The GH/IGF-1 Axis Explained
Somatostatin and Growth Hormone
75% from the liver
The proportion of circulating IGF-1 produced by hepatocytes, making the liver the primary mediator of growth hormone's systemic effects.
Le Roith et al., Endocr Rev, 2001
Le Roith et al., Endocr Rev, 2001
View as imageGrowth hormone is a 191-amino acid peptide, but calling it "the growth hormone" is misleading. Growth hormone is a signal. The molecule that does most of the growing is insulin-like growth factor 1 (IGF-1), produced primarily by the liver in response to GH stimulation. The GH/IGF-1 axis is the complete signaling system: hypothalamus to pituitary to liver to target tissues and back again through negative feedback loops. Every peptide in this chain is a potential point of intervention, and understanding the axis is essential for understanding why GH secretagogues, somatostatin analogs, and recombinant GH therapy each work differently.
Key Takeaways
- Growth hormone acts on the liver to produce IGF-1, which mediates roughly 75% of GH's growth-promoting effects through endocrine, paracrine, and autocrine signaling (Le Roith et al., 2001)
- GH secretion is pulsatile, with 3-5 major pulses per day, controlled by the opposing actions of GHRH (stimulatory) and somatostatin (inhibitory) from the hypothalamus
- IGF-1 circulates bound to IGFBP-3 and ALS in a ternary complex with a half-life of approximately 20 hours, compared to free IGF-1's half-life of about 10 minutes
- CJC-1295, a long-acting GHRH analog, increased mean IGF-1 levels by 1.5-3 fold with sustained elevation for 6-14 days after a single injection (Sackmann-Sala et al., 2009)
- The axis declines with age: GH secretion drops approximately 14% per decade after age 30, a process called somatopause (Frutos et al., 2007)
- GH and IGF-1 have both overlapping and independent effects, with GH directly promoting lipolysis and insulin resistance while IGF-1 promotes glucose uptake and protein synthesis
The Signaling Cascade
The GH/IGF-1 axis operates as a four-level endocrine cascade with two levels of negative feedback.
Level 1: The hypothalamus
Two neuronal populations in the hypothalamus control GH release. Growth hormone-releasing hormone (GHRH) neurons in the arcuate nucleus stimulate GH secretion. Somatostatin neurons in the periventricular nucleus inhibit it. The alternating activity of these two populations produces the characteristic pulsatile pattern of GH secretion.[1]
GHRH is a 44-amino acid peptide. Its first 29 amino acids are sufficient for full biological activity, which is why sermorelin (GHRH 1-29) works as a GH-releasing agent. Somatostatin is a 14- or 28-amino acid peptide that inhibits not only GH but also TSH, insulin, glucagon, and multiple gastrointestinal hormones.
A third input comes from ghrelin and synthetic GH secretagogues (GHRPs), which act through a separate receptor (GHSR1a) on pituitary somatotrophs. Ghrelin amplifies the GHRH signal and partially opposes somatostatin's inhibition, which is why GHRP/GHRH combinations produce synergistic GH release.
Level 2: The anterior pituitary
Somatotroph cells in the anterior pituitary synthesize, store, and release GH in response to GHRH stimulation. GH is released in pulses, with the largest pulses occurring during slow-wave sleep. Adults produce 3-5 significant GH pulses per day, with total daily secretion of 0.5-1.0 mg. Between pulses, circulating GH levels are often undetectable (<0.1 ng/mL), which is why a single random GH blood test is clinically useless for diagnosing GH deficiency.[1]
Level 3: The liver
GH binds to GH receptors (GHR) on hepatocytes and activates the JAK2-STAT5 signaling pathway, which directly stimulates IGF-1 gene transcription. The liver produces approximately 75% of circulating IGF-1. GH also stimulates hepatic production of IGFBP-3 (IGF-binding protein 3) and ALS (acid-labile subunit).[2]
Intra-portal insulin is required for full hepatic IGF-1 production. In type 1 diabetes, where portal insulin delivery is absent (replaced by subcutaneous injection), hepatic IGF-1 production is impaired even when GH levels are normal or elevated. This explains the "GH resistance" observed in poorly controlled diabetes.
