While most circulating metanephrines are derived directly from adrenal secretion, peripheral conversion of catecholamines makes a small contribution. Therefore, substances that increase endogenous catecholamine levels can result in borderline elevations of plasma metanephrines. These include monamine oxidase inhibitors (MOIs), catecholamine reuptake inhibitors, and some anesthetic gases, particularly halothane. Artifactually decreased plasma metanephrine levels may be observed when patients are already receiving metyrosine treatment.