GH Stimulation Test

Growth hormone is released in discrete bursts separated by hours of near-undetectable trough, so a random sample carries almost no diagnostic information; a healthy adult sampled between pulses looks identical to someone with severe deficiency. Provocative testing applies a standardised stimulus and samples repeatedly over one to three hours, taking the highest value reached as the measure of somatotroph reserve. The stimuli act by different routes, which is why they are not interchangeable: hypoglycaemia as a central stress signal, arginine by suppressing somatostatin tone, macimorelin by agonism at the ghrelin receptor.

The insulin tolerance test remains the reference standard, taking a peak below roughly 3 micrograms per litre as severe adult deficiency, but it is contraindicated in seizure disorder and ischaemic heart disease. The glucagon test is the usual alternative. Macimorelin, given orally, was approved by the FDA in 2017 with its own validated threshold. Paediatric practice has historically used higher cut-offs and two separate stimuli, and adiposity blunts the response enough to need body-mass-adjusted thresholds.

The decision the test supports is narrow but firm: it establishes whether the pituitary can secrete on demand. It does not measure spontaneous daily output.

The recurring error is transplanting a cut-off across tests or assays. A peak of 4 micrograms per litre means one thing after insulin and something else after macimorelin, and modern monoclonal assays calibrated to the recombinant international standard read materially lower than the older polyclonal ones that generated the historical thresholds. Wellness clinics compound this by running unvalidated response checks after a secretagogue and reporting the number against a threshold derived from a different provocation entirely.