Although Tg is routinely measured as a marker for thyroid neoplasia and an adjunct in the diagnosis of thyrotoxicosis due to destructive thyroiditis, its clinical significance under conditions of abnormal thyroid function remains unclear.<h4>Methods</h4>We investigated the association between serum Tg levels and thyroid function in 292 patients who underwent simultaneous evaluation of serum Tg, free thyroxine (FT4), and free triiodothyronine (FT3) levels in routine clinical practice. This evidence concerns the gene TG and thyrotoxicosis.