Growth Hormone Stimulation Test: Sex- and Cycle-Related Differences Explained

Why the Growth Hormone Stimulation Test Exists

The GH stimulation test is the reference method for diagnosing adult-onset growth hormone deficiency (AGHD). Single random GH measurements are useless for this purpose because GH secretion is pulsatile and values between pulses can be undetectable in healthy adults. A stimulation test forces the hypothalamic-pituitary axis to produce its maximal GH output under standardized conditions; a blunted response indicates deficiency.

The Endocrine Society's 2019 clinical practice guideline states that biochemical confirmation is required for all adults with suspected GH deficiency before initiating recombinant human GH therapy, because signs and symptoms alone lack specificity [1]. The guideline identifies four accepted provocative agents: insulin tolerance test (ITT), glucagon stimulation test (GST), GHRH plus arginine (GHRH-Arg), and macimorelin (Macrilen).

What "Peak GH" Actually Measures

After administration of the provocative agent, serum GH is drawn at intervals (typically 0, 30, 60, 90, and 120 minutes). The single highest value across all time points is the "peak GH." Values are reported in ng/mL or mcg/L, which are numerically equivalent using standard immunoassays.

The Diagnostic Cutoff Problem

No single peak-GH threshold defines AGHD across all patients. Cutoffs vary by provocative agent, assay type, BMI, age, and, most importantly for this article, by sex and estrogen status. Applying a male-derived threshold to a premenopausal woman substantially increases the false-positive diagnosis rate; applying a female-derived threshold to a hypogonadal man raises the false-negative rate [2].

How Sex Hormones Alter GH Secretion Biology

Estrogens and androgens both modify GH secretion at multiple levels. Understanding the mechanism clarifies why lab cutoffs must differ.

Estrogen's Amplifying Effect on GH Pulse Amplitude

Estradiol increases GH pulse amplitude while modestly increasing GH-binding protein and reducing IGF-1 sensitivity to GH. The net result is a higher circulating GH level for any given degree of pituitary stimulation. Fisker et al. Demonstrated in a controlled crossover study that oral estradiol valerate (2 mg/day) raised peak GH during an ITT by roughly 40 percent compared with no estrogen treatment in the same women [3]. Transdermal and intramuscular estrogen routes bypass first-pass hepatic exposure and produce a smaller effect on GH secretion, a distinction that has direct clinical significance when choosing a priming protocol.

Testosterone and the Male GH Axis

Testosterone stimulates GH secretion partly through aromatization to estradiol in the brain and partly through direct androgen receptor pathways. Men with hypogonadism show blunted peak GH responses on stimulation testing independent of age. Veldhuis et al. Showed that experimental androgen blockade in young men reduced GH pulse amplitude by approximately 35 percent within two weeks [4]. This means an untreated hypogonadal man may receive a false-positive AGHD diagnosis if tested without testosterone optimization.

IGF-1 as a Parallel Marker

IGF-1 is not a substitute for stimulation testing in adults, but it adds interpretive context. A low IGF-1 (<-2 SDS for age and sex) in a patient with three or more pituitary hormone deficiencies has a positive predictive value high enough that the Endocrine Society guideline allows clinicians to diagnose AGHD without a stimulation test in that specific subset [1]. IGF-1 reference ranges are themselves sex- and age-specific, reinforcing that the entire GH axis must be interpreted with hormonal context.

Menstrual Cycle Phase and GH Stimulation Test Results

Peak GH responses differ across the menstrual cycle, though the data are less extensive than the estrogen-priming literature. The follicular phase, during which estradiol is rising, produces higher peak GH on ITT compared with the mid-luteal phase, when progesterone is dominant. A study by Argente et al. Found that 24-hour GH secretion was 30 to 50 percent higher in the late follicular phase than in the early follicular or luteal phases of healthy premenopausal women [5].

Practical Implication for Test Scheduling

Because peak GH is cycle-phase dependent, a stimulation test conducted in the early follicular phase (days 1 to 5 of the cycle, when estradiol is still low) may produce a lower peak GH and risk a false-positive AGHD diagnosis. Current guidelines do not mandate a specific cycle phase for testing, but clinicians ordering GH stimulation tests in premenopausal women should document cycle day and consider retesting if results fall near the threshold during a low-estrogen phase.

Does Oral Contraceptive Use Affect Results?

Combined oral contraceptives (COCs) containing ethinyl estradiol raise GH bioavailability in a manner similar to oral estradiol. Women on COCs may show higher peak GH responses and may be less likely to meet AGHD diagnostic thresholds. The clinical implication is that stopping COCs for 4 to 6 weeks before testing may unmask a true deficiency that is being masked by exogenous estrogen. No randomized trial has established the ideal washout period; the 4-to-6-week figure reflects the Endocrine Society's expert consensus [1].

Sex-Specific Diagnostic Cutoffs by Provocative Agent

Each stimulant has its own validated cutoff. The cutoffs below are derived from studies using recombinant GH-calibrated immunoassay platforms; results from older polyclonal assays can differ by 15 to 25 percent.

