Peptides and Growth Hormone Testing: GH Stim Test,…

Understanding growth hormone testing is essential for anyone considering GH peptide therapy. Without objective assessment of your GH axis, you are either self-prescribing peptides without knowing if your GH is actually deficient, or you are using them without a way to evaluate response. Growth hormone testing is more nuanced than most blood work because GH itself is pulsatile and difficult to capture in a single draw — requiring either provocative testing or surrogate markers.

Why GH Testing Is Complex

Growth hormone is not secreted continuously. It is released in discrete pulses — primarily during deep sleep (slow-wave sleep), but also in response to exercise, fasting, and physiological stress. Between pulses, circulating GH is nearly undetectable even in young, healthy individuals.

This pulsatile nature means a random serum GH draw is essentially uninformative. A value of 0.1 ng/mL in the afternoon does not indicate GH deficiency — it may simply reflect an inter-pulse measurement in a normal adult.

The solution is either:

Measure IGF-1 — the downstream mediator with a long half-life that reflects integrated GH exposure over days
Perform provocative (stimulation) testing — administer a GH secretagogue and measure the peak GH response

IGF-1: The Primary GH Status Marker

IGF-1 (Insulin-like Growth Factor 1) is produced primarily in the liver in response to GH. Its 12–15 hour half-life means it reflects GH activity over the preceding 24–48 hours, not just a single moment. This makes it the standard surrogate marker for assessing GH axis status.

What IGF-1 testing tells you

Low IGF-1 relative to age-adjusted norms: Suggests GH deficiency or reduced GH axis activity. Note the caveat: IGF-1 can be low for reasons independent of GH, including:

Caloric restriction or malnutrition
Liver disease (reduced hepatic production)
Hypothyroidism
Estrogen dominance (high estradiol reduces IGF-1)
Inflammatory states

Normal IGF-1: Does not absolutely exclude episodic GH deficiency but makes severe GHD unlikely.

Elevated IGF-1: In the absence of GH peptide use, persistent supranormal IGF-1 warrants evaluation for GH-secreting pituitary adenoma (acromegaly).

How to test IGF-1 accurately

Draw fasted (ideally overnight fast) in the morning
Avoid intense exercise for 24 hours before the draw
Test consistently (same time of day, same fed/fasted state) for serial comparisons
Use the same laboratory across repeated measurements — assay methodology varies between labs

Age-adjusted reference ranges

| Age | Lower Normal (ng/mL) | Upper Normal (ng/mL) | |-----|---------------------|---------------------| | 20–29 | 115 | 355 | | 30–39 | 109 | 325 | | 40–49 | 90 | 280 | | 50–59 | 80 | 240 | | 60–69 | 65 | 200 | | 70+ | 50 | 170 |

An individual in their 50s with an IGF-1 of 60 ng/mL is substantially below the lower limit of normal for their age — a finding that would support consideration of GH replacement therapy. The same value in a 75-year-old is borderline but not clearly pathological.

Growth Hormone Stimulation Tests

When IGF-1 is low or borderline, provocative testing is used to directly assess pituitary GH secretory capacity. The premise: administering a known GH-releasing stimulus and measuring peak serum GH. A normal pituitary should respond with a GH surge exceeding the diagnostic threshold.

Insulin Tolerance Test (ITT): The Gold Standard

The insulin tolerance test is the historically definitive test for GH deficiency in adults. Insulin-induced hypoglycemia is a profound physiological stress that should trigger a robust GH (and cortisol) surge in individuals with intact pituitary function.

Procedure: Intravenous regular insulin (0.1–0.15 units/kg) is administered. Blood glucose typically falls to below 40 mg/dL (or below 50% of baseline) — adequate hypoglycemia must be confirmed. GH is measured at 0, 15, 30, 45, 60, and 90 minutes.

Diagnostic threshold: A peak GH > 5 ng/mL (some guidelines use 3 ng/mL) excludes severe GHD. Peak GH < 3 ng/mL with symptoms confirms severe GHD.

Limitation: The ITT requires physician supervision in a medical setting with resuscitation capability due to the deliberate hypoglycemia. It is contraindicated in individuals with cardiovascular disease, epilepsy, or adrenal insufficiency. It is impractical for routine use outside endocrinology clinics.

GHRH + Arginine Test: The Preferred Alternative

The GHRH + arginine stimulation test has become the most widely used alternative to the ITT for diagnosing adult GHD. It does not require hypoglycemia and has comparable diagnostic accuracy.

Procedure:

GHRH (1 mcg/kg IV) is administered at time 0
Arginine (0.5g/kg IV, maximum 30g) is infused over 30 minutes simultaneously
GH is measured at 0, 15, 30, 45, 60, 90, and 120 minutes

Arginine enhances the GH response to GHRH by suppressing somatostatin (the inhibitory regulator of GH release). The combination produces a more robust and diagnostically reliable GH peak than either agent alone.

Diagnostic thresholds (BMI-adjusted, per GROWTH study):

BMI < 25 kg/m²: Peak GH ≥ 11.5 ng/mL = normal
BMI 25–30 kg/m²: Peak GH ≥ 8.0 ng/mL = normal
BMI > 30 kg/m²: Peak GH ≥ 4.2 ng/mL = normal

BMI-adjustment is critical: obesity substantially reduces the peak GH response to any stimulus, creating false-positive diagnoses of GHD in overweight individuals without true pituitary dysfunction.

