HGH Research Guide: Human Growth Hormone and the Somatotropic Axis

• Recombinant HGH is somatropin — a single-chain protein of 191 amino acids, average molecular weight ≈22,125 Da (≈22.1 kDa), molecular formula C990H1528N262O300S7, structurally identical to the 22 kDa growth hormone of the anterior pituitary.
• It signals through the growth hormone receptor (GHR), a class I cytokine receptor with no intrinsic kinase; ligand binding activates JAK2, which phosphorylates STAT5 to drive transcription of target genes including IGF-1.
• Many of growth hormone's effects are mediated by IGF-1 (somatomedin C), a separate 70-amino-acid, ≈7.6 kDa peptide that acts on the IGF-1 receptor (a receptor tyrosine kinase) — a distinct molecule and receptor class from HGH.
• Body composition is the most studied endpoint: in the literature GH drives lipolysis and fat mobilisation (preferentially visceral), nitrogen retention and protein anabolism, with part of the early lean-mass change reflecting extracellular fluid.
• The largest GH secretory pulse is tied to slow-wave sleep; in men roughly 70% of daily GH output occurs during early sleep, and both slow-wave sleep and the nocturnal GH pulse decline with age.
• HGH is short-lived in circulation: intravenous elimination half-life ≈14–19 min (diurnally variable), while subcutaneous administration is absorption-rate-limited and shows a longer apparent half-life (~3 h in studied subjects).
• HGH differs from GH secretagogue peptides (e.g. CJC-1295, Ipamorelin): HGH is the hormone itself, whereas secretagogues act upstream to stimulate the body's own pulsatile GH release.
• Research-use-only framing applies throughout. Endogenous GH physiology is described to characterise the molecule; it is not a claim about how a research vial behaves or should be used. RetaLABS supplies HGH (somatropin), not IGF-1.

Human Growth Hormone (HGH), also known as somatotropin, is a 191-amino-acid single-chain polypeptide hormone secreted by somatotroph cells in the anterior pituitary gland. Recombinant HGH (rHGH) produced via recombinant DNA technology in E. coli or mammalian expression systems is structurally identical to pituitary-derived HGH and has been the subject of extensive pharmacological research since its introduction in the mid-1980s. The recombinant 191-amino-acid molecule supplied in research vials is the 22 kDa form; a naturally occurring ~20 kDa isoform (lacking residues 32–46) also exists in human plasma but is not the form supplied here.

Research-use-only: This guide describes the somatropin molecule and the surrounding growth-hormone physiology for Australian researchers. It is provided for laboratory research use only and is not medical advice, a treatment recommendation, or a dosing guide. Endogenous growth-hormone physiology is described here to characterise the molecule — it is not a claim about how a research vial behaves or should be used.

Endogenous GH secretion follows a pulsatile, diurnal pattern regulated by two hypothalamic neuropeptides: Growth Hormone-Releasing Hormone (GHRH), which stimulates GH release, and somatostatin, which inhibits it. GH secretion peaks during slow-wave sleep and is also influenced by exercise, fasting, and hypoglycaemia. It is useful to keep two things separate throughout this page: endogenous GH — the hormone the pituitary releases in pulses, whose physiology (receptor signalling, half-life, secretory timing) is well characterised in the literature — and the recombinant research compound, the same 191-amino-acid protein supplied in a vial for laboratory work. For the full chemistry, sequence, mass and registry identifiers see the HGH molecular profile.

RetaLABS stocks research-grade HGH 100IU (10 × 10IU vials), shipped Express Post to researchers across Australia.

The verified molecular identifiers for somatropin are summarised below. For the complete reference table — including the amino-acid sequence, disulfide bond positions and the full identifier set — see the HGH molecular profile.

GH exerts its effects through two primary mechanisms: direct receptor-mediated actions on target tissues, and indirect effects via Insulin-like Growth Factor 1 (IGF-1) produced primarily in the liver in response to GH stimulation.

The GH receptor (GHR) is a class I cytokine-receptor superfamily member that signals via JAK2/STAT5 phosphorylation upon GH binding — it has no intrinsic kinase activity of its own. Direct GH effects include lipolysis in adipose tissue, protein synthesis in muscle, and gluconeogenesis in the liver. IGF-1, by contrast, is a separate molecule: a 70-amino-acid, ≈7.6 kDa peptide (also called somatomedin C) that signals through the IGF-1 receptor (IGF-1R), a receptor tyrosine kinase activating PI3K/Akt and Ras/MAPK–ERK pathways. This receptor-class distinction — cytokine receptor for GH versus receptor tyrosine kinase for IGF-1 — is the central teaching point of the axis. The somatomedin hypothesis holds that GH stimulates hepatic IGF-1, and that IGF-1 mediates many of growth hormone's growth and anabolic effects, while certain GH actions (notably lipolysis and antagonism of insulin) are direct and IGF-1-independent.

