Diagnosing acromegaly involves 2 key blood tests: a test to check the level of the insulin-like growth factor-1 (IGF-1) and an oral glucose tolerance test (OGTT).
Doctors can't simply test for the level of growth hormone (GH) in your body because the level varies so much in one day—even in someone without acromegaly. That means that a doctor could randomly test for GH and get a normal level in a person with acromegaly. However, because of GH fluctuations, the doctor could also test GH levels in someone without acromegaly and get a level far above normal.
Therefore, doctors rely on IGF-1 and OGTT tests to help diagnose acromegaly.
Testing Insulin-like Growth Factor-1 (IGF-1)
Insulin-like growth factor-1 (IGF-1) is a hormone that's closely tied to growth hormone. GH tells the body to make IGF-1, which in turn causes tissues in your body to grow. In someone without acromegaly, a high IGF-1 level is the body's signal to stop producing GH. For a person with acromegaly, though, the body continues producing GH, regardless of high IGF-1 levels.
IGF-1 levels are much more constant throughout the day than GH levels, so doctors measure IGF-1 levels using a blood test. An elevated IGF-1 level may indicate acromegaly.
Oral Glucose Tolerance Test (OGTT)
Growth hormone levels and blood glucose levels are also connected. In someone without acromegaly, a higher blood glucose level usually causes the body to stop producing GH. Therefore, a doctor will purposely raise your blood glucose level using an OGTT and watch how your GH level responds.
To begin the test, your doctor will measure your blood glucose level and your GH level. You'll then drink 75 g of a very sugary drink—a glucose drink. Over the course of the next 2 hours, the doctor will test your glucose and GH levels 4 more times: 30, 60, 90, and 120 minutes after you've had the glucose drink.
If your GH level doesn't drop to below 1 ng/mL (ng/mL is the standard measurement used for GH levels) during the OGTT, you have acromegaly. Your GH level should respond to so much glucose by dropping, so if it doesn't, that tells the doctor that your body isn't listening to its own signals: The delicate feedback loop of the endocrine system has been disturbed. (To learn more about the endocrine system feedback loop and how it regulates itself, you can read our endocrine system overview.
An OGTT is also used to diagnose diabetes.
Imaging Tests to Confirm Acromegaly
Since 95% of acromegaly cases are caused by a pituitary tumor, the doctor will most likely have a magnetic resonance imaging (MRI) scan done of your pituitary gland. The MRI should show the doctor exactly where the tumor is and how big it is.
If you have an MRI done and the doctor can't see a pituitary tumor, then you may be one of those rare cases of acromegaly caused by a non-pituitary tumor. In that case, the doctor may order a computerized tomography (CT) scan of possible tumor sites (abdomen and chest are common sites). Additionally, the doctor may test your growth hormone-releasing hormone (GHRH) levels since often, non-pituitary tumors causing acromegaly make GHRH (which then causes your pituitary gland to over-produce GH).
Diagnosing acromegaly involves a combination of factors: noting physical changes, testing levels of certain hormones in the blood, and confirming the presence and size of a tumor using imaging tests.