Growth Hormone

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Category: Longevity and Aging  |  Also known as: GH, Somatotropin, Human Growth Hormone, HGH  |  Sample: Serum (fasting preferred; clinical assessment via IGF-1)

1. What This Test Measures

Growth hormone (GH) is a 191-amino-acid peptide hormone produced by somatotroph cells in the anterior pituitary gland. GH is secreted in pulsatile bursts, with the largest pulses occurring during stage N3 (deep) sleep and smaller pulses triggered by exercise, fasting, and stress. Between pulses, serum GH drops to near-undetectable levels. This pulsatile secretion pattern makes a single random serum GH measurement clinically unreliable: a value of 0.1 ng/mL could represent a patient between pulses with perfectly adequate total daily GH production, or it could represent a patient with genuine GH deficiency.

For this reason, clinical assessment of the GH axis relies primarily on IGF-1 (insulin-like growth factor 1), which is produced by the liver in direct response to GH stimulation. IGF-1 has a much longer half-life (approximately 15 hours) and remains stable throughout the day, providing a reliable integrated measure of overall GH output. When IGF-1 is low and GH deficiency is suspected, formal GH stimulation testing (insulin tolerance test, GHRH-arginine test, glucagon stimulation test) can confirm the pituitary's capacity to release GH in response to a provocative stimulus.

GH exerts its effects through two pathways: direct action on GH receptors in tissues (promoting lipolysis, inhibiting glucose uptake, stimulating protein synthesis) and indirect action through IGF-1 production (promoting cell growth, proliferation, and differentiation in bone, muscle, and other tissues). The balance between direct and IGF-1-mediated effects determines the net clinical impact of GH on body composition, metabolism, and tissue repair.

2. Why This Test Matters

• Body composition maintenance: GH is the primary hormonal driver of lean muscle mass preservation and visceral fat metabolism. GH deficiency produces sarcopenia (muscle wasting), increased visceral adiposity, and declining strength that accelerate biological aging
• Bone density: GH stimulates osteoblast activity and bone mineralization through IGF-1. Declining GH contributes to osteopenia and osteoporosis in aging adults
• Cognitive function: GH receptors are expressed throughout the brain, particularly in the hippocampus and prefrontal cortex. GH supports neuroplasticity, memory consolidation, and neuroprotection. GH-deficient adults demonstrate measurable cognitive decline that improves with GH replacement
• Immune surveillance: GH modulates thymic function, T-cell maturation, and NK cell activity. Age-related GH decline contributes to immunosenescence
• Tissue repair and recovery: GH drives the repair response to injury, exercise, and tissue damage. Declining GH slows wound healing, reduces exercise recovery capacity, and impairs the ability to adapt to physical training
• Skin integrity: GH maintains skin thickness, elasticity, and collagen production. GH decline produces the characteristic thinning and reduced elasticity of aging skin
• Cardiovascular health: GH deficiency is associated with increased cardiovascular risk, elevated LDL, reduced cardiac output, and endothelial dysfunction
• Sleep quality: the relationship between GH and sleep is bidirectional: deep sleep triggers GH release, and GH promotes restorative sleep architecture. Declining GH contributes to the sleep fragmentation of aging

3. Standard Lab Reference Range

Measure	Standard Reference Range	Notes
Random Serum GH	0.01 to 8 ng/mL	Highly variable due to pulsatile secretion; a single value is not clinically meaningful
GH Stimulation Test	Peak above 5 ng/mL	Confirms adequate pituitary GH reserve; peak below 3 to 5 ng/mL suggests GH deficiency
IGF-1 (proxy)	Age-adjusted (varies)	Primary clinical measure of GH axis function; stable, reproducible, no stimulation required

4. Optimal Functional Medicine Range

Assessment	Functional Target
IGF-1 (primary)	Upper third of age-adjusted reference range (typically 180 to 250 ng/mL for adults 30 to 60)
Random Serum GH	Not used as a standalone assessment in functional medicine; pulsatile nature makes single values unreliable
Clinical assessment	Body composition (lean mass, visceral fat), exercise recovery, sleep quality, skin integrity, cognitive function assessed alongside IGF-1

5. Growth Hormone in the Complete Aging Panel

GH axis assessment is one component of a comprehensive biological age evaluation. The complete panel includes:

Marker	What It Adds	FM Optimal
IGF-1 (GH proxy)	Integrated measure of GH axis function	Upper third of age range
DHEA-S	Adrenal reserve and neurosteroid status	Upper third of age range
Fasting Insulin	Metabolic aging; insulin suppresses GH	2 to 6 uIU/mL
hs-CRP	Inflammaging marker	Below 1.0 mg/L
Free T3	Thyroid-driven metabolic rate	Upper third of range
Total/Free Testosterone	Anabolic hormone status (men)	Age-optimized upper range
Vitamin D	Immune, bone, and hormonal cofactor	60 to 80 ng/mL

6. Symptoms Associated With Low GH Axis Function

7. What Causes Low GH Production

• Age-related somatopause: GH secretion declines approximately 14% per decade after age 30; by age 60 most adults produce 75% less GH than at age 25. This is the most common cause of low GH axis function
• Poor sleep architecture: the largest GH pulses occur during stage N3 deep sleep. Insufficient or fragmented sleep directly reduces total daily GH production. Sleep apnea, alcohol before bed, and irregular sleep schedules are significant contributors
• Hyperinsulinemia and insulin resistance: elevated fasting insulin directly suppresses GH release from the pituitary. Insulin resistance is one of the most consequential and most modifiable suppressors of GH production
• Excess visceral fat: visceral adipose tissue produces free fatty acids and inflammatory adipokines that suppress hypothalamic GHRH release and pituitary GH secretion. This creates a vicious cycle: low GH promotes visceral fat gain, and visceral fat further suppresses GH
• Sedentary lifestyle: high-intensity exercise (resistance training, HIIT) is a potent GH stimulus. Physical inactivity removes this stimulus entirely
• Chronic stress and cortisol elevation: sustained cortisol elevation suppresses GH release through hypothalamic and pituitary mechanisms
• Pituitary pathology: pituitary adenomas, head trauma, cranial radiation, and inflammatory conditions (lymphocytic hypophysitis) can directly impair somatotroph function
• Chronic inflammation: elevated hs-CRP and inflammatory cytokines suppress GH release and impair GH receptor sensitivity in target tissues

8. How to Optimize GH Axis Function

9. Related Lab Tests

10. When Testing Is Recommended

• Adults over 40 with declining body composition (loss of lean mass, increasing visceral fat) despite consistent exercise and dietary effort
• Patients with persistent fatigue, poor exercise recovery, and declining physical performance not explained by thyroid or metabolic causes
• Comprehensive longevity and biological age assessment (IGF-1 as part of an aging biomarker panel alongside DHEA-S, fasting insulin, hs-CRP)
• History of pituitary pathology, head trauma, or cranial radiation
• Patients being evaluated for or currently on GH secretagogue peptide therapy (monitoring response through IGF-1)
• Unexplained osteopenia or osteoporosis in younger adults
• Cardiovascular risk assessment when lipid elevation does not respond to standard interventions
• Evaluate alongside body composition analysis (DEXA), sleep assessment, and exercise capacity testing for complete GH axis clinical picture

11. Clinical Perspective

12. Frequently Asked Questions