gland. Other forms of hyperthyroidism are even rarer. It is important for your doctor to determine which form of hyperthyroidism you may have since the best treatment options will change depending on the underlying cause.