Growth hormone (GH, somatotropin) is a 191-amino-acid peptide synthesised and secreted by somatotroph cells of the anterior pituitary. The version known commercially and in research contexts as "HGH 191AA" refers to recombinant human growth hormone (rhGH) produced via recombinant DNA technology in E. coli or mammalian cell expression systems, with the identical primary sequence to pituitary-derived somatotropin. The "191AA" designation simply confirms the full-length, non-truncated molecule.
Why the molecular identity matters
Early growth hormone preparations (1950s–1980s) were extracted directly from human cadaveric pituitaries. This practice was discontinued globally after the discovery that some preparations transmitted Creutzfeldt-Jakob disease prions. Recombinant production, established in 1985, solved the safety problem entirely. Modern rhGH is biochemically identical to the natural molecule — same amino acid sequence, same three-dimensional fold, same receptor-binding kinetics — but produced under controlled pharmaceutical conditions without biological contamination risk.
Age-related GH decline: the physiology
GH secretion follows a pulsatile pattern driven by two hypothalamic hormones: growth hormone-releasing hormone (GHRH, stimulatory) and somatostatin (inhibitory). Peak secretion occurs during puberty and early adulthood, with the highest amplitudes during slow-wave sleep. After approximately age 30, IGF-1 — the primary downstream mediator of GH effects — declines at roughly 14% per decade.
This phenomenon was termed somatopause by researchers in the 1990s, by analogy with menopause. The functional consequences are well-documented:
- Progressive increase in adipose tissue (especially visceral)
- Decrease in lean body mass and muscle fibre cross-sectional area
- Reduced bone mineral density
- Altered lipid profiles (elevated LDL, reduced HDL)
- Changes in sleep architecture and subjective energy
GH/IGF-1 changes with ageing
| Age | IGF-1 (ng/mL) | Peak GH pulse (µg/L) | Daily GH secretion |
|---|---|---|---|
| 20–25 | 250–350 | 20–30 | ~500 µg/day |
| 35–40 | 180–250 | 10–18 | ~300 µg/day |
| 55–60 | 100–160 | 5–10 | ~140 µg/day |
| 70–75 | 60–100 | 2–5 | ~60 µg/day |
The landmark Rudman study and what followed
The scientific conversation around exogenous GH in ageing adults was opened by Daniel Rudman's 1990 paper in the New England Journal of Medicine. His trial enrolled 21 healthy men aged 61–81 with GH levels below the youthful range, randomised to 0.03 mg/kg three times weekly versus placebo for six months. The treatment group showed:
- 8.8% increase in lean body mass
- 14.4% decrease in fat mass
- 1.6% increase in lumbar spine bone density
- 7.1% increase in skin thickness
These results generated enormous scientific and public interest, and also significant over-interpretation. Rudman himself cautioned against viewing the findings as a reversal of ageing — the study was small, short-term, and measured body composition rather than functional or survival outcomes.
Approved clinical applications
Recombinant GH has well-established approved indications that provide important context for research use:
| Indication | Approval year | Key evidence |
|---|---|---|
| Paediatric GHD | 1985 | Robust RCT data; significant height gain |
| Adult GHD | 1996 | Improved body composition, QoL in deficient adults |
| HIV-associated wasting | 1996 | Lean mass preservation in catabolic states |
| Short bowel syndrome | 2003 | Nutrient absorption and body weight maintenance |
| Prader-Willi syndrome | 2000 | Body composition and muscle function improvement |
GH secretagogues: an alternative research paradigm
Rather than replacing GH exogenously, a growing body of research investigates compounds that stimulate endogenous GH secretion. This approach preserves the natural pulsatile pattern of release and avoids some complications of continuous exogenous GH (particularly IGF-1 oversaturation).
Key secretagogue classes studied:
GHRH analogues (e.g., CJC-1295, tesamorelin) — bind GHRH receptors on somatotrophs, increasing GH pulse amplitude. Tesamorelin is FDA-approved for HIV-associated lipodystrophy, providing strong clinical validation for the class.
Ghrelin mimetics / GHRPs (e.g., GHRP-6, GHRP-2, ipamorelin, hexarelin) — bind GHS-R1a receptors, synergising with GHRH to amplify GH release. Ipamorelin is notable for high selectivity with minimal cortisol or prolactin co-stimulation.
Combination protocols — the most studied research approach combines a GHRH analogue with a GHRP. The two mechanisms are synergistic: GHRH amplifies pulse amplitude, while ghrelin mimetics increase pulse frequency and suppress somatostatin. Together, the GH response can be 3–10× that of either agent alone.
GH secretagogue comparison
| Compound | Target | Half-life | Key characteristic |
|---|---|---|---|
| CJC-1295 | GHRH receptor | ~8 days (DAC form) | Sustained GHRH elevation |
| Ipamorelin | GHS-R1a | ~2 hours | High GH selectivity; minimal side effects |
| GHRP-6 | GHS-R1a | ~2 hours | Pronounced appetite stimulation (ghrelin effect) |
| Tesamorelin | GHRH receptor | ~26 min | FDA-approved; strong lipodystrophy evidence |
| MK-677 (ibutamoren) | GHS-R1a (oral) | ~24 hours | Only orally active GHS; ongoing RCTs |
Storage and analytical requirements for rhGH
Growth hormone is a protein therapeutic requiring careful handling:
- Lyophilised form — stable at −20 °C for extended periods; reconstitute with bacteriostatic water or manufacturer-specified diluent
- Reconstituted solution — store at 2–8 °C; use within 14–28 days; avoid agitation and temperature excursions
- Identity confirmation — ESI-MS should confirm the expected average mass of 22,124 Da; SDS-PAGE under reducing conditions shows a single band at approximately 22 kDa
- Bioactivity — cell-based assays (Nb2 lymphoma cell proliferation or STAT5 phosphorylation) are the standard for confirming GH receptor activity
- Aggregation — SEC-HPLC to confirm monomer content ≥95%; aggregated GH can be immunogenic