Clonidine-suppression test in epinephrine secreting pheochromocytoma: report of a case.

A patient is described in whom the diagnosis of pheochromocytoma was suspected on clinical grounds. He had normal urinary and plasma norepinephrine levels, with normal suppression of plasma norepinephrine by clonidine. However, plasma and urinary epinephrine levels were above normal, and plasma epinephrine was not suppressed but increased by clonidine. Selective venous sampling showed marked unilateral adrenal epinephrine excess. A large epinephrine secreting pheochromocytoma was surgically removed. This case suggests that, in pheochromocytomas releasing predominantly epinephrine, normal norepinephrine release from axon terminals of sympathetic postganglionic neurons is maintained and is suppressed by clonidine administration. It further reaffirms the value of plasma epinephrine estimations in the diagnosis of pheochromocytoma and indicates that normal suppression of plasma norepinephrine by clonidine does not preclude the presence of a predominantly epinephrine secreting tumor.