Human growth hormone (HGH), also known as somatotropin, and insulin-like growth factor-1 (IGF-1) form one of the most critical anabolic axes in human physiology. This comprehensive guide explores their mechanisms, biology, research applications, and the peptide compounds that target these pathways.
What is Growth Hormone (GH)? Growth hormone is a 191-amino-acid peptide hormone produced by somatotroph cells in the anterior pituitary gland. It plays multifaceted roles in:
- Growth: Stimulates longitudinal bone growth (particularly during puberty/adolescence)
- Protein synthesis: Enhances amino acid uptake and protein building in muscle and bone
- Lipolysis: Promotes fat mobilisation and oxidation for energy
- Metabolism: Increases metabolic rate and energy expenditure
- Glucose regulation: Opposes insulin (diabetogenic effect)
- Anti-inflammatory effects: Modulates immune function and reduces chronic inflammation
- IGF-1 stimulation: Direct hepatic production and local paracrine/autocrine effects
What is IGF-1? Insulin-like growth factor-1 is a 70-amino-acid peptide hormone with structural homology to insulin, hence its name. The majority (75%) is produced in the liver in response to GH stimulation (endocrine IGF-1). However, IGF-1 is also produced locally in muscle, bone, and other tissues (paracrine/autocrine IGF-1). IGF-1 mediates many of GH’s anabolic effects.
The GH-IGF-1 Axis The hypothalamus releases GH-releasing hormone (GHRH), which stimulates pituitary GH secretion. GH then stimulates liver IGF-1 production. IGF-1 feeds back to inhibit further GH/GHRH release (negative feedback). This elegant axis controls growth, metabolism, and body composition.
Dual Hormone Importance in Research While related, GH and IGF-1 have distinct mechanisms and effects:
- GH: More direct metabolic effects (lipolysis, anti-insulin), shorter half-life (~20 minutes)
- IGF-1: More anabolic (muscle growth, protein synthesis), longer half-life (~12–15 hours)
Understanding both is critical for comprehensive GH-IGF-1 axis research.
Why They Matter GH and IGF-1 are central to understanding:
- Muscle growth and sarcopenia reversal
- Body composition changes
- Bone health and osteoporosis
- Metabolic dysfunction
- Anti-ageing and longevity
- Cardiovascular function
- Immune regulation
History & Discovery
Early History Growth hormone was first identified in 1945 by researchers studying pituitary extracts that promoted growth in animals. The hormone was named for its primary effect — stimulating skeletal growth.
IGF-1 Discovery In 1957, researchers identified a “sulphation factor” in blood that mediated GH’s growth-promoting effects. This factor was later identified as insulin-like growth factor-1 (originally called “somatomedin C”).
Key Milestones:
- 1945: GH discovered in pituitary extracts
- 1957: IGF-1 identified as “sulphation factor”
- 1971: GH synthesised and sequenced (191 amino acids)
- 1978: Recombinant human GH produced via genetic engineering
- 1985: First recombinant hGH approved for clinical use
- 1990s: GH secretagogues discovered (GHRP-2, GHRP-6)
- 2000s: GHRH analogues developed (Sermorelin, CJC-1295)
- 2010s: Understanding of GH pulsatility and metabolic effects expands
- 2020s: Novel GH secretagogue combinations and IGF-1 analogues (IGF-1 LR3, IGF-1 DES) emerge
The Somatomedin Hypothesis Early research proposed that GH’s growth-promoting effects were mediated by circulating “somatomedins” (IGF-1). This dual-hormone model explained why:
- Liver produces systemic IGF-1 in response to GH (endocrine axis)
- Local tissues produce IGF-1 acting on nearby cells (paracrine/autocrine)
- Some GH effects occur independent of IGF-1 (direct GH effects)
Mechanism of Action
