What is being tested?This test measures the amount of metanephrines that are excreted in the urine over a 24-hour period. Metanephrines are the inactive
metabolites of the catecholamines epinephrine (adrenaline) and norepinephrine. Catecholamines are a group of similar hormones produced in the nervous system and in the medulla (central portion) of the adrenal glands. The adrenal glands are small, triangular organs located on top of each kidney. The primary catecholamines are dopamine, epinephrine (adrenaline), and norepinephrine. These hormones are released into the bloodstream in response to physical or emotional stress. They help transmit nerve impulses in the brain, increase glucose and fatty acid release (for energy), dilate bronchioles (small air passages in the lungs), and dilate the pupils. Norepinephrine also constricts blood vessels (increasing blood pressure) and epinephrine increases heart rate and
metabolism. After completing their actions, the hormones are metabolized to form inactive compounds. Dopamine becomes homovanillic acid (HVA), norepinephrine breaks down into normetanephrine and vanillylmandelic acid (VMA), and epinephrine becomes metanephrine and VMA. Both the hormones and their metabolites are excreted in the urine.
Urine metanephrine testing measures the amount of both metanephrine and normetanephrine. These metabolites are usually present in the urine in small fluctuating amounts that increase appreciably during and shortly after the body is exposed to a stressor. Pheochromocytomas and other neuroendocrine tumors, however, can produce large amounts of catecholamines, resulting in greatly increased concentrations of the hormones and their metabolites in both the blood and urine. The catecholamines that pheochromocytomas produce can cause persistent hypertension (high blood pressure) and/or bouts or episodes of severe hypertension. This can cause symptoms such as headaches, palpitations, sweating, nausea, anxiety, and tingling in the extremities.
About 90% of pheochromocytomas are located in the adrenal glands. While a few are cancerous, most are benign – they do not spread beyond their original location - although most do continue to grow. Left untreated, the symptoms may worsen as the tumor grows and, over a period of time, the hypertension that the pheochromocytoma causes may damage body organs, such as the kidneys and heart, and raise the risk of an affected patient having a stroke or heart attack.
The metanephrine test can be used to help detect the presence of pheochromocytomas. Although only about 800 cases a year are diagnosed in the U.S. according to the National Cancer Institute, it is important to diagnose and treat these rare tumors because they cause a potentially curable form of hypertension. In most cases, the tumors can be surgically removed and/or treated to significantly reduce the amount of catecholamine being produced and to reduce or eliminate associated symptoms and complications.
