HGH Fragment 176-191 is the unmodified 16-residue C-terminal segment of human growth hormone, reproduced synthetically and studied for the lipolytic activity attributed to that region of the parent hormone. It is the native fragment — the sequence exactly as it appears in hGH — as distinct from AOD-9604, which is the same region with an engineering change. We supply research-grade HGH Fragment 176-191 as lyophilized powder in sealed vials, for laboratory use only.
Despite the name, this peptide is not growth hormone and does not function as growth hormone. Intact somatropin is a prescription-only 191-residue biologic, listed separately in our prescription biologics reference section.
What is HGH Fragment 176-191, precisely
Human growth hormone is a 191-residue, four-helix-bundle protein that signals by dimerising its receptor. Research published from the 1980s and 1990s reported that its effects on adipose tissue could be reproduced by a short C-terminal region, and that this region did not require the receptor-binding surfaces used by the intact hormone. HGH Fragment 176-191 is that region synthesised on its own: 16 residues, containing the intramolecular disulfide loop present in the parent protein.
The distinction between a fragment and an analog is worth stating clearly. An analog is a modified version of a molecule intended to improve its properties. A fragment is a genuine sub-sequence of a larger protein, and its behaviour cannot be assumed to resemble that of the whole — a fragment can retain one activity, lose another, and acquire behaviour the parent never had, because removing it from the protein removes the structural context that constrained it.
Origin and structure
- Sequence. Human growth hormone residues 176-191, unmodified — no added residues, no acylation, no protective substitutions.
- Disulfide bridge. An intramolecular Cys-Cys bond forms the loop that the reported activity depends on. The two sulfur atoms in the formula reflect it.
- Identifiers. CAS 66004-57-7, formula C78H125N23O23S2, molecular weight 1817.12 Da.
- No stabilising modifications. Because nothing has been added to resist proteases or slow clearance, this peptide behaves as a short-lived species in any biological system.
Note the mass relationship to AOD-9604: 1817.12 Da versus 1815.08 Da, a difference of roughly two daltons. That closeness is analytically important, since a low-resolution mass measurement cannot reliably distinguish the two.
There is also a naming problem that affects literature searching. Some sources number the fragment 176-191 and others 177-191, and the two numbering conventions are used inconsistently across papers, catalogs and patents. Where a study reports activity for a growth hormone C-terminal fragment, confirm the exact residue span and whether any residue was added before treating that study as evidence about the material in your vial. This is not pedantry: the difference between the two conventions is precisely what separates the native fragment from AOD-9604.
How the fragment is thought to work
The proposed mechanism mirrors that described for AOD-9604 and carries the same uncertainty. Preclinical work has reported that the fragment promotes lipolysis and reduces lipogenesis in adipose tissue, observed in isolated adipocyte preparations and in rodent models. Beta-3 adrenergic receptor signalling has been implicated in several papers, though as a permissive or indirect contributor rather than through direct receptor agonism, and knockout experiments have complicated the picture.
What the fragment reportedly does not do is as significant as what it does. It has not been reported to raise IGF-1, to promote linear growth, or to produce the glucose-related effects associated with intact growth hormone. That separation is the reason the fragment exists as a research tool at all: it allows a laboratory to ask whether a lipolytic signal attributed to growth hormone can be elicited without engaging the growth hormone receptor and the GH-IGF axis.
What research has examined
- Adipocyte and cell studies. Glycerol and free fatty acid release from isolated adipocytes, lipogenic enzyme activity, and lipolytic signalling readouts. This is the substance of the evidence base.
- Rodent studies. Adiposity and body composition in obese mouse and rat models, typically with IGF-1 measured as a negative control endpoint to confirm the dissociation from growth hormone signalling.
- Comparative work. Studies placing the native fragment against the tyrosine-extended AOD-9604, examining whether the modification alters activity or only handling.
