Hypothyroidism, as a drug side effect, is uncommonly complicated, because drugs may interfere at different steps in thyroid hormone homeostasis [31,32], including synthesis and secretion from the thyroid gland, thyroid hormone metabolism, absorption of thyroid hormone (especially in patients subject to pre-existed hypothyroidism), etc. While drug induced hypothyroidism is not observed in clinical trials very often, in several independent studies with relatively small samples, unexpected high frequencies of hypothyroidism are found among patients accepting sunitinib treatment [33–36]. This evidence concerns the gene TG and hypothyroidism.