Secondly, we analyzed circulating FGF21 in patients with Graves’ disease that were treated with radioiodine (RAI), in three different thyroid function states, namely hyperthyroidism at clinic referral, hypothyroidism induced by RAI treatment that developed abruptly within a six week interval between clinic visits, and euthyroidism after l-thyroxine treatment. This evidence concerns the gene FGF21 and hypothyroidism.