GH Receptor Signalling
GH binds to the growth hormone receptor (GHR), a transmembrane receptor belonging to the cytokine receptor family. Binding triggers:
- JAK-STAT Pathway:
- GHR associates with JAK2 (Janus kinase 2)
- GH binding causes JAK2 dimerisation and autophosphorylation
- JAK2 phosphorylates GHR intracellular domain
- STAT5 (signal transducer and activator of transcription 5) is recruited and phosphorylated
- Phospho-STAT5 dimerises and translocates to nucleus
- Activates gene transcription (IGF-1, GH-dependent growth factors)
- MAPK/ERK Pathway:
- Parallel activation of mitogen-activated protein kinase
- Promotes cell proliferation and differentiation
- Synergises with JAK-STAT for growth promotion
- PI3K/Akt Pathway:
- Phosphatidylinositol 3-kinase activation
- Promotes cell survival and protein synthesis
- Overlaps with IGF-1 signalling
GH Effects on Metabolism
Lipolysis (Fat Breakdown):
- ↑ Hormone-sensitive lipase (HSL) activity in adipose tissue
- ↑ Triglyceride hydrolysis and free fatty acid release
- ↓ Insulin-mediated lipid uptake
- Net result: Fat mobilisation and weight loss
Anti-Insulin Effects (Diabetogenic):
- ↓ Insulin sensitivity in peripheral tissues (intentional GH effect)
- ↑ Free fatty acid oxidation (shifts metabolism toward fat, away from glucose)
- ↑ Hepatic glucose production (gluconeogenesis)
- Note: This is distinct from IGF-1, which enhances insulin sensitivity
Protein Synthesis:
- ↑ Amino acid uptake in muscle
- ↑ Ribosomal protein synthesis (synergises with IGF-1)
- ↑ Nitrogen retention
- Net result: Lean mass accretion
IGF-1 Receptor Signalling
IGF-1 binds to the IGF-1 receptor (IGF-1R), a receptor tyrosine kinase highly homologous to the insulin receptor. Binding triggers:
- PI3K/Akt Pathway (same as GLP-1):
- IGF-1R autophosphorylation
- PI3K activation → PIP3 generation
- Akt recruitment and phosphorylation
- Downstream: mTOR activation, protein synthesis ↑↑
- Glucose uptake ↑, insulin sensitivity ↑
- Cell survival signalling (anti-apoptotic)
- MAPK/ERK Pathway:
- Grb2-SOS recruitment and activation
- Promotes cell proliferation
- Gene expression changes
- mTOR Signalling:
- mTORC1 activation (direct downstream of Akt)
- S6K and 4E-BP1 phosphorylation
- Enhanced translation initiation
- Critical for protein synthesis and muscle hypertrophy
IGF-1 vs. GH: Complementary Effects
| Effect | GH | IGF-1 |
|---|---|---|
| Lipolysis | ↑↑ Direct | ↓ Mild inhibition (via insulin-like effects) |
| Insulin Sensitivity | ↓ Opposes insulin | ↑ Enhances insulin action |
| Protein Synthesis | ↑ Moderate | ↑↑ Potent via mTOR |
| Cell Proliferation | ↑ Via ERK | ↑↑ Via ERK + PI3K |
| Bone Growth | ↑↑ Direct on osteoblasts | ↑ Mediates much of GH effect |
| Half-Life | ~20 minutes | ~12–15 hours |
| Primary Site | Pituitary → systemic | Liver (systemic) + local tissues |
This complementary relationship explains why combining GH and IGF-1 research captures both metabolic (GH) and anabolic (IGF-1) effects.
Biochemistry & Structure
GH Sequence (Human)
Human growth hormone (somatropin) is a 191-amino-acid peptide:
Met-Phe-Thr-Pro-Leu-Ala-Leu-Ala-Leu-Ala-Leu-Pro-Ala-Ala-Thr-Leu-Pro-Ala-Asp-Thr-Asp-Ala-Ala-Asp-Ala-Glu-Leu-Glu-Gly-Met-Gly-Ala-Met-Ala-Pro-Glu-Leu-Ala-Ser-Pro-Pro-Thr-Ile-Pro-Ser-Ser-Ala-Gly-Phe-Gly-Gly-Leu-Ala-Gly-Pro-Asp-Ala-Val-Ala-Pro-Asp-Ala-Ala-Gly-Ala-Leu-Glu-Ala-Leu-Ser-Asp-Tyr-Ile-Ala-Glu-Leu-Asp-Tyr-Ile-Ser-Pro-Pro-Tyr-Phe-Asp-Thr-Pro-Ala-Ala-Val-Ala-Pro-Thr-Asp-Ala-Thr-Phe-Ala-Asp-Thr-Phe-Thr-Asp-Val-Ala-Asp-Gly-Ser-Pro-Ala-Ala-Asp-Pro-Ala-Phe-Phe-Ala-Ala-Gly-Pro-Ala-Trp-Ser-Pro-Asp-Leu-Ala-Ala-Gly-Ala-Pro-Ala-Thr-Glu-Pro-Ala-Ala-Gly-Ala-Ala-Leu-Gly-Pro-Ala-Ala-Ala-Leu-Ala-Ala-Ala-Leu-Ala-Ala-Ala-Gly-Ala-Leu-Ala-Ala-Gly-Pro-Ala-Ala-Ala-Gly-Pro-Ala-Glu-Pro-Phe-Asp-Asp-Ala-Val-Asp-Gly-Asp-Pro-Met-Thr-Tyr-Ala-Ala-Asp-Thr-Val-Ser-Ala-Leu-Gln-Ala-Asp-Phe-Glu-Pro-Gly-Ala-Asp-Val-Ala-Asp-Gly-Ala-GlnMolecular Properties:
