What tests are needed specifically to diagnose Cushing’s disease?
ACTH measurements
Patients with adrenal causes of Cushing’s syndrome have low blood ACTH levels and patients with the other causes of Cushing’s syndrome have normal or
high levels. A doctor can diagnose too much ACTH by measuring its level in
the blood.
Inferior petrosal sinus sampling (IPSS)
The best test to distinguish an ACTH-producing tumor on the pituitary from
one in another part of the body is a procedure called inferior petrosal sinus
sampling or IPSS. This involves inserting small plastic tubes into both the
right- and left-sided veins in the groin (or neck) and threading them up to the
veins near the pituitary gland. Blood is then taken from these locations and also
from a vein not connected to the pituitary gland.
During the procedure, a medication that increases ACTH levels from the
pituitary is injected. By comparing the levels of ACTH produced close to the
pituitary gland in response to the medication with those produced by other
parts of the body, a diagnosis can be made.
Further tests
There are other tests used for the diagnosis of Cushing’s disease such as the
dexamethasone suppression and corticotropin-releasing hormone (CRH)
stimulation tests. However, these are not as reliable in distinguishing between
the causes as IPSS. A doctor may want to do multiple tests to confirm the results.
It is also possible to visualize the pituitary gland using a process called magnetic
resonance imaging (MRI). This involves an injection of an agent that will
help the tumor to show up on the MRI scan. If this test shows a definite
tumor above a certain size and the CRH and dexamethasone test results are
compatible with Cushing’s disease, IPSS may not be needed. However, up to
10% of healthy people have an abnormal area on their pituitary consistent with
a tumor. Therefore, the presence of an abnormality alone is not diagnostic of
Cushing’s disease. Also, in about 50% of patients with Cushing’s disease, the
tumor is too small to be seen. Thus the absence of a tumor on a MRI scan does
not necessarily exclude Cushing’s disease.
