HGH Deficiency Diagnosis, Markers & Axis Assessment

Why GH Cannot Be Directly Measured for Diagnosis

The fundamental challenge in diagnosing GH deficiency is that GH is secreted in discrete pulses — 6–12 per day in healthy adults, with the largest pulse during slow-wave sleep and values near zero between pulses. A random blood sample for GH will likely be drawn during an interpulse trough, where GH levels are undetectable even in healthy individuals. This means a low or undetectable random serum GH level is entirely normal and provides no diagnostic information about GH deficiency whatsoever.

This is fundamentally different from diagnosing, for example, hypothyroidism — where TSH and FT4 can be measured at any time and reflect the steady-state thyroid axis. GH deficiency diagnosis requires either indirect inference from IGF-1 (which integrates GH exposure over ~15 hours) or direct provocative testing that stimulates maximal GH secretion under standardised conditions and measures whether the GH secretory reserve is adequate.

Step 1: IGF-1 as a Screening Marker

Serum IGF-1 — produced by the liver in direct proportion to cumulative GH stimulation — is the best available screening marker for GH axis status. Unlike GH itself, IGF-1 is stable in circulation with a half-life of ~15 hours and does not fluctuate with meals, exercise, or time of day in a way that makes random sampling unreliable. A serum IGF-1 result therefore provides an integrated picture of GH axis activity over the preceding 12–24 hours.

| IGF-1 Result (age-adjusted) | Interpretation | Next Step | | --- | --- | --- | | IGF-1 > −1 SDS (well within normal range) | GH deficiency very unlikely (<5% probability) | No provocative testing required in low pre-test probability setting | | IGF-1 −1 to −2 SDS (low-normal) | Indeterminate — possible GH deficiency | Provocative testing indicated if clinical suspicion is moderate-high | | IGF-1 < −2 SDS (below normal) | Suggestive of GH deficiency | Provocative testing required for confirmation (except 3+ pituitary hormone deficiencies — see below) | | IGF-1 < −3 SDS in context of 3+ pituitary hormone deficiencies + pituitary pathology | GH deficiency confirmed without provocative testing | GH replacement can commence — provocative testing not required per Endocrine Society guidelines |

Important limitations of IGF-1 as a GH deficiency marker: IGF-1 is reduced by malnutrition, hypothyroidism, poorly controlled diabetes, liver disease, and inflammatory illness — all of which can produce low IGF-1 independent of GH axis status. Conversely, obesity is associated with reduced GH secretion but normal or high-normal IGF-1 due to GH-independent hepatic IGF-1 regulation. These confounders must be excluded before low IGF-1 is attributed to GH deficiency.

Step 2: Provocative Stimulation Testing

For most adults with suspected GH deficiency, diagnosis requires a provocative stimulation test — a standardised pharmacological challenge that stimulates maximal GH secretion, with serial GH measurements over 1–3 hours. An adequate peak GH response rules out GH deficiency; a blunted or absent response confirms it. The key diagnostic variable is the peak GH concentration achieved during the test.

| Test | Stimulus | Peak GH Cut-off (Deficiency) | Sensitivity / Specificity | Key Considerations | | --- | --- | --- | --- | --- | | Insulin Tolerance Test (ITT) | IV insulin 0.1–0.15 U/kg → hypoglycaemia (BG <2.2 mmol/L required) | <3 µg/L = severe deficiency; <5 µg/L = deficiency (some guidelines) | Sensitivity ~96%; Specificity ~92% — gold standard | Contraindicated in seizure disorder, cardiovascular disease, elderly, severe hypopituitarism; requires physician supervision; gold standard but highest risk | | GHRH + Arginine (GHRH-Arg) | IV GHRH 1 µg/kg + IV arginine 0.5 g/kg over 30 min | BMI-adjusted: <9 µg/L (BMI <25); <4 µg/L (BMI 25–30); <1 µg/L (BMI >30) | Sensitivity ~95%; Specificity ~91% | Safe and well-tolerated; BMI adjustment essential; not valid in hypothalamic disease (GHRH acts at pituitary, bypasses hypothalamic defect); preferred test in most European guidelines | | Glucagon Stimulation Test (GST) | IM glucagon 1 mg (1.5 mg if >90 kg); samples at 90, 120, 150, 180, 210 min | <3 µg/L = deficiency | Sensitivity ~97%; Specificity ~88% | Preferred alternative when ITT contraindicated; risk of prolonged hypoglycaemia 3–4 hours post-injection — glucose monitoring required; long test duration (3.5 hours) | | GHRP-2 Stimulation Test | IV GHRP-2 100 µg bolus; samples at 0, 15, 30, 45, 60, 90 min | <9 µg/L = deficiency (Japan-validated cut-off) | Sensitivity ~97%; Specificity ~88% (Japanese validation data) | Approved diagnostic test in Japan; safe, rapid, well-tolerated; raises cortisol and prolactin (monitor); less validated in Western populations than ITT or GHRH-Arg | | Macimorelin Stimulation Test | Oral macimorelin 0.5 mg/kg; samples at 0, 30, 45, 60, 90 min | <2.8 µg/L = deficiency | Sensitivity ~87%; Specificity ~96% | First FDA-approved oral GH stimulation test (Macrilen, 2017); avoids IV access; most convenient; lower sensitivity than ITT; rising adoption in US endocrinology practice | | Arginine Alone | IV arginine 0.5 g/kg over 30 min | <0.4 µg/L | Lower sensitivity (~70%) than combination tests | Rarely used as sole test due to inferior diagnostic performance; superseded by GHRH-Arg combination |

