Somatropin is the recombinant DNA-derived form of human growth hormone (hGH), a 191-amino acid single-chain polypeptide secreted by somatotroph cells of the anterior pituitary gland. It is identical in amino acid sequence to endogenous human GH and has replaced cadaveric pituitary-derived GH since the mid-1980s.

Overview

Human growth hormone is the most abundant anterior pituitary hormone, secreted in pulsatile bursts with the largest peaks occurring during deep slow-wave sleep. Somatropin replicates these effects when administered exogenously. The GH-IGF-1 axis is central to its biological activity: GH binds to GH receptors (GHR) on hepatocytes, stimulating production and release of IGF-1, which mediates many of GH's anabolic and growth-promoting effects through endocrine, paracrine, and autocrine mechanisms.

Somatropin is FDA-approved under multiple brand names including Norditropin (Novo Nordisk), Genotropin (Pfizer), Humatrope (Eli Lilly), Saizen (EMD Serono), and Omnitrope (biosimilar). Approved indications include pediatric GH deficiency, adult GH deficiency, Turner syndrome, Prader-Willi syndrome, chronic renal insufficiency, short bowel syndrome, and AIDS-related wasting. Off-label use in anti-aging, sports performance, and body composition optimization remains controversial and is prohibited in competitive sports by WADA.

Unit conversion: 1 mg of somatropin ≈ 3 IU (International Units). Most clinical dosing references use mg, while some older protocols and non-pharmaceutical sources reference IU.

Mechanism of Action

Somatropin exerts its effects through both direct and indirect (IGF-1-mediated) pathways:

• GH receptor activation: Somatropin binds to homodimeric GH receptors (GHR) on target cells, activating the JAK2-STAT5 signaling cascade that regulates gene transcription for growth, metabolism, and differentiation (Herrington et al., 2000)
• IGF-1 stimulation: Hepatic GHR activation stimulates production and secretion of IGF-1 and IGFBP-3 into the circulation. IGF-1 mediates many of GH's anabolic effects on skeletal muscle, bone, and cartilage through IGF-1 receptor tyrosine kinase signaling
• Lipolysis: GH directly stimulates hormone-sensitive lipase activity in adipocytes, promoting triglyceride hydrolysis and fatty acid mobilization. This effect is independent of IGF-1 and accounts for GH's fat-reducing properties (Moller & Jorgensen, 2009)
• Protein synthesis: GH increases amino acid uptake and protein synthesis in skeletal muscle through both direct (mTOR pathway) and IGF-1-mediated mechanisms, promoting positive nitrogen balance
• Insulin antagonism: GH opposes insulin action on glucose metabolism, increasing hepatic glucose output and reducing peripheral glucose uptake. This diabetogenic effect is dose-dependent

Research

Safety Profile

Somatropin side effects are generally dose-related and more common at supratherapeutic doses:

• Fluid retention: Edema, carpal tunnel syndrome, and arthralgias are the most common adverse effects, occurring in 10-30% of adults initiating therapy. These are dose-dependent and typically resolve with dose reduction.
• Insulin resistance: GH increases fasting glucose and reduces insulin sensitivity in a dose-dependent manner. Monitoring of glucose metabolism is recommended, especially in patients with pre-existing diabetes risk.
• Arthralgias/Myalgias: Joint and muscle pain, particularly in hands, wrists, and knees, occurs frequently at initiation and with dose escalation.
• Carpal tunnel syndrome: Results from fluid retention and soft tissue swelling in the carpal tunnel.
• Theoretical cancer risk: Elevated IGF-1 has been epidemiologically associated with increased risk of prostate, breast, and colorectal cancers. Long-term safety data from GH replacement registries (KIMS, HypoCCS) have not shown increased cancer incidence at replacement doses, but monitoring is prudent.
• Contraindications: Active malignancy, active proliferative or severe non-proliferative diabetic retinopathy, critical illness.

Subpopulation Research

• GH-deficient adults: Well-characterized improvements in body composition, bone density, lipid profile, and quality of life with replacement therapy (Salomon et al., 1989).
• Healthy elderly: Rudman et al. (1990) demonstrated body composition benefits, but subsequent meta-analyses show higher adverse event rates in non-deficient elderly (Liu et al., 2007).
• Pediatric: Established efficacy for GHD, Turner syndrome, Prader-Willi, SGA, idiopathic short stature. Final height gain depends on age at initiation, dose, and duration.
• HIV wasting: FDA-approved (Serostim) for AIDS-associated wasting and cachexia.
• Athletes: Increased lean mass but no consistent improvement in strength, power, or exercise capacity (Liu et al., 2008). Prohibited by WADA.

Pharmacokinetic Profile

Ongoing & Future Research

• Long-acting GH formulations (somapacitan/Sogroya, lonapegsomatropin/Skytrofa) enabling weekly instead of daily injection.
• Investigation of GH in traumatic brain injury recovery and cognitive function.
• Combination protocols with GH secretagogues to optimize IGF-1 axis with minimized exogenous GH dose.
• Ongoing debate about GH and longevity -- paradoxically, GH/IGF-1 axis reduction extends lifespan in animal models, while GH deficiency reduces quality of life in humans.