Thyroid & Adrenal
The following procedure is most useful in establ-ishing the diagnosis of pheochr-omocytoma:
|A||Regitine (phentolamine) test|
|D||Urinary catecholamine measurements|
a. Pheochromocytomas are diagnosed by testing 24-hour urine samples for catecholamines and their metabolites, as well as by determining plasma metanephrine levels.
b. Urinary metanephrines are 98% sensitive and are also highly specific for pheochromocytomas, whereas vanillylmandelic acid (VMA) measurements are slightly less sensitive and specific.
c. False-positive VMA tests may result from ingestion of caffeine, raw fruits or medications (alpha-methyl dopa).
d. Fractionated urinary catecholamines (norepinephrine, epinephrine, and dopamine) are also sensitive but less specific for pheochromocytomas.
e. Both epinephrine and norepinephrine should be measured because tumors often secrete one or the other hormone.
f. Sensitivities of 85% and specificities of 95% have been reported using cutoff values of 2000 pg/mL for norepinephrine and 200 pg/mL for epinephrine.
g. A threefold increase or stimulated level of greater than 2000 pg/mL 1 to 3 minutes after the administration of 1 to 2 mg of glucagon intravenously suggests a pheochromocytoma, with an 80% sensitivity and 100% specificity.
h. Clonidine is an agent that suppresses neurogenically mediated catecholamine excess but not secretion from pheochromocytomas.
i. Chromogranin A is a monomeric, acidic protein that is stored in the adrenal medulla and released along with catecholamine hormones.
j. Plasma metanephrines are the most reliable tests to identify pheochromocytomas, with sensitivity approaching 100%