Old test. Previous to MRI and good chemical test to measure catecholamine levels, the Clonidine suppression test was done to look for a rare tumor called pheochomocytoma. Pheochromocytomas release catecholamines like Epinephrine that raise heart rate and bp. The Clonidine test blood levels of catecholamines measured before and after giving clonidine. If there was no change, that was positive for pheo.
Pheochromocytoma. Normally, a dose of Clonidine will suppress production of catecholamines (like norepinephrine) but if a catecholamine secreting tumor is present, Clonidine will fail to suppress it. Thus the Clonidine suppression test is used to diagnose pheochromocytoma (a catecholamine secreting tumor).