TB-500 in 2026: FDA List Status, Full-Length Thymosin Beta-4 vs the LKKTETQ Fragment, CAS and Expected Mass

On the FDA compounding page with content current as of April 22, 2026, TB-500 (listed as thymosin beta-4 fragment LKKTETQ) is in the table of substances nominated but withdrawn, not the current Category 2 table, after the FDA placed it in Category 2 in September 2023. TB-500, as Pepta Labs lists it, is full-length N-acetylated thymosin beta-4 (INN timbetasin): a 43-residue peptide with the formula C212H350N56O78S, a molecular weight of 4963.44 Da, CAS 77591-33-4, PubChem CID 16132341 and UNII 2D5MRE3SSY. It is not the seven-residue fragment Ac-LKKTETQ (C38H68N10O14, 889.0 g/mol), even though PubChem and the FDA compounding page apply the name TB-500 to that fragment. This page sets out both records, the masses a TB-500 mass spectrum should show, the 15.99 Da methionine oxidation peak, and the dated regulatory status. The TB-500 10 mg vial is listed from $56.73.

Is TB-500 the full thymosin beta-4 or the LKKTETQ fragment?

The name TB-500 is attached to two different molecules, so the CAS number, formula and mass on a listing or report identify the material, not the name. The Pepta Labs listing uses the full-length values: CAS 77591-33-4, C212H350N56O78S and 4963.44 Da belong to 43-residue N-acetylated thymosin beta-4. PubChem resolves the bare name TB-500 to a seven-residue acetylated fragment, Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln (Ac-LKKTETQ), with the formula C38H68N10O14 and a molecular weight of 889.0 g/mol. The FDA compounding page dated April 22, 2026 also uses the fragment wording: its entry reads "Thymosin beta-4, fragment (LKKTETQ), also known as TB-500".

The fragment is a short stretch of the full chain. Aligning the two sequences puts LKKTETQ at positions 17 to 23 of the 43-residue peptide. In the fragment the acetyl group sits on the leucine; in the full-length peptide it sits on the serine at position 1. The two records differ by 36 residues and by 4,074.44 in mass (4963.44 minus 889.0), and the full-length molecule weighs about 5.6 times as much as the fragment (calculated from 4963.44 and 889.0). That gap is far outside any instrument tolerance, so the mass on a report settles which molecule was tested.

Two practical checks follow from the table. A listing or report that pairs the name TB-500 with CAS 77591-33-4 but quotes 889 g/mol, or quotes C38H68N10O14 next to the 43-residue sequence, mixes the two records and is worth a question before ordering. A product sold as a "TB-500 fragment" should carry the fragment's formula and a mass near 889 g/mol, not 4963. Other cases where one name points to two registry entries are collected in peptide registry discrepancies, and the naming conventions behind "Ac-" and "-OH" are explained in peptide sequence notation.

Is TB-500 related to thymosin alpha-1?

Thymosin alpha-1 and thymosin beta-4 share the historical name thymosin, not a sequence, formula or registry number. Thymosin alpha-1 (INN thymalfasin) is a 28-residue N-acetylated peptide, Ac-SDAAVDTSSEITTKDLKEKKEVVEEAEN-OH, C129H215N33O55, 3108.26 Da, CAS 62304-98-7. TB-500 as listed is the 43-residue thymosin beta-4 record above. The two are compared in full, record by record, in the thymosin alpha-1 guide.

What are the TB-500 sequence, formula, molecular weight and CAS records?

TB-500 on the Pepta Labs listing is the 43-residue sequence Ac-SDKPDMAEIEKFDKSKLKKTETQEKNPLPSKETIEQEKQAGES-OH, formula C212H350N56O78S, molecular weight 4963.44 Da, CAS 77591-33-4. The formula was re-summed from the sequence for this page (43 residue units, one water, one acetyl group) and matches the listing exactly. Every value below is the listing value or a registry value, and the source is named in the row.

