Mild TPO deficiency, which causes a partial IOD (PIOD), is an autosomal recessive disorder characterized by late-onset goiter, elevated free triiodothyronine (FT3)/free thyroxine (FT4) ratio, normal or slightly elevated thyroid stimulating hormone (TSH), and increased thyroglobulin (Tg) (2). This evidence concerns the gene TG and hyperinsulinemic hypoglycemia, familial, 4.