Hypothyroidism is classified as primary when it is caused by failure of the stimulated thyroid gland to release adequate amounts of thyroxine and triiodothyronine, secondary when it is caused by inadequate pituitary secretion of thyrotropin (thyroid-stimulating hormone), and tertiary when it is due to insufficient production of thyrotropin-releasing hormone by the hypothalamus. Primary hypothyroidism is much more common than the other 2 forms. In the United States, primary hypothyroidism is most often the result of chronic autoimmune thyroiditis or of surgical or radioiodine ablation of an overactive thyroid gland (Kaplan 1999; Tews et al 2005; Devdhar et al 2007). Other causes include iodine deficiency, iodine excess, congenital conditions, and drugs or environmental chemicals that impair hormone biosynthesis, such as lithium, amiodarone, sulfonamides, sulfonylureas, carbamazepine, oxcarbazepine, phenytoin, valproate, interleukins, sunitinib and other tyrosine kinase inhibitors, propylthiouracil, methimazole , polychlorinated biphenyls, bisphenol-A, and polybrominated diphenyl esters (Simko et al 2004; Vainionpaa et al 2004; Cooper 2005; Tews et al 2005; Devdhar et al 2007; Zoeller 2007; Biondi and Cooper 2008; Laurberg 2009; Lossius et al 2009; Hamnvik et al 2011). Antineoplastic agents can cause hypothyroidism by a variety of mechanisms (Hamnvik et al 2011). Secondary and tertiary hypothyroidism can result from tumors, granulomatous disease, irradiation, surgery, hemorrhage, and infarction.