This chapter discusses thyroid cancer, where fluctuations in the levels of TSH because of dietary deficiency of iodine may cause large goiters. Prolonged TSH stimulation may be a factor in causing the abnormal gland to undergo malignant change. Glutathione-S transferase polymorphisms are now being associated with higher risk of developing differentiated thyroid cancers (DTCs). Familial thyroid cancer has been frequently reported in the literature as part of a more complex hereditable disease and rarely as an independent familial syndrome. A diagnosis of papillary cancer is based on the nuclear morphology of a thyroid neoplasm, where follicular carcinomas have histologic features that overlap with follicular adenoma, making diagnosis difficult. Follicular carcinoma can broadly be divided into minimally invasive, widely invasive and Hurthle cell variant.