Clonidine is a drug that was initially developed to treat hypertension. Unlike the ACE inhibitor drugs that are also used for reducing blood pressure, clonidine works by a way of fooling the brain into suppressing the adrenal gland production of adrenaline and noradrenaline. The drug works by closing the negative feedback loop of the adrenal cortex. A negative feedback loop is a mechanism by which the brain sends signals to an organ to, perhaps, secrete a certain hormone, the elevated level or even the presence of this hormone then would be picked up by the brain as evidence that the initial signal worked. This is a closed negative feedback loop. If the feedback loop was broken, then the brain would continue to send signals to the organ.
Clonidine uses the negative feedback loop to reduce hypertension by simulating the abundance of adrenaline and noradrenaline. This causes the brain to shut down production of corticotropins, which cause the secretion of adrenaline and noradrenaline – thus, reducing heart rates and blood pressure. There is one condition in which this series of events may not happen and that is a cancer of the adrenal medulla called Pheochromocytoma. This is where the clonidine suppression test is used.
The clonidine suppression test for plasma levels of adrenaline and noradrenaline is done to confirm the presence of a pheochromocytoma. A normal adrenal medulla would react to the effect of clonidine and reduce the levels of the two hormones; however, in an abnormal medulla, the clonidine would have no effect and this indicates that there is in fact a tumor. There is another even simpler but dangerous test, which is to gently press the adrenal gland but this can suddenly elevate the levels of the hormones.
The clonidine suppression test is performed by taking a blood sample before the administration of clonidine. The clonidine is then administered and then three hours after the tests, the levels of adrenaline and noradrenaline are measured. No changes would indicate the presence of a pheochromocytoma.
There are no special preparations needed while undergoing this test and it is important that the patient is in as relaxed a state as possible. This is because adrenaline that is naturally secreted should not interfere with the measurements taken. It is also important that any corticosteroid medication that is currently being taken is avoided prior to the test.