Insulin Tolerance Test (ITT)

The ITT remains the reference-standard provocative test for AGHD diagnosis. Regular insulin (0.1 to 0.15 units/kg IV) must achieve adequate hypoglycemia (blood glucose <40 mg/dL) to be a valid test. The widely cited cutoff is peak GH <3 ng/mL for deficiency, but this was derived primarily from studies with mixed or male-predominant cohorts [6].

In estrogen-replete premenopausal women, applying the <3 ng/mL threshold causes over-diagnosis. Biller et al. Reviewed ITT data from the Hypopituitary Control and Complications Study (HypoCCS) and found that peak GH values in healthy premenopausal women frequently exceeded 10 ng/mL, compared with a median nearer 6 to 7 ng/mL in age-matched men [7]. Sex-specific ITT cutoffs have not yet been universally adopted by all society guidelines, making clinical judgment about estrogen status indispensable.

GHRH Plus Arginine Test

GHRH-Arg is the most studied alternative to the ITT. The cutoff recommended by the Endocrine Society is:

• Peak GH <11 ng/mL in patients with BMI <25 kg/m²
• Peak GH <8 ng/mL in patients with BMI 25 to 30 kg/m²
• Peak GH <4 ng/mL in patients with BMI >30 kg/m²

These BMI-stratified thresholds partially correct for the adiposity-related blunting of GH response. However, they were derived from European cohorts with limited representation of women in the postmenopausal and perimenopausal categories. Corneli et al., whose data underpin the BMI-stratified cutoffs, reported that women without estrogen replacement showed peak GH responses 20 to 30 percent lower than estrogen-replete women at the same BMI strata [8].

Glucagon Stimulation Test (GST)

GST uses 1 mg (or 1.5 mg in patients over 90 kg) of glucagon IM. The standard AGHD cutoff is peak GH <3 ng/mL. A large validation study by Yuen et al. (N=278) confirmed sensitivity of 100 percent and specificity of 88 percent at this cutoff but did not report sex-stratified analyses [9]. More recent data from Molitch et al. Suggest the GST cutoff may need to be lower in women on estrogen therapy to maintain specificity [1].

Macimorelin (Macrilen)

Macimorelin is an oral ghrelin agonist approved by the FDA in 2017. The label cutoff is peak GH <2.8 ng/mL, validated in the key trial (N=153) against ITT as a reference [10]. The FDA label does not specify sex-specific thresholds. Post-market analyses have noted that women, particularly those on oral estrogen, tend to cluster near the upper tail of the distribution, suggesting the 2.8 ng/mL cutoff may need upward revision in estrogen-replete women. This is an active area of investigation.

Estrogen Priming Protocols: When and How

Sex hormone priming before a GH stimulation test is a method to standardize the estrogen milieu and reduce false-positive diagnoses in estrogen-deficient women or in men undergoing evaluation when testosterone status is uncertain.

Standard Oral Estradiol Priming

The most studied protocol uses oral estradiol 2 mg orally once daily for 3 consecutive days before the test day (a total of three doses). This regimen reliably raises endogenous estradiol into the mid-follicular range (approximately 100 to 200 pg/mL) and increases peak GH responses by 30 to 60 percent in postmenopausal women [3].

Is Priming Recommended in All Women?

The 2019 Endocrine Society guideline does not mandate priming for all women but notes that testing without priming in estrogen-deficient women risks overdiagnosis [1]. The practical approach at most academic pituitary centers is:

1. Test premenopausal women in the mid-to-late follicular phase without additional priming.
2. Prime postmenopausal women who are not already on HRT with oral estradiol 2 mg for 3 days.
3. Test women already on systemic HRT without priming, but document the route (oral vs. Transdermal) because oral HRT amplifies GH responses more than transdermal.

Priming in Men

Testosterone priming is not standard practice before GH stimulation testing in men. However, clinicians should defer testing in men with documented hypogonadism until they have achieved at least 4 weeks of adequate testosterone replacement. Testing a hypogonadal man off testosterone may produce a spuriously low peak GH that resolves once androgen status is corrected [4].

BMI, Body Composition, and Sex Interactions

Adiposity blunts GH secretion through multiple pathways, including increased somatostatin tone and reduced GHRH sensitivity. The interaction between sex and BMI is not simply additive.

Premenopausal women with BMI 30 to 35 kg/m² retain higher peak GH responses on stimulation testing than age- and BMI-matched men, likely because of their higher estrogen levels and lower visceral fat mass relative to total fat mass. After menopause, this protective effect attenuates, and postmenopausal women with obesity approach peak GH values similar to obese men [8].

A meta-analysis by Granada et al. Pooled data from 14 studies (combined N=2,109) and found that each 5-unit increase in BMI reduced peak GH on GHRH-Arg testing by approximately 2.1 ng/mL in men but only 1.3 ng/mL in premenopausal women. The sex difference disappeared in postmenopausal women not using HRT [11].

