Growth Hormone Stimulation Test: Which Tests to Order Alongside

What a GH Stimulation Test Actually Measures

The GH stimulation test (also called a GH provocation test or stim test) quantifies the pituitary's ability to release growth hormone when given a standardized pharmacologic trigger. Baseline GH levels are nearly useless on their own because GH is secreted in pulses, with troughs that can sit at undetectable levels in healthy individuals 1.

During the test, a provocative agent forces the pituitary to mount a GH surge. The clinician then draws timed serum samples (typically at 0, 30, 60, 90, and 120 minutes) and records the peak value. A blunted peak signals that the somatotroph cells cannot respond adequately.

The 2011 Endocrine Society Clinical Practice Guideline defines severe adult GH deficiency (AGHD) as a peak GH <5 mcg/L on the insulin tolerance test or <3 mcg/L on the GHRH-arginine test 1. Pediatric cutoffs vary by assay and country but generally use a threshold of <10 mcg/L on two separate provocative tests 2. These numbers lose meaning without context from companion labs. That context is exactly what the paired panel provides.

IGF-1 and IGFBP-3: The Non-Negotiable Companions

Order a serum IGF-1 (insulin-like growth factor 1) on the same draw as the stim test baseline. IGF-1 reflects the liver's integrated response to circulating GH over the preceding 24 hours, making it a stable proxy for GH status where the single time-point stim peak is a snapshot 3.

An IGF-1 below the age- and sex-adjusted reference range strongly supports GH deficiency, especially in lean patients. In the 2011 Endocrine Society guideline, the authors state: "A low serum IGF-1 level, when measured under appropriate conditions, raises the suspicion of GHD, and a very low level (<84 mcg/L) is highly predictive of AGHD" 1.

IGFBP-3 (insulin-like growth factor binding protein 3) adds sensitivity in pediatric evaluations, where it correlates with GH secretory capacity more reliably than IGF-1 alone in children under five 2. In adults, IGFBP-3 is less discriminating but still worth drawing when the clinical picture is ambiguous.

A normal IGF-1 does not exclude GH deficiency outright. Obesity, uncontrolled diabetes, and hepatic disease all influence IGF-1 independently of GH status 3.

Thyroid Function Panel: The Mandatory Pre-Test Check

Hypothyroidism suppresses the GH axis. A patient with untreated or under-treated hypothyroidism can fail a stim test even if their pituitary somatotrophs are intact. The Endocrine Society explicitly recommends documenting adequate thyroid replacement before performing provocative GH testing 1.

Order free T4 and TSH at minimum. Free T3 adds value when you suspect T4-to-T3 conversion problems (common in patients on levothyroxine monotherapy or those with non-thyroidal illness). In patients with known or suspected central hypothyroidism, TSH is unreliable because the pituitary itself is the broken link. Free T4 becomes the primary gauge.

If the free T4 comes back low, correct it and wait a minimum of six weeks on stable replacement before running the GH stim test. Testing earlier risks a false-positive diagnosis of GH deficiency.

Morning Cortisol and ACTH: Safety and Diagnostic Overlap

A morning cortisol (drawn between 7:00 and 9:00 AM) and ACTH belong on the panel for two reasons. First, hypocortisolism blunts the GH response in a way that mimics true GH deficiency, creating the same confounding risk as hypothyroidism 4. Second, if you are using the insulin tolerance test as your provocateur, undiagnosed adrenal insufficiency makes insulin-induced hypoglycemia dangerous.

The 2016 Endocrine Society guideline on adrenal insufficiency recommends using a morning cortisol <3 mcg/dL as virtually diagnostic and >15 mcg/dL as effectively excluding primary adrenal insufficiency 4. Values between 3 and 15 mcg/dL require a cosyntropin stimulation test for clarification.

If the patient is already scheduled for an ITT, you get cortisol data for free. The ITT is the gold standard for both GH and cortisol reserve testing because it stresses both axes simultaneously. A cortisol peak >18 mcg/dL during the ITT confirms adequate adrenal reserve, while a peak <18 mcg/dL suggests adrenal insufficiency requiring further evaluation 1.

Prolactin: Screening for Pituitary Mass Effect

Order a serum prolactin. Hyperprolactinemia (typically >100 ng/mL) points toward a prolactinoma, while mildly elevated prolactin (25 to 100 ng/mL) can signal stalk effect from a non-functioning pituitary adenoma compressing the infundibulum 5.

Either finding changes management. A prolactinoma gets dopamine agonist therapy rather than surgery in most cases. Stalk-effect hyperprolactinemia demands imaging with a gadolinium-enhanced pituitary MRI. Both scenarios also raise the probability of combined pituitary hormone deficiencies, which brings us to gonadotropins.

Dr. Beverly Biller, associate professor of medicine at Harvard Medical School, has noted: "Patients with structural hypothalamic-pituitary disease and deficiency of at least one pituitary hormone, along with a low IGF-1, have a greater than 95% chance of being GH deficient" 1. That single-test threshold only applies when the paired labs confirm multi-axis disease.

LH, FSH, and Sex Steroids: Gonadal Axis Assessment

Gonadotropins (LH and FSH) and sex steroids (testosterone in males, estradiol in females) round out the anterior pituitary survey. Low LH and FSH paired with low testosterone or estradiol indicate central hypogonadism, which, combined with GH deficiency, strongly suggests structural pituitary pathology 6.

In premenopausal women, the timing of the blood draw matters. LH and FSH vary across the menstrual cycle, so draw in the early follicular phase (cycle days 2 to 5) whenever possible. In men, draw testosterone between 7:00 and 10:00 AM to capture the diurnal peak. Two low morning testosterone readings (<300 ng/dL by most lab reference ranges) confirm biochemical hypogonadism per the AUA/Endocrine Society threshold 6.

