Methoxynorepinephrine is a catecholamine hormone that is usually released from sympathetic nerve endings, acts on alpha receptors, and has vasoconstrictive effects. Methoxynorepinephrine can be detected by urine specimen and blood specimen, usually plasma free methoxynorepinephrine concentration, the normal reference value range is 0.6-0.9 nmol/L, and urinary methoxynorepinephrine is 3.0-3.8 μmol/L. Elevated methoxynorepinephrine indexes are most commonly seen in pheochromocytoma. Pheochromocytoma of the adrenal medulla produces norepinephrine and epinephrine, and methoxynorepinephrine is a product of norepinephrine and epinephrine. As a result, pheochromocytomas can present with high levels of methoxynorepinephrine, which is usually measured at more than two times the normal high limit. In contrast, a decreased methoxynorepinephrine index is most often seen as a result of abnormal neurologic function. Because the sympathetic nerve excitability is reduced or dysfunctional, it will lead to lower norepinephrine in the blood. For example, hypothyroidism decreases the basal metabolic rate, the sympathetic nerves are not excited, and their production of catecholamine hormones decreases, resulting in low methoxynorepinephrine.