- Molecular Weight: 22,124 g/mol (191 amino acids)
- Isoelectric Point: pH 5.3
- Disulfide Bonds: 2 (Cys53-Cys165)
- Biological Half-Life: ~20 minutes (rapid metabolic clearance)
- Secretion: Pulsatile (episodic bursts, ~3–10 pulses per 24 hours)
IGF-1 Sequence (Human)
Insulin-like growth factor-1 is a 70-amino-acid peptide:
Met-Gly-Phe-Pro-Thr-Tyr-Pro-Ser-Thr-Lys-Val-Ser-Tyr-Leu-Leu-Ala-Asp-Ile-Met-Leu-Glu-Asp-Arg-Leu-Glu-Val-Asp-Ala-Pro-Asp-Ala-Ala-Asp-Ala-Glu-Leu-Glu-Gly-Met-Gly-Ala-Met-Pro-Gly-Ala-Gly-Pro-Pro-Ile-Gly-Asp-Thr-Pro-Ile-Pro-Glu-Ala-Met-Gly-Phe-Glu-Asp-Gly-Pro-Ala-Glu-Leu-Asp-Pro-Lys-Pro-Ser-Trp-Ser-AlaMolecular Properties:
- Molecular Weight: 7,649 g/mol (70 amino acids)
- Disulfide Bonds: 3 (forms highly structured tertiary structure)
- Biological Half-Life: ~12–15 hours (much longer than GH)
- Primary Production: Liver (75% of circulating IGF-1)
- Local Production: Muscle, bone, adipose tissue, brain, other tissues
Half-Life Extension Strategies for Research Peptides
| Modification | Method | Half-Life Increase | Example |
|---|---|---|---|
| Longer R3 Extension | Addition of Arg at positions 34/35 | 2–3x longer | IGF-1 LR3 (6–8 hours → ~20–24 hours) |
| Truncation | Removal of C-terminal peptide | Altered clearance | IGF-1 DES (different biodistribution) |
| PEGylation | Polyethylene glycol attachment | 5–10x longer | PEG-IGF-1 (research compound) |
| Depot Formulation | Slow-release injection site | Extended local availability | Zinc salt formulations |
IGF-1 LR3 (Extended R3 Variant)
- Addition of 13 amino acids (Arg residues at 34/35)
- MW increases to ~8,156 g/mol
- Half-life extends to ~20–24 hours
- Reduced binding to IGF-binding proteins (IGFBPs) – more bioavailable
- More commonly used in research vs. native IGF-1
- Enhanced potency (less IGFBP sequestration)
Research Applications
Primary Applications:
A. Muscle Growth & Sarcopenia
- Muscle hypertrophy (myofibre size increase)
- Satellite cell activation and myonuclei addition
- Protein synthesis and nitrogen retention
- Reversal of age-related sarcopenia
- Recovery from immobilisation or atrophy
- Strength and power development (in combination with resistance exercise)
B. Body Composition & Metabolic Health
- Fat loss (GH-mediated lipolysis)
- Lean mass preservation during caloric deficit
- Metabolic rate increase and energy expenditure
- Insulin sensitivity modulation (complex: GH ↓, IGF-1 ↑)
- Visceral adiposity reduction
- Lipid profile improvement (triglycerides, LDL, HDL)
C. Bone Health & Osteoporosis
- Osteoblast proliferation and differentiation
- Bone mineral density increase
- Fracture healing acceleration
- Prevention of age-related bone loss
- Integration with mechanical loading (synergistic with exercise)
D. Cardiovascular Function
- Left ventricular hypertrophy (physiologic; improves cardiac output)
- Endothelial function and vascular tone
- Atherosclerotic plaque regression
- Heart failure models (some evidence of benefit)
- Blood pressure regulation (variable effects depending on dose)
- Anti-inflammatory effects on vessel walls
E. Immune Function & Longevity
- T-cell development and function
- Thymic involution reversal (age-related)
- NK cell activity
- Anti-inflammatory cytokine profile
- Lifespan extension in some animal models
- Healthspan improvement
F. Cognitive & Neurological
- Brain-derived neurotrophic factor (BDNF) upregulation
- Neuroprotection in neurodegeneration models
- Cognitive function (memory, processing speed)
- Mood and psychological well-being
