Growth Hormone Stimulation Test: When to Order and What Results Mean

What Is a Growth Hormone Stimulation Test?

A GH stimulation test (also called a provocative GH test) measures how much growth hormone the pituitary gland can produce when chemically prompted. The pituitary normally secretes GH in pulses, making random single blood draws unreliable for diagnosis. A pharmacologic stimulus forces a measurable peak that clinicians can compare against established thresholds.

The test works by injecting or administering a secretagogue (insulin, glucagon, arginine, clonidine, L-DOPA, or the oral agent macimorelin) and drawing serial blood samples over 2 to 4 hours. Each agent provokes GH release through a different pathway. Insulin triggers hypoglycemia-mediated counter-regulatory GH release. Glucagon stimulates GH through mechanisms that are still being characterized but likely involve hypothalamic GH-releasing hormone (GHRH) pathways 1. Arginine suppresses somatostatin tone, removing the brake on GH secretion. The Endocrine Society's 2011 clinical practice guideline remains the primary reference for adult GHD testing protocols, while the 2016 Pediatric Endocrine Society consensus refined recommendations for children 2.

GH is measured at baseline and at 30-minute intervals. The single highest value across all time points is reported as the "peak GH." A peak below the diagnostic cutoff, confirmed on two separate tests using different agents, supports a GHD diagnosis.

When Should Clinicians Order This Test?

The stimulation test is not a screening tool. Order it only after clinical suspicion and preliminary biochemical evidence point toward GH deficiency. Screening with a serum IGF-1 level comes first, and a low age- and sex-adjusted IGF-1 raises the probability enough to justify provocative testing.

In children, the 2016 consensus guidelines recommend GH stimulation testing when a child demonstrates:

• Height more than 2 standard deviations below the mean for age and sex
• Growth velocity below the 25th percentile sustained over 6 to 12 months
• Declining height percentile crossing two or more major centile lines
• Delayed bone age by 2 or more years
• Low or low-normal IGF-1 and IGFBP-3 for age

A single failed provocative test is insufficient. Two tests with two different agents are standard before diagnosing isolated GHD. The exception: children with a documented CNS lesion, pituitary surgery, cranial irradiation, or multiple pituitary hormone deficiencies may need only one confirmatory test 3.

In adults, the Endocrine Society guideline states that GH stimulation testing is appropriate for patients with 1:

• Known hypothalamic-pituitary disease (tumor, surgery, radiation)
• Childhood-onset GHD requiring re-evaluation after growth is complete
• Traumatic brain injury or subarachnoid hemorrhage (GHD prevalence reaches 20% to 25% in TBI survivors at 12 months, per a 2005 JCEM study, N=102) 4
• At least one other documented pituitary hormone deficiency plus a low IGF-1

The guideline explicitly discourages GH stimulation testing in otherwise healthy adults with normal pituitary anatomy, even if IGF-1 is borderline low. Age, BMI, and estrogen status all affect IGF-1 interpretation and must be factored in before proceeding.

Which Stimulation Agent to Use

Not every agent is appropriate for every patient. The choice depends on patient age, comorbidities, and local availability.

Insulin tolerance test (ITT) is the reference standard for adults. Adequate hypoglycemia (blood glucose <40 mg/dL or <2.2 mmol/L) must be achieved for the test to be valid. A peak GH <3 ng/mL on ITT confirms severe adult GHD 1. The test is contraindicated in patients with seizure disorders, coronary artery disease, and those over age 65. A physician must remain present during the entire procedure.

Glucagon stimulation test (GST) serves as the primary alternative when ITT is contraindicated. The Endocrine Society accepts a peak GH <3 ng/mL on GST as diagnostic. A 2015 meta-analysis (N=817 across 12 studies) found GST sensitivity of 97% and specificity of 88% at the 3 ng/mL cutoff 5. Nausea is the most common side effect, occurring in roughly 30% of patients.

Macimorelin (Macrilen) received FDA approval in December 2017 as the first oral GH stimulation agent for adults. A peak GH <2.8 ng/mL is diagnostic. The AACE 2019 position statement endorsed macimorelin as equivalent to ITT for adult GHD diagnosis 6. The key trial (N=157) demonstrated 87% sensitivity and 96% specificity versus ITT as the comparator, with 94% reproducibility on repeat testing 7. This simplicity (single oral dose, three blood draws over 90 minutes, no hypoglycemia risk) makes it the most patient-friendly option.

In children, clonidine and arginine are the most commonly used agents. Clonidine is favored for outpatient testing because of its favorable safety profile. A peak GH <10 ng/mL is the traditional pediatric cutoff, though some centers have proposed lowering this to 7 ng/mL based on data showing high false-positive rates at the 10 ng/mL threshold 8. Insulin is occasionally used in adolescent patients but requires close monitoring.

Understanding Normal and Abnormal Results

Interpretation hinges on the peak GH level, not baseline values. A normal response rules out GHD regardless of what the pre-stimulation GH level shows.

Normal peak GH values by test and population:

| Test | Population | Normal Peak GH | |------|-----------|---------------| | ITT | Adults | ≥5 ng/mL (some use ≥3 ng/mL for severe GHD cutoff) | | GST | Adults | ≥3 ng/mL | | Macimorelin | Adults | ≥2.8 ng/mL | | Clonidine or arginine | Children | ≥10 ng/mL (≥7 ng/mL at some centers) |

A low peak GH (below cutoff) means the pituitary failed to mount an adequate GH response. Two failed tests confirm the diagnosis. Causes of true GHD include pituitary tumors (most commonly nonfunctioning adenomas), craniopharyngiomas, prior pituitary surgery, cranial radiation, Sheehan syndrome, lymphocytic hypophysitis, genetic causes (GH1, GHRHR mutations), and idiopathic GHD.

