Very small, functional pituitary tumors that produce excessive amounts of adrenocorticotropic hormone (ACTH) can be difficult to detect on an MRI scan. Sometimes ACTH levels may be high due to a tumor located somewhere else in the body. For these reasons, doctors may order a pituitary venous blood sampling test, also called petrosal sinus sampling.
For this test, Neuro Interventional Radiologists place tiny catheters in the veins of the inner thighs or arms. The physician then guides the catheter(s) up to an area at the base of the brain that holds two veins that drain blood from the pituitary gland. A blood sample is taken from the pituitary veins and compared to a sample taken from a vein in the arm. If the pituitary blood sample has comparatively high levels of ACTH, it is likely that a person has a pituitary tumor. If blood taken from a vein in the arm has a high level, then a tumor elsewhere in the body may be the problem. Sometimes a drug called corticotrophin releasing hormone (CRH) is injected to exaggerate the difference between blood draining the pituitary and the arm blood sample. This procedure is performed using mild sedation, and people can usually go home the same day.