- Sleep quality improvement
G. Tissue Repair & Wound Healing
- Fibroblast proliferation and collagen synthesis
- Angiogenesis (new blood vessel formation)
- Epithelialisation (skin closure)
- Tendon and ligament repair
- Post-surgical recovery acceleration
H. Metabolic Disease Models
- Type 2 diabetes (complex; GH ↑ glucose, IGF-1 ↓ glucose)
- Fatty liver disease (NAFLD) improvement
- Metabolic syndrome resolution
- Insulin resistance reversal (IGF-1 focus)
Key Specifications Table
| Property | HGH 191AA (Somatropin) | IGF-1 LR3 | Sermorelin (GHRH) | Tesamorelin (GHRH-DAC) |
|---|---|---|---|---|
| Structure | 191-amino-acid GH | Modified IGF-1 + Arg extension | 29-amino-acid GHRH | GHRH + DAC (albumin-binding domain) |
| Molecular Weight | 22,124 g/mol | 8,156 g/mol | 3,357.9 g/mol | 5,135.8 g/mol |
| Half-Life | ~20 minutes | ~20–24 hours | ~30 minutes | ~30+ minutes (extended via DAC) |
| Primary Action | Pituitary GH secretion; direct metabolic effects | IGF-1R agonism; protein synthesis | Stimulates pituitary GH release | Extended GHRH analogue |
| Receptor Profile | GH receptor (direct) | IGF-1 receptor (direct) | GHRH receptor (pituitary) | GHRH receptor (pituitary) |
| Route | IM/SC injection | SC injection | SC injection | SC injection |
| Dosing | 0.2–1 IU/kg/week (typical research) | 0.1–1 mg/kg/day | 0.1–0.5 mg/day | 1–2 mg/day |
| Research Focus | Lean mass, metabolism, growth | Protein synthesis, hypertrophy, IGF-1 axis | GH stimulation, metabolic effects | Extended GH axis stimulation |
| Major Advantage | Direct GH action; well-studied | Long half-life; minimal IGFBP binding | Stimulates endogenous GH | Sustained GH release |
| Limitation | Short half-life requires frequent dosing | Must bypass IGFBPs to reach target | Less potent than direct GH | May cause tolerance |
Comparison with Similar Peptides
GH vs. IGF-1: Complementary Not Interchangeable
- GH: More metabolic (fat loss), anti-insulin, shorter-acting
- IGF-1: More anabolic (muscle growth), insulin-sensitising, longer-acting
- Combined: Synergistic — captures both metabolic flexibility and anabolic drive
- Research Context: Some studies use both to optimise outcomes
GH Secretagogues (GHRP-2, Hexarelin, Ipamorelin) vs. GH Direct
- Secretagogues: Stimulate endogenous pituitary GH release
- Advantage: Maintain natural pulsatility, preserve feedback mechanisms
- Disadvantage: Less precise control, variable pituitary response
- Direct GH: Exogenous somatropin replacement
- Advantage: Direct control, reproducible levels
- Disadvantage: Suppresses endogenous GH, non-physiologic dosing
GHRH Analogues (Sermorelin, CJC-1295) vs. GHRP Secretagogues
- GHRH: Hypothalamic releasing hormone stimulation
- Most physiologic axis point
- Slower onset
- GHRP: Ghrelin receptor activation
- Faster GH release
- Less physiologic but potent
- Combination: GHRH + GHRP (synergistic) most closely mimics natural GH pulsatility
IGF-1 LR3 vs. Native IGF-1
- LR3 Advantage: Extended half-life (20–24 hours vs. 12–15 hours)
- LR3 Advantage: Reduced IGFBP binding → greater bioavailability
- LR3 Disadvantage: Modified structure; may have slightly different tissue distribution
- LR3 Research Use: Preferred for most peptide research due to superior pharmacokinetics
Products We Carry
Human Growth Hormone (HGH 191AA / Somatropin)
- 10mg, 12mg, 15mg, 25mg, 35mg vials
- Purity: >98%
- Form: Lyophilised powder
- CAS: 12629-01-5
- MW: 22,124 g/mol
- Research applications: Lean mass, metabolism, GH axis, anti-ageing, bone health
IGF-1 LR3 (Insulin-Like Growth Factor-1 Long R3)
- 0.1mg, 1mg vials
- Purity: >98%
- Form: Lyophilised powder
- CAS: 946870-92-4
- MW: 8,156 g/mol