The GH/IGF-1 axis exhibits negative feedback: IGF-1 suppresses both hypothalamic GHRH secretion and pituitary GH secretion, establishing a regulated homeostatic loop. Because GH itself is short-lived in circulation while circulating IGF-1 is largely carried in a long-half-life IGF-1/IGFBP-3/ALS ternary complex, IGF-1 is frequently measured as a stable surrogate biomarker for GH-axis activity in both preclinical and clinical study designs. RetaLABS supplies HGH (somatropin) and does not sell IGF-1; the IGF-1 material here is comparative and educational. For the full side-by-side molecular comparison see HGH vs IGF-1, and for the chemistry of the somatropin molecule itself see the HGH molecular profile.

Growth hormone has been examined across several research domains. Each of the deeper topics below has its own dedicated spoke article; the summaries here are deliberately concise overviews that link out for full detail.

Body composition

Body composition is the most extensively studied endpoint in GH research. In the literature, GH stimulates lipolysis and fat mobilisation — the most prominent metabolic effect of GH — with a preferential reduction of visceral (central) adipose tissue over subcutaneous fat. GH also drives nitrogen retention and protein anabolism (chiefly by stimulating protein synthesis), which is reflected as an increase in lean body mass in controlled studies of GH-deficient adults. An important nuance is that part of the early lean-mass change reflects extracellular fluid (sodium and water retention) rather than solid tissue, so early "lean mass" gain should not be equated with muscle. Effects in GH-sufficient individuals differ substantially from those in GH-deficiency states. For the full evidence base, study-specific magnitudes and the GH-direct versus IGF-1-mediated breakdown, see the HGH and body composition research.

Metabolic and cardiovascular

Beyond body composition, GH/IGF-1 axis research has examined metabolic and cardiovascular profiles, particularly in adult GH deficiency (AGHD), which is associated with increased cardiovascular morbidity and an adverse lipid profile. GH is a counter-regulatory hormone that antagonises insulin action — supraphysiological GH exposure increases insulin resistance and hepatic glucose output, partly via the rise in free fatty acids from lipolysis. This diabetogenic potential is relevant to research protocol design wherever glucose homeostasis is a primary endpoint, and it contrasts with IGF-1, which has insulin-like actions.

Sleep and the nocturnal GH pulse

Endogenous GH secretion is tightly coupled to sleep architecture. The largest GH secretory pulse is typically associated with the onset of slow-wave (deep NREM) sleep, producing a nocturnal surge shortly after sleep onset; in men, roughly 70% of daily GH output occurs during early sleep. The amount of slow-wave sleep is quantitatively associated with the amount of GH secreted, and both slow-wave sleep and the nocturnal GH pulse decline with age beginning in early-to-mid adulthood. This is endogenous secretory physiology, not a property of any research compound. For the mechanism (GHRH stimulation during somatostatin withdrawal), the age-related figures and the full citation set, see the HGH and sleep research.

In the literature, the intravenous elimination half-life of the 22 kDa GH molecule is short and diurnally variable — approximately 14–19 minutes (shorter in the morning, longer in the evening). Subcutaneous administration behaves differently: it is absorption-rate-limited, so the apparent half-life is governed by the slow absorption from the injection site rather than by elimination (flip-flop kinetics), giving a longer apparent half-life — on the order of ~3 hours in studied subjects. GH is cleared by both renal (glomerular filtration and tubular degradation) and receptor-mediated routes, and a substantial fraction of circulating GH is carried bound to growth hormone binding protein, which buffers the free pool. Because GH is short-lived while it drives IGF-1 — itself carried in a long-half-life ternary complex (~16 h or more) — IGF-1 is often used as a stable readout of GH-axis activity. These parameters describe endogenous GH and clinically administered somatropin in studied subjects; they are not claims about how a research vial behaves. For the full PK breakdown see the HGH pharmacokinetics reference.

A common point of confusion is the difference between HGH and GH secretagogue peptides. HGH (somatropin) is the growth hormone molecule itself — the 191-amino-acid protein that binds the GH receptor directly. GH secretagogues such as CJC-1295 (a GHRH analogue) and Ipamorelin (a ghrelin-receptor / GH-secretagogue-receptor agonist) act one step upstream: rather than being the hormone, they stimulate the pituitary to release its own GH, working with the body's pulsatile secretory machinery. The two approaches are therefore mechanistically distinct — direct hormone versus endogenous-release stimulus — which matters for how each is framed in research. For the full comparison, including the GHRH and ghrelin-pathway mechanisms, see HGH vs CJC-1295/Ipamorelin.

Adult Growth Hormone Deficiency (AGHD) is the best-characterised clinical model for GH research. It arises from pituitary pathology, hypothalamic damage, or as a consequence of treatment for pituitary tumours, and produces a metabolic phenotype including increased visceral adiposity, reduced muscle mass, dyslipidaemia, and reduced bone density.

International consensus statements and guidance provide established diagnostic and treatment parameters for AGHD that serve as useful reference frameworks for researchers designing GH intervention studies. Paediatric GH deficiency models (linear growth failure) represent a distinct research area with different endpoint considerations — height velocity, bone age advancement, and final adult height prediction. These GH-deficiency findings should be kept conceptually separate from the research compound RetaLABS supplies: the science concerns the biology of the endogenous hormone as studied in deficient populations, not product performance.

