Patients with Allan-Herndon-Dudley syndrome (AHDS) exhibit characteristic serum TH abnormalities (high triiodothyronine [T3], low thyroxine [T4], and reverse T3, with normal or slightly elevated thyrotropin) accompanied by severe and irreversible neurological deficits (7), presumably due to reduced TH availability to neural cells. Here, TH is linked to Allan-Herndon-Dudley syndrome.