Identifying concurrent hypogonadism also matters therapeutically. Testosterone replacement in GH-deficient males can amplify the IGF-1 response to subsequent GH therapy, and estrogen status in women affects GH dose requirements because oral estrogen increases hepatic GH resistance 7.

Comprehensive Metabolic Panel and Lipids: Capturing Downstream Effects

GH deficiency produces a recognizable metabolic signature: elevated LDL cholesterol, reduced HDL, increased visceral adiposity, and impaired glucose tolerance. A comprehensive metabolic panel (CMP) and fasting lipid panel at baseline serve both diagnostic and monitoring purposes 8.

In a meta-analysis of 16 studies (N=468 GH-deficient adults), total cholesterol averaged 16% higher and LDL averaged 20% higher in untreated AGHD compared to matched controls 8. These lipid derangements improve with GH replacement, giving you a quantifiable treatment response marker.

Fasting glucose and HbA1c warrant inclusion because GH replacement can worsen insulin resistance in the first months of therapy. Baseline values let you track this and adjust the GH dose or add metformin as needed.

Bone Age (Pediatric) and DEXA (Adult): Structural Assessments

For children, a bone age X-ray of the left hand and wrist should accompany the stim test workup. Delayed bone age relative to chronological age supports GH deficiency as a cause of short stature and helps predict remaining growth potential 2.

For adults, a DEXA scan (dual-energy X-ray absorptiometry) captures the reduced bone mineral density that accompanies prolonged GH deficiency. AGHD patients have a 2- to 5-fold increased fracture risk compared to age-matched peers, and the 2011 Endocrine Society guideline recommends baseline DEXA before starting replacement 1.

Choosing Your Provocative Agent: How the Protocol Shapes the Panel

The choice of stim test agent affects which additional labs you need and how you interpret the results.

Insulin Tolerance Test (ITT). Requires confirmed morning cortisol adequacy or willingness to assess cortisol simultaneously. Contraindicated in patients with seizure disorders, ischemic heart disease, or age >65. Glucose must be monitored every 5 minutes during the hypoglycemic phase; blood glucose must drop below 40 mg/dL for the test to be valid 1.

Glucagon Stimulation Test (GST). An alternative when the ITT is contraindicated. The peak GH cutoff is <3 mcg/L for severe deficiency. Nausea occurs in roughly 30% of patients. Glucagon also stimulates cortisol, so it can double as an adrenal reserve test, though it is less validated than the ITT for that purpose 9.

Macimorelin Oral Test. FDA-approved in December 2017, macimorelin is an oral ghrelin agonist with a diagnostic accuracy of 92% (sensitivity 87%, specificity 96%) validated against the ITT in a 314-patient registration trial 10. The cutoff is a peak GH <2.8 mcg/L. No IV access required. Avoid strong CYP3A4 inhibitors during the test window, which means checking the patient's medication list beforehand.

GHRH-Arginine Test. Historically popular, but GHRH (Geref) was discontinued in the U.S. market. Still used in Europe and other regions. BMI-stratified cutoffs apply: peak GH <11.5 mcg/L for BMI <25, <8.0 for BMI 25 to 30, and <4.2 for BMI >30 11.

The Effect of BMI on Stim Test Interpretation

Obesity dampens the GH response to all provocative agents. In a study of 322 healthy volunteers, peak GH on the ITT fell by 50% in subjects with BMI >30 compared to normal-weight controls 11. This overlap between obesity-related blunting and true GH deficiency is one of the most common diagnostic traps in endocrinology.

For patients with BMI >30, the AACE 2019 consensus recommends using the macimorelin test with its fixed 2.8 mcg/L cutoff (which was validated in a cohort that included obese subjects) or applying BMI-adjusted cutoffs for the GHRH-arginine protocol 12. Pairing the stim result with a definitively low IGF-1 and at least one additional pituitary hormone deficiency increases diagnostic confidence in obese patients.

Always order a fasting insulin alongside the CMP in obese patients undergoing GH evaluation. Hyperinsulinemia suggests that the metabolic phenotype may partly explain a low IGF-1 through GH receptor resistance rather than true GH deficiency 3.

When to Add Pituitary MRI to the Workup

Imaging is not a lab, but it is part of the paired diagnostic package. Any confirmed GH deficiency in an adult or child (outside of idiopathic isolated GH deficiency in a child with no red flags) warrants a gadolinium-enhanced MRI of the sella turcica 1. The scan looks for pituitary adenomas, craniopharyngiomas, empty sella, stalk thickening, or infiltrative disease.

Order the MRI before starting GH replacement. An unsuspected tumor changes the treatment algorithm entirely.

Complete Paired Lab Checklist

For clinicians building order sets, here is the full panel to draw alongside or immediately before a GH stimulation test:

1. IGF-1 (age/sex-adjusted)
2. IGFBP-3 (especially pediatric cases)
3. Free T4 and TSH
4. Morning cortisol (7:00 to 9:00 AM) and ACTH
5. Prolactin
6. LH and FSH
7. Testosterone (males, morning draw) or estradiol (females, early follicular)
8. Comprehensive metabolic panel
9. Fasting lipid panel
10. Fasting glucose and HbA1c
11. Fasting insulin (if BMI >30)
12. CBC (screens for chronic disease confounders)
13. Bone age X-ray (pediatric)
14. DEXA scan (adult)

This panel captures pituitary reserve across all anterior axes, metabolic consequences of GH deficiency, and confounders that can produce false-positive stim test results.