A high baseline GH before stimulation may suggest acromegaly or a GH-secreting pituitary adenoma. If random GH levels exceed 10 ng/mL and IGF-1 is elevated, the appropriate next step is an oral glucose tolerance test (OGTT) for GH suppression, not a stimulation test. In acromegaly, GH fails to suppress below 1 ng/mL after 75 g of oral glucose, according to the 2014 Endocrine Society acromegaly guideline 9.

Obesity is a major confounder. BMI above 30 blunts the GH response to all provocative agents, increasing the false-positive rate for GHD diagnosis. The 2019 AACE position statement recommends using BMI-adjusted cutoffs or preferring macimorelin in obese patients because its diagnostic accuracy was preserved across BMI categories in the registration trial 6.

How to Prepare for the Test

Preparation is straightforward but strict compliance matters. Failing to fast or stopping medications at the wrong time can invalidate results, wasting a half-day procedure.

Patients should fast for 8 to 12 hours (water is allowed). The test must start in the morning, ideally between 7:00 and 9:00 AM, to minimize the effect of diurnal GH variation. Strenuous exercise should be avoided for 24 hours beforehand because exercise independently raises GH levels.

Several medications interfere with GH secretion and may need to be held:

• Glucocorticoids in supraphysiologic doses suppress GH. Replacement doses for adrenal insufficiency should be continued.
• Estrogen (particularly oral estrogen) lowers IGF-1 and may blunt GH responses. The Endocrine Society advises interpreting results cautiously in women on oral estrogen 1.
• Opioids and dopamine agonists alter GH dynamics. Withdrawal timelines should be discussed with the ordering endocrinologist.

An IV line is placed before the stimulus is given. Serial blood draws occur at 0, 30, 60, 90, and sometimes 120 minutes (the exact schedule varies by agent). For ITT, glucose monitoring is required at every draw. Symptomatic hypoglycemia (sweating, tremor, tachycardia) confirms adequate stimulus. If glucose does not drop below 40 mg/dL, the test is non-diagnostic and must be repeated.

How Growth Hormone Deficiency Is Managed After Diagnosis

Confirming GHD is the gateway to recombinant human GH (rhGH) therapy, so accurate testing directly affects treatment decisions. The Endocrine Society recommends initiating rhGH at 0.2 to 0.3 mg/day in adults (lower in older patients, higher in women on oral estrogen) and titrating based on IGF-1 levels and clinical response every 1 to 2 months 1.

In children, the FDA has approved multiple GH formulations, and more recently, lonapegsomatropin (Skytrofa), a once-weekly GH approved in 2021, has expanded options. The pediatric dosing range is 0.024 to 0.034 mg/kg/day for daily formulations 10.

Dr. Beverly Biller, a neuroendocrinologist at Massachusetts General Hospital and co-author of the Endocrine Society guideline, has stated: "The decision to test should always follow a clinical algorithm. Provocative testing without prior biochemical screening leads to unnecessary procedures and false-positive diagnoses" 1.

Post-treatment monitoring includes IGF-1 every 6 to 12 months, lipid panels, body composition assessments, and bone density scans at baseline and 2 years. The goal is to maintain IGF-1 in the middle of the age-appropriate reference range.

Can You Lower or Raise Growth Hormone Levels?

Lowering GH is relevant in acromegaly, not GHD. Treatment for excess GH includes transsphenoidal surgery (remission rate 80% to 90% for microadenomas), somatostatin analogs (octreotide LAR, lanreotide), the GH receptor antagonist pegvisomant, and radiation therapy. The 2014 Endocrine Society guideline provides a detailed treatment algorithm 9.

Raising GH physiologically is possible through:

• Sleep optimization. Approximately 70% of daily GH secretion occurs during slow-wave sleep. A 2000 study in the Journal of Clinical Endocrinology & Metabolism (N=149) found that slow-wave sleep disruption reduced GH secretion by 75% 11.
• High-intensity exercise. Resistance training and sprint intervals produce acute GH spikes of 300% to 500% above baseline.
• Body-fat reduction. Visceral adiposity suppresses GH output. Each unit increase in BMI is associated with a 6% reduction in 24-hour integrated GH concentrations.
• Fasting. A 24-hour fast roughly doubles mean GH secretion.

These lifestyle strategies do not treat true GHD caused by structural pituitary damage or genetic conditions. They primarily benefit individuals whose GH axis is intact but suboptimally stimulated by metabolic factors.

The Endocrine Society's 2006 scientific statement cautions: "GH secretagogues and over-the-counter GH-releasing supplements have not demonstrated clinically meaningful increases in serum GH or IGF-1 in controlled trials and should not be used as a substitute for approved GH therapy in patients with documented deficiency" 12.

Retesting and Special Populations

Not every childhood GHD diagnosis persists into adulthood. The Endocrine Society recommends retesting after final height is achieved, with the exception of patients who have:

• Three or more pituitary hormone deficiencies
• Documented genetic cause of GHD
• Structural hypothalamic-pituitary defects on MRI
• History of CNS tumor or cranial irradiation

In these cases, a low IGF-1 alone suffices to confirm ongoing deficiency without repeating provocative tests 1.

For patients who underwent childhood cranial radiation, GHD may develop years after treatment. The GH axis is the most radiation-sensitive of all pituitary axes, with deficiency occurring in up to 100% of patients receiving doses exceeding 30 Gy over a 5-year follow-up period 13. Annual IGF-1 screening with provocative testing when IGF-1 falls below the reference range is appropriate in this population.

Pregnancy is an absolute contraindication to GH stimulation testing (and to GH therapy). GH therapy is discontinued upon confirmation of pregnancy, and retesting is deferred to the postpartum period.