A number of factors that influence thyroid function or growth can result in a goiter.

1. Iodine deficiency. Iodine is essential for the production of thyroid hormones. If a person does not get enough dietary iodine, hormone production drops and the pituitary gland signals the thyroid to make more. This increased signal results in thyroid growth. In the United States, this cause is uncommon because of iodine added to table salt.

2. Hashimoto’s disease. Hashimoto’s disease is an autoimmune disorder, an illness caused by the immune system attacking healthy tissues. The damaged and inflamed tissues of the thyroid don’t produce enough hormones (hypothyroidism). When the pituitary gland detects the decline and prompts the thyroid to create more hormones, the thyroid can become enlarged.

3. Graves’ disease. Another autoimmune disorder called Graves’ disease occurs when the immune system produces a protein that mimics TSH. This rogue protein prompts the thyroid to overproduce hormones (hyperthyroidism) and can result in thyroid growth.

4. Thyroid nodules. A nodule is the irregular growth of thyroid cells that form a lump. A person may have one nodule or several nodules (multinodular goiter). The cause of nodules is not clear, but there may be multiple factors — genetics, diet, lifestyle and environment. Most thyroid nodules are noncancerous (benign).

5. Thyroid cancer. Thyroid cancer is less common than other cancers and generally treatable. About 5% of people with thyroid nodules are found to have cancer.

6. Pregnancy. A hormone produced during pregnancy, human chorionic gonadotropin (HCG), may cause the thyroid gland to be overactive and enlarge slightly.

7. Inflammation. Thyroiditis is inflammation of the thyroid caused by an autoimmune disorder, bacterial or viral infection, or medication. The inflammation may cause hyperthyroidism or hypothyroidism.

8. A lack of dietary iodine. Iodine is found primarily in seawater and in the soil in coastal areas. In the developing world in particular, people who don’t have enough iodine in their diets or access to food supplemented with iodine are at increased risk.

9. Being female. Women are more likely to develop a goiter or other thyroid disorders.

10. Pregnancy and menopause. Thyroid problems in women are more likely to occur during pregnancy and menopause.

11. Age. Goiters are more common after age 40.

12. Family medical history. Family medical history of goiters or other thyroid disorders increases the risk of goiters. Also, researchers have identified genetic factors that may be associated with an increased risk.

13. Medications. Some medical treatments, including the heart drug amiodarone (Pacerone) and the psychiatric drug lithium (Lithobid), increase your risk.

14. Radiation exposure. Your risk increases if you’ve had radiation treatments to your neck or chest area.