Is TB-500 the same as thymosin beta-4?

Not necessarily. Doping-control laboratories that analysed TB-500 found the N-acetylated fragment Ac-LKKTETQ, residues 17 to 23 of thymosin beta-4, and PubChem and the US FDA use the name for that fragment. Some sellers use the same name for full-length thymosin beta-4, a 43-residue protein. Only identity testing by mass spectrometry shows which one a particular vial contains.

What mass should a TB-500 COA show?

It depends on the molecule. Ac-LKKTETQ is C38H68N10O14 with an average mass of 889.0 g/mol and monoisotopic mass of 888.49 Da. Full-length thymosin beta-4 is C212H350N56O78S at 4963 g/mol average and 4960.49 Da monoisotopic. The certificate should name which molecule it compared against. A mass near neither value points to a different peptide entirely.

Why does it matter which TB-500 molecule I have?

Because purity and amount are measured against a reference of a specific molecule. If the laboratory calibrates with the wrong one, the milligram result is wrong even when the analysis was done well. The two also have different impurities, such as oxidised methionine in the full protein, so identity decides which reference and which impurity checks apply.

How do I know if my TB-500 is real?

First decide what real means for your vial: the fragment or the full protein, as the label or seller claims. Then test a sample for identity by mass spectrometry, purity by HPLC and amount against a matching reference. A seller's certificate is only useful if it names the molecule, the batch and the laboratory, and can be verified with the issuer.

One of the 30 compound testing profiles

TB-500

TB-500 is sold as two different molecules, the 7-residue fragment Ac-LKKTETQ (C38H68N10O14, average mass 889.0 g/mol) and full-length thymosin beta-4 (43 residues, C212H350N56O78S, average mass 4963 g/mol); a test should first establish which one is in the vial, then measure it against the matching reference.

TB-500 is a trade name: the product analysed in doping-control work contained N-acetylated LKKTETQ, a synthetic copy of residues 17 to 23 of the actin-binding protein thymosin beta-4 (Ho et al., Journal of Chromatography A, 2012), and the US FDA uses the name for that fragment (FDA, bulk substances with safety risks).

Test your TB-500

Identity card

Formula: Not a single molecule (see note)
Average mass: Not published
Monoisotopic mass: Not published
Length: Not applicable
Modifications: None listed
Also sold as: Thymosin beta-4 fragment, TB4
Note: The name covers two molecules, so no single identity is given. Fragment: N-acetyl-LKKTETQ (Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln-OH), PubChem CID 62707662, C38H68N10O14, average mass 889.0 g/mol, monoisotopic mass 888.49 Da, 7 residues, N-terminal acetyl and free C-terminal acid; PubChem lists TB-500 and TB500 as synonyms of this record. Full-length thymosin beta-4: UniProt P62328, 43 residues (SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES) after removal of the initiator methionine, with N-acetylserine at the N-terminus; PubChem CID 45382195 gives C212H350N56O78S, average mass 4963 g/mol and monoisotopic mass 4960.49 Da. The single sulfur is Met6.

Source: PubChem CID 62707662, TB-500 (Ac-LKKTETQ), PubChem CID 45382195, Thymosin beta 4, UniProt P62328, Thymosin beta-4 (human), Esposito S et al. Synthesis and characterization of the N-terminal acetylated 17-23 fragment of thymosin beta 4 identified in TB-500. Drug Test Anal. 2012;4(9):733-8.

Compare the mass printed on a certificate with the values above. The mass checker explains charge states and which mass a lab usually reports.

What usually goes wrong with TB-500. Documented failure modes, with sources.

Which molecule is in the vial

A doping-control laboratory identified the N-acetylated 17 to 23 fragment of thymosin beta-4, Ac-LKKTETQ, in TB-500 by liquid chromatography with high-resolution mass spectrometry (Esposito et al., Drug Testing and Analysis, 2012), and a racing laboratory describes the same acetylated peptide as TB-500's key ingredient (Ho et al., 2012). Listings also apply the name, or "TB4", to full-length thymosin beta-4, a 43-residue protein (UniProt P62328). A certificate that does not state which one it looked for, and which mass it found, has not answered the first question.

