Tesamorelin vs Follistatin-344.

Tesamorelin is a synthetic 44-amino-acid analog of human growth hormone-releasing hormone (GHRH/GRF 1-44), stabilized by a trans-3-hexenoic acid group attached to its N-terminus. It is one of the few research peptides in its class that holds full FDA approval, marketed as Egrifta (and the reformulated Egrifta SV / Egrifta WR) by Theratechnologies for a narrow, specific indication. Originally designated TH9507, it is a secretagogue rather than a hormone itself.

Follistatin-344 (FS-344) is an alternatively spliced isoform of human follistatin, a naturally occurring secreted glycoprotein that acts as a high-affinity antagonist of several TGF-beta superfamily ligands. The "344" refers to a 344-amino-acid precursor variant; relative to the longer FS-315 serum isoform, it lacks the C-terminal acidic tail and was selected for therapeutic use partly to reduce off-target heparin/cell-surface binding. In gene-therapy programs it is the FS344 transgene that is delivered, not an injected peptide product, although it is now marketed in gray-market channels as a "research peptide."

Tesamorelin binds the GHRH receptor on the anterior pituitary, stimulating the synthesis and pulsatile release of the body's own growth hormone (GH), which in turn raises hepatic and circulating insulin-like growth factor-1 (IGF-1). Because it works upstream by amplifying endogenous GH secretion, it largely preserves physiological feedback and pulsatility, unlike direct exogenous GH administration. The N-terminal hexenoyl modification confers resistance to degradation (including by dipeptidyl peptidase-IV) and extends its half-life relative to native GHRH. The downstream rise in GH/IGF-1 drives lipolysis, with a notable effect on visceral (intra-abdominal) adipose tissue.

Follistatin works by binding and neutralizing myostatin (GDF-8) and related ligands such as activin A, GDF-11, and several BMPs, preventing them from engaging activin type II receptors. Because myostatin is a dominant negative regulator of skeletal muscle mass, removing this brake promotes satellite-cell activation, myofiber hypertrophy, and reduced fibrosis. Critically, follistatin neutralizes a broader set of ligands than myostatin-only blockade, which is why follistatin overexpression produces larger muscle gains in animals than myostatin knockout alone. The foundational biology traces to McPherron, Lawler and Lee (Nature, 1997), who showed myostatin loss roughly doubles muscle mass in mice.

Human evidence is unusually strong for one specific population: HIV patients with lipodystrophy. Two pivotal phase 3 randomized, placebo-controlled trials and their pooled analysis (Falutz et al., JAIDS 2010 and J Clin Endocrinol Metab 2010) showed roughly a 15-18% reduction in visceral adipose tissue over 26 weeks, with regain after discontinuation, supporting the FDA approval. Stanley et al. (JAMA 2014; PMID 25038357) and a randomized NAFLD trial (Stanley et al., Lancet HIV 2019; PMID 31611038) further demonstrated reductions in liver fat and a lower rate of fibrosis progression in HIV-associated fatty liver disease. Crucially, nearly all rigorous human data come from HIV-positive cohorts; use for general anti-aging, body recomposition, or NAFLD in HIV-negative people is extrapolation and has not been established in large controlled trials. Subcutaneous fat and total weight are largely unaffected, and effects reverse when treatment stops.

Human evidence is limited to two small, open-label AAV1-delivered FS344 gene-therapy trials from Nationwide Children's Hospital (Mendell and colleagues), not to any injected-peptide product. A Phase 1/2a trial in Becker muscular dystrophy (6 subjects; Mol Ther 2015, PMID 25322757) reported six-minute-walk gains in some treated patients (e.g., +58 m and +125 m in two subjects) with histological evidence of reduced fibrosis and fiber hypertrophy. A companion sporadic inclusion body myositis trial (6 subjects; Mol Ther 2017, PMID 28279643) reported improved annualized six-minute-walk distance versus untreated controls, though responses were heterogeneous and the comparison used a non-randomized matched control group. These are early-phase, unblinded, very small studies; large-animal support comes from a nonhuman-primate follistatin gene-delivery study (Kota et al., Sci Transl Med 2009, PMID 20368179). No randomized controlled trial, and no trial of FS-344 as a standalone injectable peptide, has demonstrated efficacy. The sIBM functional claims also drew a published methodological critique in Molecular Therapy.

The most consistent documented concerns are glucose-related: because it raises GH/IGF-1, tesamorelin can worsen insulin sensitivity and glucose tolerance, and IGF-1 levels are monitored in clinical practice. Common adverse effects in trials included injection-site reactions, arthralgia, myalgia, peripheral edema, and paresthesia; hypersensitivity reactions have occurred. It is contraindicated in pregnancy and in people with active malignancy, since elevated IGF-1 is a theoretical tumor-growth concern, and in those with disrupted hypothalamic-pituitary axis (e.g., pituitary tumor/surgery, head irradiation). The NIH LiverTox database assigns it a low likelihood of causing clinically apparent liver injury. Long-term safety beyond the trial windows, and safety in non-HIV populations, remains poorly characterized.

In the two small gene-therapy trials, intramuscular AAV1.FS344 was reported as generally well tolerated over follow-up exceeding two years, but these cohorts are far too small to characterize real risk. Because follistatin broadly inhibits TGF-beta/activin signaling, theoretical and preclinical concerns include effects on reproductive tissues (follistatin was first identified as an inhibitor of FSH secretion), the pituitary-gonadal axis, vascular and cardiac remodeling, and possible influence on tumor biology, none of which are adequately resolved in humans. Gray-market "follistatin-344 peptide" products carry the additional, unquantified hazards of unverified identity, purity, sterility, and the fundamental mismatch that human data come from a delivered gene, not an injected protein. There is no established human safety profile for self-administered FS-344.

FDA-approved (2010, as Egrifta; reformulated as Egrifta SV in 2019 and the F8 formulation Egrifta WR in 2023) solely to reduce excess visceral abdominal fat in HIV-infected adults with lipodystrophy. All other uses are off-label, and it is a prescription drug, not a dietary supplement.

Follistatin-344 is not approved by the FDA (or any major regulator) for any indication; it has only been studied investigationally as an AAV-delivered gene therapy and is sold elsewhere strictly as a research-use-only chemical, not a medicine. Myostatin-pathway inhibition is also of interest to anti-doping bodies, and follistatin/myostatin inhibitors fall under WADA's prohibited categories.