A goiter is an abnormal swelling in the neck caused by an enlarged thyroid gland. It can become quite large. The problem occurs in at least 5% of the population worldwide. The most common cause of a goiter is lack of iodine, a chemical which the thyroid uses to produce its hormones. About 100 million people don’t get enough iodine in their diets, but the problem has been solved in North America and Western Europe by adding iodine to salt. Even with the right amount of iodine, the thyroid gland can swell, creating a goiter. This can occur in any type of thyroid disease, including hyperthyroidism, hypothyroidism, thyroiditis, and thyroid cancer. However, many goiters develop with normal thyroid hormone levels and do not require treatment.