In Australia, growth hormone (somatropin) supplied as a registered therapeutic good is a Schedule 4 (Prescription Only Medicine) substance, regulated by the Therapeutic Goods Administration (TGA) under the Therapeutic Goods Act 1989 (Cth) and the Poisons Standard. The operative concept in that framework is intended and represented use: a substance falls under therapeutic-goods jurisdiction when it is presented or likely to be regarded as suitable for therapeutic use in humans.

Material supplied, labelled and documented strictly for in-vitro laboratory research — and not represented as suitable for human therapeutic use — occupies a different regulatory space from approved therapeutic goods. RetaLABS supplies HGH (somatropin) on a research-use-only basis: it is not a medicine, supplement or food, carries no therapeutic claims, and is intended for laboratory research only. This is general information, not legal advice. For the broader regulatory framework see the research peptides legal status in Australia guide.

RetaLABS HGH 100IU is supplied as 10 × 10IU lyophilised vials and ships Express Post Australia-wide within 1–2 business days of confirmed payment. Payment via Bitcoin, Litecoin, or Monero — discreet packaging as standard.

General handling notes for the lyophilised research compound:

• Reconstitute with bacteriostatic water for multi-dose vials; sterile water for single-use only
• Inject the water slowly along the vial wall — do not inject directly onto the powder
• Gently swirl until dissolved — do not shake or vortex; the protein is sensitive to mechanical degradation
• Store lyophilised vials at 2–8°C (refrigerator) — unlike most peptides, lyophilised HGH does not require −20°C storage
• Keep reconstituted solution at 2–8°C and protect from light; avoid temperature cycling

The reconstitution calculator helps work out concentrations for laboratory preparation, and the Peptide Reconstitution & Storage Guide covers general protocol notes. For laboratory research use only — no dosing or administration guidance is provided.

Somatropin / somatotropin

Recombinant human growth hormone; a single-chain 191-amino-acid protein (≈22.1 kDa). "Somatotropin" is the UniProt protein name for the same molecule.

GHR (growth hormone receptor)

The class I cytokine receptor that GH binds; it has no intrinsic kinase and signals through associated JAK2.

IGF-1

Insulin-like growth factor 1 (somatomedin C); a 70-amino-acid, ≈7.6 kDa peptide produced largely in the liver in response to GH that mediates many of GH's anabolic effects via the IGF-1 receptor.

IGFBP

IGF binding protein; one of six principal high-affinity carrier proteins that bind circulating IGF-1 and modulate its half-life, transport and bioavailability.

JAK2

Janus kinase 2; the receptor-associated kinase activated when GH binds GHR, which then phosphorylates STAT proteins.

STAT5

Signal transducer and activator of transcription 5; phosphorylated by JAK2 downstream of GHR, it drives transcription of GH target genes including IGF-1.

GH secretagogue

A compound (e.g. a GHRH analogue or ghrelin-receptor agonist) that stimulates the pituitary to release its own GH, acting upstream of the hormone rather than being the hormone.

Somatomedin

Historical name for the GH-dependent growth-mediating factors; somatomedin C is IGF-1, the basis of the "somatomedin hypothesis."

Recombinant

Produced by recombinant DNA technology in an expression system (e.g. E. coli or mammalian cells), yielding a protein structurally identical to the native human sequence.

Primary literature underpinning the research summarised on this page:

• Le Roith D, Bondy C, Yakar S, Liu JL, Butler A. The somatomedin hypothesis: 2001. Endocr Rev. 2001;22(1):53–74. PMID 11159816.
• Brooks AJ, Waters MJ. The growth hormone receptor: mechanism of activation and clinical implications. Nat Rev Endocrinol. 2010;6(9):515–525. PMID 20664532.
• Waters MJ, Brooks AJ. JAK2 activation by growth hormone and other cytokines. Biochem J. 2015;466(1):1–11. PMID 25656053.
• Møller N, Jørgensen JOL. Effects of growth hormone on glucose, lipid, and protein metabolism in human subjects. Endocr Rev. 2009;30(2):152–177. PMID 19240267.
• Bartke A, Sun LY, Longo V. Somatotropic signaling: trade-offs between growth, reproductive development, and longevity. Physiol Rev. 2013;93(2):571–598. PMID 23589828.
• Carroll PV, Christ ER, Bengtsson BÅ, et al. Growth hormone deficiency in adulthood and the effects of GH replacement: a review. J Clin Endocrinol Metab. 1998;83(2):382–395. PMID 9467546.
• Giovannini L, Tirabassi G, Muscogiuri G, Di Somma C, Colao A, Balercia G. Impact of adult growth hormone deficiency on metabolic profile and cardiovascular risk. Endocr J. 2015;62(12):1037–1048. PMID 26300280.

Continue across the HGH research cluster: HGH molecular profile, HGH vs CJC-1295/Ipamorelin, HGH vs IGF-1, HGH and body composition, HGH and sleep, and the HGH pharmacokinetics. Research-grade vials: HGH 100IU kit, HGH 240IU kit, and HGH 400IU kit.