GH levels are measured following overnight fasting. However, a single measurement of elevated GH does not confirm a diagnosis of acromegaly, because growth hormone secretion from the pituitary occurs in spurts, and the concentration of GH in the blood may vary greatly at different times. In addition, a patient with acromegaly may have a normal GH level at any time.

A more accurate measurement of GH can be obtained under conditions in which GH is normally suppressed. An oral glucose tolerance test is recommended to diagnose acromegaly. In healthy people, ingestion of 75 g of sugar glucose lowers the level of GH in the blood below 1 µg/L. This reduction does not occur in patients with acromegaly. According to the most recent concensus conference on acromegaly, a glucose tolerance test is the most reliable method for confirming a diagnosis of acromegaly.²

IGF-I is a hormone that causes bone and tissue growth; it is produced in the liver under the influence of GH. IGF-I signals the pituitary to reduce GH production. However, if GH production continues despite negative feedback mechanisms in the body, the level of IGF-I in the blood will also increase, leading to bone growth and organ enlargement.

IGF-I levels are much more stable over the course of the day than GH levels, and elevation of this hormone almost always indicates the presence of acromegaly. However, it should be noted that the level of IGF-I in the blood is 2 to 3 times higher than normal in pregnant women, can be abnormally low in people with poorly controlled diabetes, and is lower than normal in older people.

After a diagnosis of acromegaly has been made, imaging techniques (eg, CT or MRI scans) are used to locate the pituitary tumour causing GH overproduction. If a pituitary tumour cannot be seen using these imaging studies, a non-pituitary tumour of the chest, abdomen, or pelvis may be the cause of GH overproduction. Measurement of growth-hormone releasing hormone (GHRH), which stimulates the pituitary to make GH, along with CT scans of possible tumour sites, can be used to diagnose non-pituitary tumours.

MRI scans of pituitary tumours causing acromegaly. Right-sided microadenoma, coronal section (a). Invasive macroadenoma extending into the cavernous sinus and suprasellar region, coronal section (b). Macroadenoma with suprasella extension, sagittal section (c). Wass J (ed). Handbook of Acromegaly. UK: BioScientifica Ltd, p. 27, 2001.