A goiter is an enlarged thyroid gland, which is a butterfly-shaped gland in the front of the neck. A goiter often causes a noticeable swelling in the neck.

A goiter can be caused by a lack of iodine in the diet, a tumour or nodule on the gland, a thyroid disease, or, in rare cases, cancer. In many parts of the world, goiters are most common in people who have hypothyroidism, in which the thyroid gland does not produce enough thyroid hormone, related to a low-iodine diet. In Canada, most people with goiters have chronic autoimmune thyroiditis (Hashimoto's thyroiditis).

An enlarged thyroid may produce normal amounts of thyroid hormone, or it may produce less- or greater-than-normal amounts. Treatment for a goiter depends on what is causing it.