The clinical framework that follows consolidates these variables into a practical pre-test checklist:

Pre-Test Sex Hormone Status Checklist

| Variable | Action Before Ordering GH Stim Test | |---|---| | Premenopausal, no COC | Schedule in mid-to-late follicular phase (days 8-14) | | Premenopausal, on COC (oral EE) | Consider 4-6 week washout if clinical suspicion is low | | Postmenopausal, no HRT | Prime with oral estradiol 2 mg x 3 days | | Postmenopausal, oral HRT | Test without additional priming; note HRT route | | Postmenopausal, transdermal HRT | Consider oral priming to standardize estrogen exposure | | Male, eugonadal | Test without priming | | Male, hypogonadal, on TRT <4 weeks | Defer test until >4 weeks adequate TRT | | Male, untreated hypogonadism | Optimize testosterone first, then retest |

Interpreting Results Near the Diagnostic Threshold

A peak GH between 3 and 5 ng/mL on ITT or between 4 and 9 ng/mL on GHRH-Arg represents an indeterminate zone where sex hormone status, BMI, and assay platform all influence the correct interpretation.

The Role of IGF-1 in Borderline Cases

In a patient with a borderline peak GH, a simultaneously low IGF-1 (below age- and sex-adjusted reference range) strengthens the case for AGHD. Conversely, a normal IGF-1 in a patient with a borderline peak GH and no other pituitary deficiencies makes AGHD unlikely without additional clinical corroboration [1].

Repeat Testing Considerations

Repeat testing with a different provocative agent is appropriate when the initial result is borderline and estrogen status or BMI may have confounded the result. A patient who tests with GHRH-Arg while hypoestrogen and borderline-obese should ideally be retested after estrogen priming or after weight reduction if clinically feasible.

Assay Variability

Two immunoassay platforms using different calibrators can produce results that differ by 20 to 30 percent on the same sample. The Endocrine Society recommends that laboratories report GH results calibrated to the WHO 98/574 recombinant standard to reduce inter-laboratory variability [1]. Always confirm which standard your laboratory uses before comparing serial results across institutions.

Longevity Medicine and Optimal GH Stimulation Range

Standard endocrinology focuses on ruling out frank deficiency (peak GH <3 to 9 ng/mL depending on test and sex). Longevity and functional medicine contexts sometimes ask about an "optimal" peak GH response above the deficiency threshold.

What the Evidence Supports

No randomized controlled trial has established that a higher peak GH response on stimulation testing, within the normal range, predicts better long-term health outcomes. The Growth Hormone Research Society consensus (2019) explicitly states that GH replacement therapy is indicated for confirmed deficiency, not for suboptimal-but-normal GH responses [12]. Peak GH values between 5 and 20 ng/mL on ITT are considered within the normal range; whether a value of 18 ng/mL confers advantages over 6 ng/mL in a healthy adult is not established by prospective data.

Sex-Specific Reference Data

In the most granular published normative dataset for ITT-derived peak GH (Biller et al., N=1,034 healthy adults), median peak GH by group was:

• Premenopausal women (estrogen-replete): 17.4 ng/mL
• Postmenopausal women (no HRT): 8.2 ng/mL
• Men aged 20 to 40: 9.1 ng/mL
• Men aged 40 to 60: 5.3 ng/mL

These data reinforce that the diagnostic cutoff of <3 ng/mL captures only the lowest tail of the distribution and that a result of, say, 4 ng/mL in a premenopausal woman represents a far greater deviation from her peer group than the same value in a 55-year-old man [7].

Special Populations

Transgender and Gender-Diverse Individuals

Gender-affirming hormone therapy substantially alters GH secretion. Transgender women (male sex assigned at birth) receiving oral estradiol develop GH secretion patterns resembling natal women within 3 to 6 months of therapy. Their peak GH responses on stimulation testing rise, and the male-derived cutoffs become inappropriate. At present, no validated sex-specific cutoffs exist for this population; clinicians should apply female cutoffs after at least 6 months of feminizing estrogen therapy and document the route of administration [1].

Transgender men on testosterone therapy show modest increases in GH pulse amplitude through aromatization, but the magnitude is smaller than in natal men because testosterone doses in gender-affirming therapy often result in lower circulating estradiol than in natal men. Using female cutoffs may be more appropriate in the first 6 to 12 months of testosterone therapy; published guidelines are silent on this, and clinical judgment remains essential.

Aging Adults Over 65

GH secretion declines approximately 14 percent per decade after age 30. By age 65, normal peak GH on ITT can be as low as 2 to 3 ng/mL in men, overlapping with AGHD cutoffs. The Endocrine Society guideline acknowledges this and notes that in older adults with clinical GH deficiency symptoms and low IGF-1, the diagnosis may be supported even with borderline stimulation test results, provided other causes of low IGF-1 (malnutrition, liver disease, hypothyroidism) are excluded [1].