Метанефрин

Free metanephrine and free normetanephrine in blood supports the evaluation of suspected pheochromocytoma and related catecholamine-secreting neuroendocrine tumors. Measurement of plasma free metanephrines helps confirm biochemical excess and is also used to monitor response after therapy or tumor recurrence. This assay, often referred to as plasma free metanephrines or fractionated plasma metanephrines, is ordered when characteristic clinical features are present and to assess treatment effectiveness over time.

Metanephrine and normetanephrine are O-methylated metabolites formed from epinephrine and norepinephrine, respectively. Catecholamines are synthesized in the adrenal glands, are released with physical exertion or emotional stress, and modulate neurotransmission, energy substrate mobilization, bronchodilation, and pupillary dilation. Norepinephrine promotes vasoconstriction with blood pressure elevation, while epinephrine increases heart rate and overall metabolic activity. After exerting their effects, dopamine is converted to homovanillic acid, norepinephrine to normetanephrine and vanillylmandelic acid, and epinephrine to metanephrine and vanillylmandelic acid; modern platforms distinguish free from conjugated forms in circulation. Compared with direct catecholamine measurement, which is limited by rapid in vivo degradation, quantifying plasma free metanephrines provides more stable biochemical evidence of catecholamine production. Low concentrations are present in health and may rise with or after stress. Pheochromocytomas and other neuroendocrine tumors can markedly increase catecholamine output, leading to elevated metanephrine and normetanephrine and resulting in sustained or episodic hypertension. Associated manifestations include headache, diaphoresis, nausea, anxiety, and paresthesias. Most pheochromocytomas arise within the adrenal glands and are typically benign but slow growing. Without treatment, progressive catecholamine excess can worsen hypertension and contribute to end-organ injury, including renal and cardiac damage, with increased risks of myocardial infarction and stroke. Early identification enables surgical removal and/or medical therapy to reduce catecholamine levels, alleviate symptoms, and limit complications. Testing generally includes both free metanephrine and free normetanephrine to improve diagnostic yield.