Notes: Metanephrine-d3 is intended for use as an internal standard for the quantification of metanephrine (Item No. 15408) by GC- or LC-MS. Metanephrine is an inactive metabolite of epinephrine (Item No. 21245).{54827} It is formed from epinephrine by catechol-O-methyl transferase (COMT).{25427} Urinary and plasma levels of metanephrine are increased in patients with pheochromocytoma, an adrenal medullary neuroendocrine tumor.{5331,32483}