If a doctor suspects acromegaly, he or she can measure the GH level in the blood after a patient has fasted overnight to determine if it is elevated. However, a single measurement of an elevated blood GH level is not enough to diagnose acromegaly, because GH is secreted by the pituitary in spurts and its concentration in the blood can vary widely from minute to minute. At a given moment, a patient with acromegaly may have a normal GH level, whereas a GH level in a healthy person may be five times higher.
Because of these problems, more accurate information can be obtained when GH is measured under conditions in which GH secretion is normally suppressed. Physicians often use the oral glucose tolerance test to diagnose acromegaly, because ingestion of 75 g of the sugar glucose lowers blood GH levels less than 2 ng/ml in healthy people. In patients with GH overproduction, this reduction does not occur. The glucose tolerance test is the most reliable method of confirming a diagnosis of acromegaly.
Physicians also can measure IGF-1 levels in patients with suspected acromegaly. As mentioned earlier, elevated GH levels increase IGF-1 blood levels. Because IGF-1 levels are much more stable over the course of the day, they are often a more practical and reliable measure than GH levels. Elevated IGF-1 levels almost always indicate acromegaly. However, a pregnant woman's IGF-1 levels are two to three times higher than normal. In addition, physicians must be aware that IGF-1 levels decline in aging people and may be abnormally low in patients with poorly controlled diabetes mellitus.
After acromegaly has been diagnosed by measuring GH or IGF-1, imaging techniques, such as computed tomography (CT) scans or magnetic resonance imaging (MRI) scans of the pituitary are used to locate the tumor that causes the GH overproduction. Both techniques are excellent tools to visualize a tumor without surgery. If scans fail to detect a pituitary tumor, the physician should look for non-pituitary tumors in the chest, abdomen, or pelvis as the cause for excess GH. The presence of such tumors usually can be diagnosed by measuring GHRH in the blood and by a CT scan of possible tumor sites.
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