An amount measured against the wrong reference

An amount test compares the sample's signal with a reference standard of known content. If the vial holds the fragment and the laboratory calibrates against full-length thymosin beta-4, or the reverse, the milligram figure is wrong however carefully the work was done, and with a more than fivefold difference in mass the error is not small. The report should name the reference compound, and it should match the molecule the identity test found.

Acetylated or not

The doping-control work found the N-acetylated peptide in TB-500 (Ho et al., 2012), while the biological activities reported for this region were described for the unacetylated form, LKKTETQ (Rahaman et al., Journal of Chromatography B, 2024). The acetyl group changes the formula and the mass, so an accurate mass measurement shows at once whether a vial sold as one form contains the other.

Oxidised methionine in full-length thymosin beta-4

Vials that contain the full protein have a weak point at the single methionine, position 6. Thymosin beta-4 prepared from bovine lung was accompanied by a derivative in which Met6 had become the sulfoxide, presumably by autoxidation (Heintz et al., European Journal of Biochemistry, 1994). Methionine sulfoxide adds 16 Da (Butré et al., Molecules, 2023). The 7-residue fragment has no methionine.

Counter-ions and aggregation

The US FDA names the potential for aggregation and peptide-related impurities among its concerns for the fragment (FDA, bulk substances with safety risks). Ac-LKKTETQ carries two lysines, and the number of trifluoroacetate counter-ions in synthetic peptides tracks the number of positive charges (Erckes et al., Pharmaceuticals, 2025). On a peptide under 900 g/mol, each counter-ion is a noticeable share of the weighed powder.

What we check in TB-500. Silent, with captions.

Which tests matter most

LC-MSAnswers the first question: fragment near 889 g/mol, full-length protein near 4963 g/mol, or something else.
HRMSSeparates acetylated from unacetylated LKKTETQ and detects a Met6 sulfoxide in full-length thymosin beta-4 by accurate mass.
HPLC-UVGives purity once the molecule is known, and shows synthesis by-products and oxidised forms as separate peaks.
Counter-ion contentTwo lysines on a small fragment make counter-ions a large share of the weight; this test lets the amount be stated as peptide.

Reading TB-500 results

Because the two molecules differ about 5.6-fold in mass (889.0 g/mol against 4963 g/mol in PubChem), the same milligram label means very different numbers of molecules depending on which one is in the vial. An amount test reports milligrams of the molecule actually identified, against a matching reference; the amount testing lesson explains the comparison.

The fragment is the easier of the two to test: small, synthetic and with a specific mass. Full-length thymosin beta-4 is a 43-residue protein with an oxidation-prone methionine, which makes impurity work and amount assignment more demanding.

Public Tested.ph results

No public TB-500 records yet

Each public TB-500 record will appear here with its identity, HPLC purity and measured amount against the label, once the first vial is tested.

FIG 13Amount against the label

Questions about testing TB-500

Is TB-500 the same as thymosin beta-4?: Not necessarily. Doping-control laboratories that analysed TB-500 found the N-acetylated fragment Ac-LKKTETQ, residues 17 to 23 of thymosin beta-4, and PubChem and the US FDA use the name for that fragment. Some sellers use the same name for full-length thymosin beta-4, a 43-residue protein. Only identity testing by mass spectrometry shows which one a particular vial contains.
What mass should a TB-500 COA show?: It depends on the molecule. Ac-LKKTETQ is C38H68N10O14 with an average mass of 889.0 g/mol and monoisotopic mass of 888.49 Da. Full-length thymosin beta-4 is C212H350N56O78S at 4963 g/mol average and 4960.49 Da monoisotopic. The certificate should name which molecule it compared against. A mass near neither value points to a different peptide entirely.
Why does it matter which TB-500 molecule I have?: Because purity and amount are measured against a reference of a specific molecule. If the laboratory calibrates with the wrong one, the milligram result is wrong even when the analysis was done well. The two also have different impurities, such as oxidised methionine in the full protein, so identity decides which reference and which impurity checks apply.
How do I know if my TB-500 is real?: First decide what real means for your vial: the fragment or the full protein, as the label or seller claims. Then test a sample for identity by mass spectrometry, purity by HPLC and amount against a matching reference. A seller's certificate is only useful if it names the molecule, the batch and the laboratory, and can be verified with the issuer.

Sources

Last reviewed 6 Oct 2026.