Level 4: Target tissues
IGF-1 reaches target tissues through the circulation (endocrine action) and is also produced locally by many tissues (paracrine/autocrine action). It acts through the IGF-1 receptor (IGF-1R), a tyrosine kinase receptor structurally similar to the insulin receptor.
Target tissue effects include:
- Bone growth plates: IGF-1 stimulates chondrocyte proliferation and differentiation, driving longitudinal bone growth in children
- Skeletal muscle: IGF-1 promotes protein synthesis, satellite cell activation, and myofiber hypertrophy
- Adipose tissue: GH directly promotes lipolysis (independent of IGF-1), while IGF-1 improves insulin sensitivity
- Brain: IGF-1 supports neuronal survival, synaptogenesis, and myelination
- Kidney: IGF-1 increases renal plasma flow and glomerular filtration rate
The Feedback Loops
Two negative feedback loops prevent runaway GH/IGF-1 signaling.
Long loop: Circulating IGF-1 feeds back to both the hypothalamus and the pituitary. At the hypothalamus, IGF-1 suppresses GHRH gene expression and stimulates somatostatin secretion. At the pituitary, IGF-1 directly inhibits GH gene transcription and GH secretion from somatotrophs.[1]
Short loop: GH itself feeds back to the hypothalamus to stimulate somatostatin release, creating a more rapid negative feedback independent of IGF-1.
These feedback loops are why exogenous recombinant GH (somatropin) suppresses endogenous GH secretion, while GH secretagogues like CJC-1295 work within the natural feedback system. GHRH analogs stimulate GH release but the resulting IGF-1 increase activates normal feedback, limiting the peak response. This distinction has practical consequences for the safety and efficacy profile of different GH-modifying interventions.
The IGF-1 Transport System
Free IGF-1 has a circulating half-life of about 10 minutes. This is too short for a hormone that needs to reach distant tissues. The solution: binding proteins.
More than 95% of circulating IGF-1 is bound in a ternary complex consisting of IGF-1, IGFBP-3, and ALS. This 150 kDa complex cannot cross capillary walls easily, creating a circulating reservoir with a half-life of approximately 20 hours. IGF-1 is released from the complex by proteolysis of IGFBP-3, primarily by matrix metalloproteinases and pregnancy-associated plasma protein-A (PAPP-A).[2]
Six IGF-binding proteins (IGFBP-1 through IGFBP-6) modulate IGF-1 bioavailability in different tissues and physiological states. IGFBP-1, for example, is rapidly regulated by insulin: after a meal, rising insulin suppresses IGFBP-1, increasing free IGF-1 levels. During fasting, IGFBP-1 rises, sequestering IGF-1 and reducing its growth-promoting effects.
Daniel et al. (2025) found that semaglutide and sitagliptin differentially affected circulating IGFBP levels over long-term treatment, suggesting that metabolic drugs can modulate IGF-1 bioavailability through binding protein regulation rather than through direct GH or IGF-1 changes.[3]
How Peptide Drugs Interact with the Axis
Different classes of GH-related peptides intervene at different points in the axis, producing distinct pharmacological profiles.
GHRH analogs (Level 1 intervention)
CJC-1295 is a modified GHRH analog with a drug affinity complex (DAC) that extends its half-life to 5-8 days. Sackmann-Sala et al. (2009) demonstrated that a single subcutaneous injection of CJC-1295 produced sustained GH/IGF-1 axis activation, with mean IGF-1 levels increasing 1.5-3 fold and remaining elevated for 6-14 days.[4]
GHRH analogs work within the natural pulsatile system: they increase the amplitude of GH pulses rather than creating continuous elevation. The negative feedback loops remain functional, which is why GHRH analogs produce more physiological GH/IGF-1 profiles compared to exogenous recombinant GH.
GH secretagogues (Level 1 intervention, different receptor)
GHRPs (GHRP-2, GHRP-6, hexarelin) and the oral secretagogue MK-677 act through the ghrelin receptor (GHSR1a). Bowers et al. (2004) showed that sustained GH secretagogue administration produced prolonged elevation of both pulsatile GH secretion and IGF-1 levels, with the response magnitude depending on the specific compound and dosing regimen.[5]
The synergy between GHRH and GHRP pathways is well established. Combined GHRH + GHRP administration produces GH release that exceeds the sum of individual responses, because GHRH directly activates somatotrophs while GHRPs reduce somatostatin inhibition and amplify the GHRH signal.