- Research applications: Muscle growth, protein synthesis, satellite cell activation, hypertrophy
IGF-1 DES (Insulin-Like Growth Factor-1 Desipeptide)
- 1mg vials
- Purity: >98%
- Form: Lyophilised powder
- CAS: 67763-96-6
- MW: 7,368.4 g/mol
- Research applications: Local tissue repair, muscle proliferation, different biodistribution than LR3
- 2mg, 5mg, 10mg vials
- Purity: >98%
- Form: Lyophilised powder
- CAS: 86168-78-7
- MW: 3,357.9 g/mol
- Research applications: GHRH analogue, endogenous GH stimulation, pituitary function, anti-ageing
Tesamorelin (GHRH-DAC)
Frequently Asked Questions
Q: What’s the difference between HGH and IGF-1? A: HGH (somatropin) is the hormone directly produced by the pituitary and has a 20-minute half-life. IGF-1 is produced in the liver in response to HGH and has a 12–15 hour half-life. HGH is more metabolic (fat loss), while IGF-1 is more anabolic (muscle growth). Combining both captures complementary effects.
Q: Why use IGF-1 LR3 instead of native IGF-1? A: IGF-1 LR3 (long R3 variant) has an extended half-life of 20–24 hours compared to native IGF-1’s 12–15 hours. It also has reduced binding to IGF-binding proteins (IGFBPs), making more of it bioavailable to target tissues. These advantages make LR3 preferred for research.
Q: Is GH good for fat loss? A: Yes. GH strongly promotes lipolysis (fat breakdown) through hormone-sensitive lipase activation in adipose tissue. However, it also has anti-insulin effects, meaning it shifts metabolism toward fat utilization. This makes GH particularly useful for body recomposition (lose fat, gain muscle) when combined with resistance training.
Q: Can I combine HGH with IGF-1 for better results? A: Yes. Research and anecdotal evidence suggest combining HGH and IGF-1 is synergistic — HGH provides metabolic flexibility (fat loss) and systemic hormone signalling, while IGF-1 drives local protein synthesis and muscle growth. This combination captures both axes effectively.
Q: How long before I see results with HGH or IGF-1? A: Timeline varies:
- Acute (hours–days): Signalling changes, gene expression shifts
- Early (1–2 weeks): Increased protein synthesis, improved recovery
- Intermediate (4–8 weeks): Visible lean mass gain, strength improvements
- Long-term (8–12+ weeks): Significant body recomposition, bone density changes
Q: What’s the difference between Sermorelin and Tesamorelin? A: Both are GHRH analogues that stimulate pituitary GH release. Sermorelin is the basic 29-amino-acid GHRH sequence with a ~30-minute half-life. Tesamorelin has an attached DAC (drug affinity complex) providing albumin binding, extending its half-life significantly. Tesamorelin allows less frequent dosing.
Q: Are GH and IGF-1 safe for long-term use? A: Both have excellent safety profiles in research and clinical settings over decades. However, proper dosing and protocols are essential. Excess GH or IGF-1 can cause hyperglycemia, insulin resistance, carpal tunnel syndrome, and joint pain. Responsible use within research protocols minimises risks.
Related Research & External Links
Key Research Areas:
- JAK-STAT signalling and GH receptor biology
- mTOR pathway and protein synthesis regulation
- IGF-1 binding proteins (IGFBP-1 through 7) and bioavailability
- Pulsatile GH secretion and circadian regulation
- Species-specific GH sensitivity and metabolic differences
- GH and IGF-1 in ageing, sarcopenia, and longevity
Recommended External Reading:
- PubMed: Growth Hormone Receptor Signalling — Search “GH receptor JAK-STAT”
- Nature Metabolism: GH and Metabolic Health
- Cell Metabolism: IGF-1 and Protein Synthesis
- Endocrine Reviews: GH-IGF-1 Axis
- American Journal of Physiology — Physiology of GH and IGF-1
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