Causes

Several conditions can cause nodules to develop in your thyroid gland, including:

• Overgrowth of normal thyroid tissue. An overgrowth of normal thyroid tissue is sometimes referred to as a thyroid adenoma. It's unclear why this occurs, but it's not cancerous and isn't considered serious unless it causes bothersome symptoms from its size.

  Some thyroid adenomas lead to hyperthyroidism.

• Thyroid cyst. Fluid-filled cavities (cysts) in the thyroid most commonly result from degenerating thyroid adenomas. Often, solid components are mixed with fluid in thyroid cysts. Cysts are usually noncancerous, but they occasionally contain cancerous solid components.
• Chronic inflammation of the thyroid. Hashimoto's disease, a thyroid disorder, can cause thyroid inflammation and result in enlarged nodules. This often is associated with hypothyroidism.
• Multinodular goiter. The term goiter is used to describe any enlargement of the thyroid gland, which can be caused by iodine deficiency or a thyroid disorder. A multinodular goiter contains multiple distinct nodules within the goiter, but its cause is less clear.

• Thyroid cancer. The chances that a nodule is cancerous are small. However, a nodule that is large and hard or causes pain or discomfort is more worrisome. You will likely want to have it checked by your doctor.

  Certain factors increase your risk of thyroid cancer, such as a family history of thyroid or other endocrine cancers and having a history of radiation exposure from medical therapy or from nuclear fallout.

• Iodine deficiency. Lack of iodine in your diet can sometimes cause your thyroid gland to develop thyroid nodules. But iodine deficiency is uncommon in the United States, where iodine is routinely added to table salt and other foods.