Somatostatin analogs (Level 1/2 intervention)
Octreotide, lanreotide, and pasireotide suppress GH secretion by mimicking somatostatin at pituitary somatostatin receptors. They are used clinically to treat acromegaly (GH excess) and neuroendocrine tumors. Their mechanism is the pharmacological amplification of the axis's natural brake.
Recombinant GH (Level 3 bypass)
Somatropin (recombinant human GH) bypasses the hypothalamic-pituitary regulation entirely. It increases IGF-1 directly by stimulating hepatic production. Because it suppresses endogenous GH secretion through feedback, the pituitary becomes quiescent during treatment. Stopping exogenous GH requires the pituitary to resume normal pulsatile secretion, which may take time.
The Somatopause: Age-Related Axis Decline
GH secretion declines approximately 14% per decade after age 30. By age 60, many adults have GH secretion rates comparable to GH-deficient patients. This age-related decline is termed the somatopause.[6]
The mechanism involves both increased somatostatin tone and decreased GHRH drive. Frutos et al. (2007) reviewed evidence that GH secretagogues could partially reverse age-related GH decline by overriding the increased somatostatin inhibition, offering a potential approach to the somatopause that works with the natural axis rather than replacing it.[6]
Whether reversing the somatopause produces clinical benefit is contested. Higher IGF-1 levels in older adults have been associated with both beneficial effects (muscle mass, bone density, cognitive function) and potential risks (cancer susceptibility). The axis evolved to decline with age, and the question of whether that decline is a deficiency or a protective adaptation remains open.
GH vs. IGF-1: Overlapping but Distinct Effects
A common misconception is that GH works entirely through IGF-1. Liver-specific IGF-1 gene deletion studies in mice demonstrated that eliminating 75% of circulating IGF-1 did not produce the severe growth retardation expected, because GH continued to act directly on tissues through GHR signaling.[2]
The direct effects of GH (independent of IGF-1) include:
- Lipolysis: GH directly activates hormone-sensitive lipase in adipocytes, promoting fat breakdown. This is why GH therapy reduces body fat even when IGF-1 levels are only modestly elevated.
- Insulin resistance: GH directly antagonizes insulin signaling in muscle and liver. This is a side effect of GH therapy and a feature of acromegaly.
- Diabetogenic effects: By promoting lipolysis and insulin resistance simultaneously, GH shifts metabolism toward fat oxidation and away from glucose utilization.
IGF-1, by contrast, is structurally similar to insulin and has insulin-like metabolic effects: it promotes glucose uptake, protein synthesis, and cell proliferation. The GH/IGF-1 axis therefore contains an internal metabolic tension. GH makes you insulin resistant. IGF-1 makes you insulin sensitive. The balance between the two determines the net metabolic outcome.
Van den Berghe et al. (1997) examined how critical illness disrupts this balance, finding that the somatotropic axis undergoes profound changes during severe illness, with GH resistance, low IGF-1, and altered IGFBP profiles, creating a catabolic state despite elevated GH levels.[7]
Novel Peptide Research in IGF-1 Biology
Beyond traditional GH-axis drugs, newer peptide research explores IGF-1 biology from different angles. Burgdorf et al. (2023) identified an IGFBP-2-derived peptide that promotes neuroplasticity and rescues cognitive deficits in animal models, suggesting that fragments of IGF-1 binding proteins have independent biological activity beyond their role as IGF-1 carriers.[8]
This finding reframes the IGF-binding proteins from passive transport molecules to active signaling peptides with their own receptor interactions and downstream effects, opening a new dimension of the GH/IGF-1 axis that is only beginning to be explored.
The Bottom Line
The GH/IGF-1 axis is a four-level endocrine cascade from hypothalamus to pituitary to liver to target tissues, regulated by two negative feedback loops. GH stimulates hepatic IGF-1 production, but GH and IGF-1 have distinct and sometimes opposing metabolic effects. Different peptide interventions (GHRH analogs, GH secretagogues, somatostatin analogs, recombinant GH) tap into different levels of the axis with different pharmacological profiles. The axis declines with age, and whether reversing that decline is beneficial or risky remains an active research question.