Thyroid nodules are lumps that can form in the thyroid gland. They are the most common endocrine condition in the United States, according to the American Thyroid Association. However, estimates of their incidence vary widely. The American Cancer Society reports that at least half of the general population develop them and usually do not even notice them, whereas the American Association of Clinical Endocrinologists states that 6 percent of women and 1 to 2 percent of men are diagnosed with thyroid nodules.

Lumps in the thyroid glands occur due to an abnormal growth of thyroid cells. Patients may have just one thyroid nodule or a collection of several nodules.

Between 90 and 95 percent of these nodules are noncancerous (benign). Such nodules may not cause symptoms and usually do not pose significant health dangers. In some cases, the nodule may produce thyroid hormones, leading to excessive levels of hormones in the bloodstream (hyperthyroidism). Nodules that produce thyroid hormones are called autonomous nodules.

Different types of noncancerous nodules include:

• Colloid nodules. These nodules result from an overgrowth of normal tissue or storage of thyroid hormones.
  
  
• Hyperplastic nodules. Marked by excessive cellular activity and include several congenital (present at birth) types. These nodules may sometimes produce thyroid hormones.
  
  
• Adenomas. Benign nodules that include follicular and and Hurthle cell types. Adenomas sometimes produce thyroid hormones.
  
  
• Thyroid cysts. Nodules filled with blood or other fluid. Some cysts contain solid components (complex cysts). These are more likely to be cancerous than fluid-filled cysts.
  
  
• Inflammatory nodules. Nodules that develop as a result of inflammation of the thyroid gland (thyroiditis), particularly Hashimoto’s thyroiditis.

If a number of nodules are present, they can cause an enlargement of the thyroid gland (goiter). Enlargement due to the presence of several nodules is known as a multinodular goiter.

In uncommon cases, thyroid nodules may be cancerous (malignant). For this reason, patients are urged to have nodules checked by a physician. Cancer of the thyroid is usually highly treatable, especially if it is caught early.