Growth Hormone (GH / Somatotropin)

Growth hormone (GH), also called somatotropin, is a 191-amino-acid polypeptide hormone synthesized and secreted by somatotroph cells in the anterior pituitary gland. It is the primary driver of linear growth during childhood and adolescence, and continues to regulate body composition, tissue repair, metabolism, and organ function throughout adulthood.

Structure and secretion

GH is released in a pulsatile pattern — large bursts interspersed with near-zero troughs. The largest pulse occurs during the first phase of slow-wave sleep (typically 60-90 minutes after sleep onset). Additional pulses occur during exercise, fasting, and stress. This pulsatile pattern is not cosmetic — the episodic exposure is necessary for GH to exert its full biological effects. Continuous GH exposure (as from exogenous GH administered by constant infusion) produces a different and less favorable response profile.

Secretion is controlled by two hypothalamic hormones:

• GHRH (Growth Hormone-Releasing Hormone) — a 44-amino-acid peptide that stimulates GH release
• Somatostatin (GHIH) — a 14-amino-acid peptide that inhibits GH release

The alternation of GHRH stimulation and somatostatin inhibition produces the pulsatile secretion pattern. A third regulator, ghrelin (produced primarily in the stomach), provides an additional stimulatory signal through the GHS-R1a receptor.

The GH-IGF-1 axis

GH exerts many of its effects indirectly through insulin-like growth factor 1 (IGF-1). When GH reaches the liver, it stimulates hepatic IGF-1 production. Circulating IGF-1 then mediates many of the growth-promoting and anabolic effects attributed to GH:

• Protein synthesis — IGF-1 activates the mTOR pathway, promoting muscle protein synthesis
• Bone growth — IGF-1 stimulates chondrocyte proliferation at the epiphyseal plate (in growing individuals) and osteoblast activity
• Fat metabolism — GH directly promotes lipolysis (fat breakdown) independent of IGF-1, mobilizing fatty acids for energy
• Tissue repair — IGF-1 and GH together accelerate wound healing, connective tissue synthesis, and organ maintenance

IGF-1 also provides negative feedback to the pituitary, suppressing further GH release when levels are sufficient. This creates a self-regulating loop.

Age-related GH decline

GH secretion peaks during puberty and begins declining in the early 20s. By age 60, total daily GH output is approximately 25% of peak adolescent levels — a phenomenon termed "somatopause." This decline correlates with:

• Increased visceral adiposity
• Decreased lean muscle mass (sarcopenia)
• Reduced bone mineral density
• Thinner, less elastic skin
• Slower wound healing
• Decreased exercise capacity

Whether this decline is a cause of aging or merely a marker remains debated. GH replacement therapy in elderly adults has shown mixed results — improvements in body composition but also increased side effects (edema, joint pain, insulin resistance, theoretical cancer concern).

The peptide connection

Most peptides in the growth hormone secretagogue (GHS) category work by stimulating the body's own GH production rather than replacing it with exogenous GH. This approach offers a theoretical advantage: it preserves the natural pulsatile release pattern and maintains the hypothalamic-pituitary feedback loop.

GHRH analogs (Sermorelin, CJC-1295) mimic GHRH, directly stimulating somatotroph cells to synthesize and release GH. Sermorelin is a 29-amino-acid fragment of GHRH (the first 29 residues are sufficient for full biological activity). CJC-1295 is a GHRH analog with a Drug Affinity Complex (DAC) that extends its half-life from minutes to days.

Ghrelin mimetics (Ipamorelin, GHRP-2, GHRP-6, Hexarelin) activate the GHS-R1a receptor, the same receptor that endogenous ghrelin uses. These peptides amplify GH pulses through a pathway complementary to GHRH. Ipamorelin is the most selective — it increases GH without significantly affecting cortisol, prolactin, or aldosterone.

Combination protocols (CJC-1295 + Ipamorelin) exploit the synergy between GHRH and ghrelin pathways. Simultaneous stimulation of both inputs produces a larger GH pulse than either alone, mimicking the physiological state where GHRH and ghrelin peaks coincide.

GH vs. secretagogues: the key distinction

Exogenous recombinant GH (somatropin) directly elevates circulating GH levels, bypassing pituitary control. GH secretagogues stimulate the pituitary to produce more of its own GH. The practical differences:

| Factor | Exogenous GH | GH Secretagogues |

|--------|-------------|-------------------|

| Release pattern | Supraphysiological, non-pulsatile (unless micro-dosed) | Amplified natural pulses, pulsatile |

| Feedback | Suppresses endogenous GH production | Maintains feedback regulation |

| IGF-1 elevation | Dose-dependent, can be excessive | Generally moderate, proportional |

| Side effects | Higher risk at therapeutic doses (edema, CTS, insulin resistance) | Generally milder at typical doses |

| Regulatory status | Prescription (approved for GH deficiency, wasting) | Research-only (most peptides) or approved (tesamorelin for HIV lipodystrophy) |

| Cost | Higher | Lower |

Clinical applications

GH itself is FDA-approved for:

• Pediatric GH deficiency
• Adult GH deficiency (confirmed by stimulation testing)
• Turner syndrome, Prader-Willi syndrome, chronic renal insufficiency
• HIV-associated wasting
• Short bowel syndrome

Tesamorelin (a GHRH analog) is FDA-approved specifically for reducing excess abdominal fat in HIV-associated lipodystrophy.

Sermorelin was previously FDA-approved for pediatric GH deficiency diagnosis and treatment but has been discontinued from the market as a pharmaceutical product. It remains available through compounding pharmacies.

Key terminology

• Somatotropin — another name for GH
• Somatotrophs — the pituitary cells that produce GH
• Somatopause — age-related decline in GH secretion
• Secretagogue — any substance that stimulates secretion of another substance (in this context, a substance that stimulates GH secretion)
• GHS-R1a — growth hormone secretagogue receptor type 1a, the ghrelin receptor