- Human data. There is no substantial published human trial programme for the unmodified fragment. The clinical work in this space was conducted with AOD-9604, and it did not reproduce the preclinical weight findings. Any claim of human efficacy for the native fragment is unsupported by published trial evidence.
Our article on the growth hormone lipolytic fragment traces how the two versions diverged.
Choosing between the native fragment and its tyrosine-extended counterpart comes down to what the experiment is asking. If the question concerns the native sequence as it exists within growth hormone — for instance whether the region has intrinsic activity when liberated from the parent protein — the unmodified fragment is the correct reagent, because any added residue introduces a variable that the parent protein does not contain. If the question concerns comparability with the published AOD-9604 literature, or if analytical detection at 280 nm is needed, the modified version is the better fit. Running both arms is common in comparative work, and it is the only way to establish whether the tyrosine is analytically convenient or pharmacologically consequential.
Forms and sizes we supply
| Attribute | Specification |
|---|---|
| Form | Lyophilized powder, sealed glass vial |
| Sizes | 5 mg, 10 mg |
| Purity | Greater than or equal to 99% by HPLC, lot-matched COA |
| CAS | 66004-57-7 |
| Molecular weight | 1817.12 Da |
| Formula | C78H125N23O23S2 |
| Residues | 16 (hGH 176-191, unmodified) |
| Intended use | Laboratory research only |
Sizes and pricing are on the HGH Fragment 176-191 product page, within the fat-loss and metabolic peptides collection.
Reconstitution and storage in a lab context
Standard concentration arithmetic: a 10 mg vial reconstituted with 2 mL of bacteriostatic water gives 5 mg/mL, equal to 5,000 mcg/mL, so 0.1 mL of that solution contains 500 mcg of peptide. A small, unmodified, water-soluble peptide is undemanding to reconstitute, but the disulfide bridge is the vulnerable feature: reducing agents, trace thiols in a poorly chosen diluent, and prolonged exposure to alkaline conditions can open the loop, and an open-loop peptide is a different molecule even though its purity certificate still reads clean.
Store sealed lyophilized powder at minus 20 degrees Celsius, protected from light and moisture; hold reconstituted solution at 2 to 8 degrees Celsius and use it within the study window. Aliquot at reconstitution to avoid repeated freeze-thaw rounds. See our reconstitution guide.
Purity, COA and how to read it
Expect an HPLC chromatogram with main-peak area percentage, mass spectrometry identity against the theoretical 1817.12 Da, net peptide content, lot number and analysis date. This peptide has two specific analytical traps. The first is the roughly two-dalton gap between it and AOD-9604, which a low-resolution instrument cannot resolve — meaning a purity certificate alone cannot prove which of the two peptides is in your vial. The second is disulfide state, where the correctly folded and reduced forms also differ by two mass units. If either distinction matters to your work, ask what resolution the analysis achieved rather than accepting a headline number. See mass spectrometry identity and how to read a COA.
Clearance is the practical constraint that shapes any in vivo design with this peptide. With no acylation, no protective substitutions and a mass under 2 kDa, it is filtered renally and degraded quickly, so systemic exposure after a single administration is brief. Published rodent work generally uses frequent administration for this reason. A study that administers once and measures a distant endpoint is unlikely to detect anything, and a negative result from such a design says more about exposure than about activity — a distinction worth making explicitly when reporting or reading results with short unmodified peptides.
Regulatory status
HGH Fragment 176-191 is not an approved medicine in any jurisdiction and has no clinical development programme behind it. It is a research chemical, supplied research use only, not approved or formulated for human or veterinary administration. It is also worth noting that its name causes recurring confusion with somatropin, which is a prescription-only biologic under an entirely different regulatory regime — the fragment is neither a substitute for it nor a legal route to its effects. See research use only, explained.
Related peptides and comparisons
The essential comparison is AOD-9604 vs HGH Fragment 176-191. For the intact hormone as a reference listing see HGH (somatropin).