The Complete Diagnostic Algorithm

#### Establish Pre-Test Probability

Identify aetiology: pituitary tumour, surgery, radiation, TBI, or idiopathic. Count confirmed pituitary hormone deficiencies (hypogonadism, hypothyroidism, adrenal insufficiency). Higher pre-test probability with pituitary pathology + multiple deficiencies warrants confirmation. Low clinical suspicion in otherwise healthy adults generally does not warrant formal testing. #### Measure Serum IGF-1 (Age- and Sex-Adjusted)

Exclude confounders (malnutrition, hypothyroidism, uncontrolled diabetes, liver disease). If IGF-1 < −2 SDS: proceed to provocative testing. If IGF-1 within normal range in a low pre-test probability subject: GH deficiency very unlikely — no further testing required. If 3+ pituitary hormone deficiencies + structural pituitary disease + IGF-1 < −3 SDS: diagnosis confirmed, no provocative test needed. #### Select Provocative Test Based on Patient Factors

ITT: gold standard — use when cardiovascular/seizure contraindications absent and hypoglycaemia can be safely managed. GHRH-Arginine: preferred when ITT contraindicated — but NOT in hypothalamic disease (gives false normal result by directly stimulating pituitary). GST: preferred alternative to ITT when GHRH-Arg not available or hypothalamic disease suspected. Macimorelin: convenient oral option — rising standard in clinical practice. GHRP-2: validated in Japan — use in research contexts where GHRP-2 pharmacology is specifically relevant. #### Interpret Peak GH Against BMI-Adjusted Cut-offs

Obesity profoundly blunts GH responses to all provocative stimuli — obese subjects have lower peak GH responses at equivalent GH secretory capacity compared to lean subjects. BMI-adjusted cut-offs (particularly for GHRH-Arg) are essential. Failure to adjust for BMI produces false-positive GH deficiency diagnoses in obese subjects with normal GH axis function. #### Pituitary MRI for Structural Assessment

All patients with confirmed AGHD should have pituitary MRI to identify structural pathology (tumour, empty sella, ectopic posterior pituitary, hypoplasia) that explains the deficiency and guides management of coexisting hormone deficiencies. In subjects with idiopathic AGHD and no structural finding on MRI, retesting after a period off GH replacement may be appropriate to confirm true persistent deficiency, as a subset of childhood-onset idiopathic GHD normalises in adulthood.

The Obesity-GH Deficiency Diagnostic Trap > Obesity independently reduces GH secretion — through increased hypothalamic somatostatin tone, reduced GHRH responsiveness, and enhanced GH clearance. Obese adults without any pituitary disease can have peak GH responses to provocative testing that fall below the cut-offs established in lean subjects, producing a false-positive GH deficiency diagnosis. The GHRH-arginine test with BMI-adjusted cut-offs (<9 µg/L for BMI <25; <4 µg/L for BMI 25–30; <1 µg/L for BMI >30) addresses this specifically. For the ITT and glucagon tests, a single lean-subject cut-off (<3 µg/L) is less affected by obesity but still imperfect. Researchers must account for BMI when designing and interpreting GH deficiency studies in metabolically overweight populations.

References

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2. Ghigo E, et al. "Consensus guidelines on the diagnosis and treatment of adults with GH deficiency: summary statement of the Growth Hormone Research Society Workshop on adult growth hormone deficiency." *J Clin Endocrinol Metab*. 1998;83(2):379–381.
3. Yuen KC, et al. "American Association of Clinical Endocrinologists and American College of Endocrinology guidelines for management of growth hormone deficiency in adults and patients transitioning from pediatric to adult care." *Endocr Pract*. 2019;25(11):1191–1232.
4. Aimaretti G, et al. "Diagnostic reliability of a single IGF-1 measurement in 237 adults with total anterior hypopituitarism." *Clin Endocrinol (Oxf)*. 2003;59(1):56–61.
5. Biller BM, et al. "Sensitivity and specificity of six tests for the diagnosis of adult GH deficiency." *J Clin Endocrinol Metab*. 2002;87(5):2067–2079.
6. Garcia JM, et al. "Macimorelin as a diagnostic test for adult GH deficiency." *J Clin Endocrinol Metab*. 2018;103(8):3083–3093.