GHRH + GHRP-2 Test: The Peptide-Specific Option

The combination of GHRH and GHRP-2 as a stimulation test is used in some research and clinical settings as an alternative to GHRH + arginine. This approach is directly relevant to peptide therapy because it tests the same receptor systems that GH peptide protocols utilize.

Procedure: GHRH (1 mcg/kg IV) + GHRP-2 (1 mcg/kg IV) are administered simultaneously. GH is measured at 0, 15, 30, 45, and 60 minutes.

Normal response: Peak GH typically exceeds 20–30 ng/mL in healthy adults, with the expected peak at 30–45 minutes.

The GHRH + GHRP-2 test is particularly informative for individuals considering peptide therapy: a robust response to this combination confirms that both GH-axis receptor systems are intact and responsive, predicting good clinical response to a GHRH/GHRP-based protocol.

A blunted response may indicate:

True pituitary GHD (requires exogenous GH, not secretagogues)
Somatostatin excess (high-stress states, obesity)
Receptor desensitization from prior GHRP use

Glucagon Stimulation Test

For individuals where neither ITT nor GHRH+arginine testing is practical, the glucagon stimulation test is an accepted alternative. Intramuscular glucagon (1mg) stimulates GH through poorly characterized mechanisms. It requires a 3–4 hour testing period and has lower diagnostic sensitivity than the GHRH+arginine test, but avoids the cardiovascular risks of the ITT.

Interpreting Results in the Context of Peptide Therapy

Who is an appropriate candidate for GH peptide therapy based on testing?

Clear indication: IGF-1 below the lower limit of the age-adjusted reference range with symptoms (fatigue, poor body composition, impaired recovery). Confirmed low peak GH on stimulation testing (if performed) strengthens the clinical indication.

Reasonable indication: IGF-1 in the lower quartile of the age-adjusted range with consistent symptoms in a middle-aged or older adult. Many clinicians will initiate a therapeutic trial of GH peptides in this context without formal provocative testing.

Requires formal testing: Suspected GHD in a younger adult (under 40) should be confirmed with provocative testing before attributing symptoms to GH deficiency and initiating therapy.

Testing during an active peptide cycle

IGF-1 is the standard monitoring marker on a peptide protocol. Do not test GH itself during an active cycle — the pulsatile nature makes random draws uninformative. IGF-1 at 4–6 weeks into a stable protocol gives a reliable assessment of GH axis activation.

Run a stimulation test only after a sufficient off-cycle period (minimum 6–8 weeks) to assess the underlying GH axis without peptide influence.

Post-cycle assessment

If you want to evaluate whether your GH axis has returned to baseline after a peptide cycle, repeat IGF-1 at 6–8 weeks off peptides. If it returns to your pre-cycle baseline, the GH axis has normalized. If IGF-1 remains elevated, continued active peptide or GH exposure should be suspected.

Practical Testing Protocol for Peptide Users

Pre-cycle panel (before starting):

IGF-1 (fasted morning draw)
IGFBP-3 (provides additional context for bioavailable IGF-1)
Fasting glucose and fasting insulin
TSH, Free T4, Free T3
Testosterone (total and free, for men)
Prolactin (especially if planning to use GHRP-2, Hexarelin)
Basic metabolic panel

On-cycle monitoring (4–6 weeks into stable protocol):

IGF-1 (primary marker)
Fasting glucose
Any markers that were borderline at baseline

Post-cycle (6–8 weeks after stopping):

IGF-1 (confirm return to baseline)
Full metabolic and hormonal panel

Frequently Asked Questions

Q: Can I get a GH stimulation test through my regular doctor? Yes, but availability varies. Endocrinologists are the appropriate specialists for formal GH testing. General practitioners can order IGF-1 testing without referral in most healthcare systems.

Q: Can I use ipamorelin or sermorelin as a "home" stimulation test before deciding whether to start a protocol? Not meaningfully. Home peptide administration lacks the controlled conditions, IV access, and serial blood draws that make formal stimulation tests diagnostically useful. IGF-1 testing after 4–6 weeks of a therapeutic trial is a more practical home-accessible evaluation approach.

Q: My IGF-1 is at the 50th percentile for my age. Do I need peptides? Not necessarily. Mid-range IGF-1 without symptoms does not indicate a clinical need. The case for GH peptide therapy is stronger when IGF-1 is in the lower quartile and consistent symptoms are present.

Q: Does MK-677 affect GH stimulation test results? Yes significantly. MK-677's 24-hour ghrelin receptor activity will produce ongoing GH stimulation that falsely elevates both IGF-1 and stimulation test peak GH values. Discontinue MK-677 for at least 4 weeks before formal GH axis testing.

Q: Why does BMI matter for interpreting GH stimulation tests? Obesity dramatically reduces GH secretion — both basally and in response to stimulation — through increased somatostatin tone and altered ghrelin receptor sensitivity. Without BMI-specific diagnostic cutoffs, overweight individuals appear GH-deficient when they may simply have GH axis suppression secondary to obesity that could reverse with weight loss.

Log your GH testing results, IGF-1 values, and peptide protocols with Optimize to track your GH axis over time.

Peptides and Growth Hormone Testing: GH Stim Test, IGF-1, and Interpreting Results