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Acromegaly
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How is Acromegaly Diagnosed?
Blood tests
If acromegaly is suspected, a doctor must measure the growth hormone (GH) level in a person's blood to determine if it is elevated. However, a single measurement of an elevated blood growth hormone (GH) level is not enough to diagnose acromegaly: Because growth hormone (GH) is secreted by the pituitary in impulses, or spurts, its concentration in the blood can vary widely from minute to minute. At a given moment, a person with acromegaly may have a normal growth hormone (GH) level, whereas a growth hormone (GH) level in a healthy person may even be five times higher.
More accurate information is obtained when growth hormone (GH) is measured under conditions that normally suppress growth hormone (GH) secretion. Healthcare professionals often use the oral glucose tolerance test to diagnose acromegaly because drinking 75 to 100 grams of glucose solution lowers blood growth hormone (GH) levels to less than 1 nanogram per milliliter (ng/ml) in healthy people. In people with growth hormone (GH) overproduction, this suppression does not occur. The oral glucose tolerance test is a highly reliable method for confirming a diagnosis of acromegaly.
Physicians also can measure insulin-like growth factor I (IGF-I) levels, which increase as growth hormone (GH) levels go up, in people with suspected acromegaly. Because insulin-like growth factor I (IGF-I) levels are much more stable than growth hormone (GH) levels over the course of the day, they are often a more practical and reliable screening measure. Elevated insulin-like growth factor I (IGF-I) levels almost always indicate acromegaly. However, a pregnant woman's insulin-like growth factor I (IGF-I) levels are two to three times higher than normal. In addition, physicians must be aware that insulin-like growth factor I (IGF-I) levels decline with age and may also be abnormally low in people with poorly controlled diabetes or liver or kidney disease.
Imaging
After acromegaly has been diagnosed by measuring growth hormone (GH) or insulin-like growth factor I (IGF-I) levels, a magnetic resonance imaging (MRI) scan of the pituitary is used to locate and detect the size of the tumor causing growth hormone (GH) verproduction. Magnetic resonance imaging (MRI) is the most sensitive imaging technique, but computerized tomography (CT) scans can be used if the patient should not have magnetic resonance imaging (MRI). For example, people who have pacemakers or other types of implants containing metal should not have an magnetic resonance imaging (MRI) scan because magnetic resonance imaging (MRI) machines contain powerful magnets.
If a head scan fails to detect a pituitary tumor, the physician should look for nonpituitary "ectopic" tumors in the chest, abdomen, or pelvis as the cause of excess growth hormone (GH). The presence of such tumors usually can be diagnosed by measuring growth hormone-releasing hormone (GHRH) in the blood and by a computerized tomography (CT) scan of possible tumor sites.
Rarely, a pituitary tumor secreting growth hormone (GH) may be too tiny to detect even with a sensitive magnetic resonance imaging (MRI) scan.
References:
National Endocrine and Metabolic Diseases Information Service
NIH Publication No. 07-3924
April 2007
www.endocrine.niddk.nih.gov/pubs/acro/acro.htm
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