Growth hormone and IGF-1 are closely linked — so closely that they’re often discussed as a single system called the GH/IGF-1 axis. But they are distinct molecules with different structures, different receptors, different tissue sources, and different biological effects. Understanding how the axis functions — and where the two molecules diverge — is essential for interpreting HGH research intelligently.
The Basic Relationship
The simplest version of the GH/IGF-1 axis is: growth hormone is released from the pituitary → travels to the liver → stimulates the liver to produce IGF-1 → IGF-1 circulates and drives growth and anabolic effects. This is the core hepatic model, and it captures the dominant pathway for IGF-1 production.
But the relationship is more nuanced than this linear model suggests. IGF-1 is also produced locally in multiple other tissues in response to GH — muscle, bone, cartilage, fat, brain, kidney — and some of this local IGF-1 acts within the tissue where it’s produced (paracrine and autocrine signalling) rather than entering the circulation. The liver dominates circulating IGF-1 levels, but local IGF-1 production may be more relevant for some tissue-specific effects.
IGF-1: Structure and Receptor
| Item | Detail |
| Full name | Insulin-like Growth Factor 1 (formerly Somatomedin C) |
| Structure | 70 amino acids, single chain; structurally similar to insulin (~50% sequence homology) |
| Primary production site | Liver (hepatocytes) — accounts for ~75% of circulating IGF-1 |
| Receptor | IGF-1 receptor (IGF-1R) — receptor tyrosine kinase, distinct from GH receptor |
| Signalling pathways | PI3K/Akt (survival, growth), MAPK/ERK (proliferation, differentiation) |
| Binding proteins | 6 IGFBPs regulate IGF-1 bioavailability in circulation |
| Half-life | ~12-15 hours (in IGFBP complex); much shorter as free IGF-1 |
How GH Drives IGF-1 Production in the Liver
When growth hormone reaches the liver, it binds to the GH receptor on hepatocytes and activates the JAK2/STAT5 signalling cascade. Activated STAT5 translocates to the nucleus and drives transcription of the IGF-1 gene — a direct GH-responsive gene. The resulting IGF-1 protein is secreted into the portal and then systemic circulation, where it circulates bound to insulin-like growth factor binding proteins (IGFBPs) that regulate its availability and half-life.
This liver-centred model means that circulating IGF-1 levels reflect the recent history of hepatic GH stimulation — which is why serum IGF-1 is used as a proxy for overall GH axis activity. A single HGH measurement captures only one point in the pulsatile secretion pattern; IGF-1 provides an integrated view of recent GH activity.
Direct GH Effects vs IGF-1-Mediated Effects
One of the most important distinctions in the GH/IGF-1 axis is separating what growth hormone does directly from what it does via IGF-1. Research using selective blockade of each component has helped map this:
| Item | Detail |
| Longitudinal bone growth | Primarily IGF-1 mediated (via growth plate chondrocytes) |
| Lipolysis in adipose tissue | Primarily direct GH effect (via GH receptor on adipocytes) |
| Muscle protein synthesis | Both direct and IGF-1 mediated |
| Liver glucose production | Direct GH effect |
| Cartilage synthesis | Primarily IGF-1 mediated |
| Insulin resistance effect | Direct GH effect (GH antagonises insulin signalling) |
| Cell survival signalling | Primarily IGF-1 mediated (PI3K/Akt) |
Feedback Regulation: Closing the Loop
The GH/IGF-1 axis operates with negative feedback at multiple levels. IGF-1 produced in the liver and other tissues circulates back to the pituitary and hypothalamus, where it suppresses GH secretion: at the pituitary, IGF-1 directly reduces somatotroph responsiveness to GHRH; at the hypothalamus, IGF-1 stimulates somatostatin release and reduces GHRH neuron activity.
This feedback means the axis is self-regulating within normal physiological ranges. Excessive GH stimulation produces elevated IGF-1, which feeds back to dampen further GH release. This regulatory loop is maintained in GHRH analogue and GHRP research, since these compounds stimulate endogenous production rather than bypassing the axis entirely.
IGFBPs: The Regulatory Layer
IGF-1 circulates almost entirely bound to one of six insulin-like growth factor binding proteins (IGFBP1-6). These binding proteins modulate IGF-1’s activity in complex ways: they extend IGF-1’s half-life by protecting it from degradation; they create a circulating reservoir that buffers acute changes in IGF-1 production; and in some contexts they inhibit IGF-1 receptor binding (reducing bioavailability), while in others they concentrate IGF-1 near target tissues (enhancing local activity).
IGFBP-3 is the dominant circulating binding protein, carrying the majority of serum IGF-1. Changes in IGFBP levels — which themselves are regulated by GH, nutrition, and other factors — can alter IGF-1 bioavailability independently of IGF-1 production. This adds a layer of regulatory complexity that purely measuring total serum IGF-1 doesn’t capture.
GH and IGF-1 are not the same thing measured differently — they are distinct molecules with distinct direct effects, connected by a regulated production axis. Research findings attributed to ‘GH/IGF-1 axis activity’ may reflect the action of either molecule, or both, depending on the tissue and the measured outcome.