Human Growth
HormoneSomatropin — Research Reference by QSC Peptides

Human Growth Hormone (HGH), or somatropin, is a 191-amino-acid, single-chain polypeptide synthesized and secreted by somatotroph cells in the anterior pituitary gland. Its secretion is pulsatile, regulated by hypothalamic GHRH (stimulatory) and somatostatin (inhibitory). QSC Peptides offers recombinant research-grade somatropin identical to the endogenous 191-aa sequence.

HGH binds the growth hormone receptor (GHR), a transmembrane class I cytokine receptor, as a 1:2 complex — one HGH molecule binds two GHR subunits sequentially via binding site 1 (high-affinity, ~1 nM Kd) and site 2 (lower affinity). Receptor dimerization transactivates the receptor-associated tyrosine kinase JAK2, which phosphorylates STAT5. Phospho-STAT5 dimerizes, translocates to the nucleus, and drives transcription of target genes including IGF-1, IGFBP-3, and acid-labile subunit (ALS).

Direct vs. IGF-1-Mediated Effects

HGH exerts both direct effects and indirect effects mediated through circulating IGF-1:

• Direct lipolysis: HGH activates hormone-sensitive lipase in adipocytes, mobilizing triglycerides to free fatty acids
• Direct insulin antagonism: HGH induces insulin resistance by inhibiting insulin receptor substrate-1 (IRS-1) signaling
• IGF-1-mediated anabolism: Liver-derived IGF-1 binds IGF-1R to activate PI3K-Akt-mTOR protein synthesis pathways
• IGF-1-mediated mitogenesis: Ras-MAPK-ERK1/2 proliferative signaling in target tissues

Pulsatile Secretion & Somatopause

Endogenous GH is secreted in 4–8 discrete pulses per 24 hours, with peak amplitude during slow-wave sleep. GH secretory amplitude declines ~14% per decade from the third decade onward (somatopause), with a corresponding fall in IGF-1. This decline forms the basis for GH axis research using QSC Peptides somatropin and GHRH analogs.

IU to mg Conversion

Per WHO International Standard: 1 mg somatropin ≈ 3 IU. A 10 IU vial contains ~3.33 mg; a 36 IU vial contains ~12 mg. QSC Peptides labels all HGH products in IU per vial.