Some potential causes of residual symptoms in patients with normal thyrotropin values have been proposed, such as the presence of symptoms caused by accompanying conditions or comorbidities, incorrect attribution and unrealistic patient expectations, awareness of chronic condition, tissue hypothyroidism (low T3 levels at the tissue or cellular level), and alterations in the conversion of T4 to T3 or in the entry of thyroid hormone into cells [19]. This evidence concerns the gene TG and hypothyroidism.