Enlargement of the thyroid gland is due to increased secretion of thyroid-stimulating hormone, which is usually secondary to diminished output of thyroid hormones. The thyroid undergoes periods of activity and regression, which can lead to the formation of nodules, some of which are solid and some of which are partially cystic (colloid cysts). This nodule formation is compounded by areas of fibrosis within the gland. Other causes of multinodular goiter include iodine deficiency and in certain circumstances, drugs that interfere with the metabolism and production of thyroxine. The typical symptom of a goiter is a painless swelling of the thyroid gland. It may be smooth or nodular, and occasionally it may extend into the superior mediastinum as a retrosternal goiter.