Colloid nodular goiter is thyroid gland enlargement that is usually caused by too little iodine in the diet.

When the thyroid gland is unable to make enough thyroid hormone, it may attempt to compensate by enlarging. Enlargement of an otherwise normal thyroid gland is called colloid nodular goiter.

A colloid nodular goiter occurs when the thyroid gland is unable to meet the metabolic demands of the body with sufficient hormone production. The thyroid gland compensates by enlarging, which usually overcomes mild deficiencies of thyroid hormone.

If the thyroid gland is then re-exposed to iodine, the nodules may produce thyroid hormone independently. Occasionally, the nodules may produce too much thyroid hormone, causing thyrotoxicosis. This is called a toxic nodular goiter.

Colloid nodular goiters are also known as endemic goiters and are usually caused by inadequate iodine in diet. They tend to occur in certain geographical areas with iodine-depleted soil, usually areas away from the sea coast. An area is defined as endemic for goiter if more than 10 % of children aged 6 to 12 years have goiters.

In the U.S., the Great Lakes, Midwest, and Intermountain regions were once known as the "goiter belt." The use of iodized table salt now usually prevents this deficiency. As a result, the incidence of colloid nodular goiters is now rare in the U.S.

Risk factors are being female, being older than 40, having an inadequate dietary intake of iodine, living in an endemic area, and having a family history of goiters.