Most writing about human growth hormone starts at the injection. This post starts one step earlier — at the test that decides whether anyone gets an injection at all. A new real-world analysis published in Pituitary in 2026 looked at how the macimorelin stimulation test actually performs outside a trial, and found that two defensible cut-offs can label the same patient differently, with body weight sitting right in the middle of the disagreement. If you've searched "what is HGH" or "HGH Australia" and landed on lean-mass claims, this is the less glamorous and far more consequential part of the story: growth hormone deficiency is a diagnosis of exclusion built on a provocation test, and the test has more wobble in it than the marketing suggests.

For background on the compound itself, see our somatropin profile and the wider peptides library.

What Is HGH, and Why Diagnosis Is the Hard Part

Somatropin is recombinant human growth hormone — a 191-amino-acid protein identical to the pituitary hormone. It is TGA-approved and PBS-subsidised in Australia for defined conditions, including severe adult growth hormone deficiency (AGHD).

The diagnostic problem is structural. Growth hormone is secreted in pulses, so a single random blood level tells you almost nothing. Clinicians instead provoke the pituitary with a stimulus and measure the peak response. The historical reference standard is the insulin tolerance test (ITT), which works by inducing hypoglycaemia — effective, but unpleasant, labour-intensive, and contraindicated in patients with seizure disorders or ischaemic heart disease. As Endotext's review of GH stimulation testing notes, safety concerns have confined the ITT to a handful of experienced centres in some countries, which is exactly why alternatives matter.

Macimorelin is one of those alternatives: an orally active ghrelin receptor agonist that stimulates GH release without hypoglycaemia. It was approved by the FDA in December 2017 for diagnosing adult GHD, with EMA authorisation following in 2019.

The 2026 Real-World Finding: Two Cut-Offs, Two Answers

The Pituitary analysis by Yadav, Hamrahian and Salvatori is a retrospective, single-centre study of adults who underwent macimorelin testing, stratified by pre-test probability of AGHD (high, intermediate, low). It set out to answer three practical questions the registration trials left open.

Which cut-off is right?

The FDA-approved peak GH threshold is 2.8 ng/mL. A post-hoc analysis of the validation data proposed 5.1 ng/mL instead. These are not cosmetic differences — applying 5.1 ng/mL classified more patients as deficient than 2.8 ng/mL. The disagreements clustered in the low pre-test probability group, which is precisely the population where a false positive is most likely to lead to unnecessary lifelong hormone replacement.

Does body weight change the answer?

Yes, and substantially. In the low pre-test probability group, BMI showed an inverse association with peak GH (Spearman ρ = −0.67, p = 0.006). Patients whose classification flipped between the two cut-offs had a mean BMI of 37.7 ± 2.7 kg/m² versus 28.2 ± 4.6 kg/m² in those whose classification agreed — a gap of roughly 9.5 kg/m². The authors' reading is that obesity blunts stimulated GH release and may therefore drive diagnostic reclassification rather than reflecting true pituitary failure.

Can the protocol be shortened?

GH values at 60 and 90 minutes were highly correlated (r = 0.775, p < 0.0001), and dropping the 90-minute sample did not change any diagnostic outcome in this cohort. That is a small but genuine efficiency finding for busy endocrine day units.

Stated plainly: this is a single-centre retrospective chart review, not a prospective validation study, and it has not been replicated. It generates hypotheses about BMI adjustment and protocol length; it does not settle them. Anyone citing the 9.5 kg/m² figure as an established correction factor is overreading it.

Is HGH Legal in Australia, and How Is Deficiency Actually Confirmed Here?

Somatropin is a Schedule 4 (Prescription Only) medicine in Australia and is border-controlled. Importing or possessing it without a valid prescription is illegal, and it is prohibited at all times in sport under the WADA List. Legal access runs through a specialist, not a website.

The Australian diagnostic gate is also more specific than most people expect — and notably, it does not currently name macimorelin. According to the PBS authority criteria circulated by the Endocrine Society of Australia, an adult seeking PBS-subsidised somatropin for severe GHD must be 18 or over, be treated by an endocrinologist, and meet one of three biochemical thresholds:

  • Insulin tolerance test with peak serum GH < 2.5 µg/L, or
  • Arginine infusion test with peak serum GH < 0.4 µg/L, or
  • Glucagon provocation test with peak serum GH < 3 µg/L

On top of the biochemistry, applicants must score 16 or higher on the QoL-AGHDA quality-of-life instrument, with that score and a baseline IGF-1 no more than 12 weeks old at the time of application. Australia, in other words, requires both a failed pituitary and documented symptom burden.

Two things are worth noticing. First, ng/mL and µg/L are numerically equivalent — so Australia's ITT threshold of 2.5 sits below the FDA's macimorelin cut-off of 2.8 and well below the proposed 5.1. Different tests carry different cut-offs for good physiological reasons, but it means the choice of test is not neutral to who qualifies. Second, because macimorelin is not among the three tests named in the PBS criteria, the 2026 debate over its cut-offs is, for now, more relevant to Australian readers as a lesson in diagnostic uncertainty than as a change to local practice. For context on how GH replacement is approached domestically, Endocrinology Today has published an Australian clinical overview of adult GH replacement.

Why This Matters for the Anti-Ageing Claims

The diagnostic literature is a useful corrective to the "HGH reverses ageing" pitch. GH replacement exists to correct a documented pituitary failure — hence the requirement for provocation testing, a specialist, and a symptom score. In healthy older adults without deficiency, controlled trials have reported roughly 2 kg of lean mass gain alongside real side effects: oedema, arthralgia, carpal tunnel symptoms and worsened insulin sensitivity. That is a very different risk-benefit calculation from treating a person whose pituitary genuinely fails a stimulation test.

The 2026 macimorelin data adds a subtler point. If higher BMI can blunt stimulated GH and push a borderline result across a threshold, then the group most likely to be misclassified as deficient overlaps with the group most likely to be seeking body-composition benefits. Careful pre-test probability assessment — imaging, pituitary history, other hormone deficiencies — does more work than any single number.

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FAQ

What is HGH used for in Australia?

Somatropin is TGA-approved and PBS-subsidised for defined conditions including paediatric growth disorders and severe adult growth hormone deficiency. It is not approved or subsidised for anti-ageing, bodybuilding or general wellness use.

How is growth hormone deficiency diagnosed?

Because GH is released in pulses, a random blood test is uninformative. Diagnosis requires a provocation test — in Australia the PBS criteria name the insulin tolerance test, arginine infusion or glucagon test, each with its own peak GH cut-off, plus a QoL-AGHDA score of 16 or more.

Is HGH legal in Australia?

Somatropin is a Schedule 4 prescription-only medicine and is border-controlled. Possessing or importing it without a valid prescription is illegal, and it is prohibited at all times in sport under the WADA List.

Does being overweight affect a growth hormone test?

A 2026 single-centre retrospective study in Pituitary found higher BMI was associated with lower peak GH on macimorelin testing and clustered among patients whose diagnosis changed between two cut-offs. The finding is unreplicated and should not be treated as a validated correction factor.

Sources

This article is independent editorial content and is not medical advice. Somatropin is TGA-approved and PBS-subsidised in Australia for defined conditions, is Schedule 4 (prescription only) and border-controlled, and is prohibited in sport by WADA; macimorelin is a diagnostic agent approved by the FDA and EMA and is not among the tests named in Australia's PBS criteria. Discuss any hormone testing or treatment with a qualified clinician.