Oral Glucose GH Suppression Test

The test exploits a normal physiological reflex: a rise in blood glucose suppresses growth hormone secretion, largely by increasing hypothalamic somatostatin tone. A standard 75 gram oral glucose load is given fasting and growth hormone sampled over the next two hours. A healthy axis drives the nadir below the assay-specific cut-off; an autonomously secreting somatotroph adenoma does not, and roughly a quarter to a third of patients with acromegaly show a paradoxical rise instead of any fall.

The cut-off has moved with the assays. Older polyclonal radioimmunoassays supported a threshold near 1 microgram per litre; current sensitive monoclonal methods read lower, and guidelines have shifted the nadir toward roughly 0.4 micrograms per litre, the exact value tied to the method. This is a case where quoting a number without its assay makes the result uninterpretable, and where a laboratory changing platforms can appear to change a patient's disease status.

The test earns its place because it distinguishes autonomy from a high-normal value. IGF-1 integrates exposure but is affected by nutrition, oestrogen, liver disease and diabetes; the suppression test asks the narrower and more specific question of whether the somatotroph still obeys its regulator. It is the confirmatory step when IGF-1 is elevated or equivocal, and it is used again after surgery to judge remission.

False positives are the practical problem, and diabetes is the main source: poorly controlled hyperglycaemia and marked insulin resistance blunt suppression in people without acromegaly, so a failed test in that setting is weak evidence. Adolescence, pregnancy, hepatic and renal impairment, and malnutrition also impair suppression. In the other direction, a suppressed nadir does not exclude acromegaly when IGF-1 is clearly and repeatedly high.