Goiter is an enlargement of the thyroid gland caused most commonly by iodine deficiency in the diet. Iodine deficiency leads to low levels of thyroid hormone production, and a reduction in thyroid hormone negative feedback on the hypothalamus and the anterior pituitary leads to a compensatory rise in the thyroid stimulating hormone (TSH). Enlargement of the thyroid gland is, therefore, caused by abnormally high circulating levels of TSH, which has a strong trophic effect on the thyroid gland.