Diagnosing Pituitary Tumors
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A thorough medical history and physical exam provides important clues for your physician to determine the cause of your symptoms. However, additional tests are necessary to confirm or rule out a specific diagnosis.
If a pituitary tumor, also known as a pituitary adenoma, is a possibility, a variety of blood and urine tests may be performed. Additionally, an MRI may be ordered to visualize the pituitary gland. These tests can be overwhelming but are critical to establishing the diagnosis and starting appropriate treatment. This discussion aims to explain why physicians order certain tests and images if a pituitary tumor is suspected.
Pituitary tumors can secrete large amounts of hormones into the blood. Depending on the type of pituitary tumor, different hormones such as prolactin (prolactinoma), growth hormone (acromegaly), adrenocorticotropic hormone (ACTH) (Cushing’s disease) or thyroid-stimulating hormone may be released. Thus, checking the blood for abnormal levels of these hormones can suggest the presence of a pituitary tumor, and provide information on the type of pituitary tumor that exists.
Venous Blood Sampling
In patients with elevated ACTH levels but a normal MRI scan, additional tests may be necessary to confirm the diagnosis. Inferior petrosal sinus sampling is a procedure to measure the level of ACTH in the veins that drain blood directly from the pituitary gland before and after administration of corticotropin-stimulating hormone (CSH). This can then be compared with the levels of ACTH in other areas of the body such as the arm or leg.
If an ACTH-secreting pituitary tumor is present, CSH will stimulate the tumor to release ACTH. This ACTH will be much more concentrated in the blood vessels near the pituitary gland than in the peripheral blood vessels of the arm or leg. A large difference in these ACTH levels suggests the presence of a pituitary tumor. During the procedure, a thin flexible tube will be placed in the veins of the arms or thighs and guided up to the veins by the pituitary gland. Patients will be sedated during this test but can go home the same day.
While some pituitary tumors produce hormones, others may produce no hormones at all but still cause problems by being too large and compressing the surrounding pituitary tissue such as the optic (vision) nerves. In rare cases, this can damage a part of the pituitary gland that stores the hormone vasopressin, also known as anti-diuretic hormone (ADH). A deficiency of ADH can cause you to make large amounts of dilute urine in a condition called diabetes insipidus.
Diabetes insipidus more commonly occurs after a surgical procedure to remove the pituitary tumor, and usually will resolve on its own. To test for diabetes insipidus, physicians may conduct urine tests to check the amount of urine produced over 24 hours, and how concentrated the urine is. If the results are unclear, physicians may perform a water deprivation study, when you are not allowed to drink water for several hours. If you are still producing large amounts of urine, this is an abnormal result that suggests a lack of ADH to retain water.
Physicians may also check the urine for the presence of elevated hormone levels. This is usually performed if the presence of an ACTH-secreting pituitary tumor is suspected. The release of ACTH increases the levels of the “stress hormone” cortisol in the body and urine. Urine may be collected over several days to ensure a more accurate result.
Just as cortisol can be measured in the urine to test for an ACTH-secreting pituitary tumor, it can also be measured in the saliva. The amount of cortisol in the patient’s saliva will be measured at night because this is the time when cortisol levels are normally low. If they are high at night, it could indicate the presence of an ACTH-secreting pituitary tumor.
Pituitary tumors, especially large ones, can compress nearby structures in the brain, leading to symptoms like headaches and vision disturbances. If a patient is experiencing these symptoms, a physician may perform visual field tests to determine the location of the visual deficits. A large pituitary tumor can cause a specific pattern of visual loss that makes it difficult to see items on the periphery (bitemporal hemianopia). These tests can be easily performed in the office, sometimes by eye specialists known as neuro-ophthalmologists.
Every patient with a suspected pituitary tumor will receive some sort of imaging test. The most common is magnetic resonance imaging (MRI) followed by computed tomography (CT) scan and positron emission tomography (PET) scan.
- MRI: An MRI can create a detailed 3D picture of the brain and pituitary gland. MRIs are often ordered with contrast to better visualize the pituitary tumor if one is present. During a pituitary MRI scan, patients can expect to lie on their back while in the scanning machine for about one hour.
- CT: A CT scan uses X-rays to obtain an image of the pituitary gland. Although it is faster to obtain a CT than an MRI, the CT image is less detailed. It may be difficult to identify a pituitary tumor on a CT unless it is particularly large.
- PET: A PET scan uses a radiotracer to identify metabolic activity in the brain and may be used in combination with an MRI to identify a pituitary tumor. This type of imaging may be used when the pituitary tumor is too small to be visualized by MRI alone.
Blood tests combined with imaging provide sufficient information to detect pituitary tumors, so it is often unnecessary to perform a tissue biopsy. However, a biopsy of the tumor is the only way to be completely certain about the diagnosis, so rarely some physicians order one before starting treatment.
Other physicians forgo a biopsy because it does not provide additional information. If surgery is part of the treatment plan, tumor that is removed is viewed under the microscope to confirm the type of pituitary tumor and check for any rare features of malignancy. If surgery is not one of the treatment options, a biopsy is usually not pursued.
Understanding why a physician orders specific tests and images when diagnosing a pituitary tumor can decrease patients’ anxiety throughout the process and empower them as a treatment plan is pursued.
- Blood and urine tests are common ways to detect hormone excess or deficiencies due to potential pituitary tumors.
- MRI is the most commonly performed imaging test to detect pituitary tumors, although very small pituitary tumors can be missed.
- Biopsies are rarely necessary to move forward with treatment of pituitary tumors.