The listing value of 4963.44 and the calculated 4963.51 describe the same molecule. The listing figure is what the formula gives with the older standard atomic weights (carbon 12.0107, hydrogen 1.00794, nitrogen 14.0067, oxygen 15.9994, sulfur 32.065); the newer abridged values (12.011, 1.008, 14.007, 15.999, 32.06) give 4963.51. A difference of 0.07 Da is a rounding convention, not a different compound. The monoisotopic mass, 4960.49 Da, is lower by about 3 Da because it counts only the lightest isotope of each element.

The acetyl group on the N-terminal serine adds 42.01 Da (monoisotopic) and is already counted in C212H350N56O78S. The counter-ion is not: an acetate salt carries acetate and residual water in the weighed solid, so the powder in the vial weighs more than the peptide alone, and the label quantity is the supplier-stated nominal quantity of that solid. The TFA vs acetate guide explains the difference, and the full table of listed compounds is in research peptide CAS numbers and molecular weights.

Which masses should a TB-500 mass spectrum show?

A TB-500 mass spectrum confirms identity when the observed ions convert back to a neutral mass of about 4963.4 Da (average) or 4960.49 Da (monoisotopic). Electrospray ionization (ESI) adds several protons to a peptide of this size, so the spectrum shows a series of multiply charged ions rather than one peak at 4963. Each ion appears at m/z = (M + z × 1.00728) / z, where z is the number of added protons, and software deconvolutes the series back to one neutral mass. The table lists the 4+ to 8+ ions; which charge states are most intense depends on the solvent and the instrument.

Which column applies depends on resolution. A low-resolution instrument cannot separate the isotope peaks of a 4963 Da ion, so it reports the centroid, which sits near the average-mass column. A high-resolution instrument resolves the isotope envelope; for C212H350N56O78S the calculated pattern puts the tallest peak two mass units above the monoisotopic peak, and the monoisotopic peak is only about 29% of the tallest, so a deconvoluted "most abundant" mass near 4962.5 is also consistent with the listed molecule. MALDI-TOF usually gives a singly charged ion, [M+H]+, near m/z 4964.45. At this size a ±0.1% tolerance is about ±5 Da.

Why can a TB-500 spectrum show a peak 15.99 Da heavier?

A peak 15.99 Da above the main mass corresponds to TB-500 with its single methionine (position 6) oxidized to methionine sulfoxide. The thioether sulfur of methionine takes up one oxygen atom, which adds 15.995 Da (monoisotopic) or 15.999 Da (average). Oxidation can occur during synthesis, during storage of the solid, or in the ion source itself. In the ESI series the shift is divided by the charge, so the oxidized ion sits 4.00 m/z above the parent at 4+ and 3.20 m/z above it at 5+. On a reversed-phase HPLC trace, the more polar sulfoxide form usually elutes slightly ahead of the main peak, and it counts as an impurity in the area-percent purity.

Which other mass shifts are worth recognising?

The shifts below are calculated from the listing mass 4963.44 Da with average residue masses. Each corresponds to a specific chemical difference from the listed sequence; none of them identifies a different compound.

Deletion sequences are the typical by-product of stepwise solid-phase synthesis, and a 43-residue chain has 42 coupling steps in which one residue can be skipped. Mass spectrometry has one blind spot here: the lysine and glutamine residues (128.095 and 128.059 Da, monoisotopic residue masses) differ by only 0.036 Da, so a low-resolution spectrum cannot tell which of the two is missing. How chains of this length are assembled is covered in how research peptides are made. The same ion calculation for every listed compound is in the expected mass table, and the two methods are compared in HPLC vs mass spectrometry.

What does the TB-500 analytical report cover?

Pepta Labs represents independent third-party HPLC purity analysis and mass-spectrometry identity confirmation, commissioned through the purchasing group on a sampled production run from the source. Pepta Labs does not test in-house. The report is sent by email on request, before you order if you want to see it first, through the report request form. Because testing is commissioned by the purchasing group, the client field may name the group or source rather than Pepta Labs, and supplier-identifying fields are redacted. Vial labels identify the compound, quantity and research-use designation. They do not carry lot or batch numbers. Sterility, endotoxin or LAL, potency, pharmaceutical-grade, cGMP, contaminant-panel and vial fill-quantity testing are not claimed. A step-by-step reading of a report is in how to verify a peptide COA.

What is the FDA status of TB-500 in 2026?

As of the FDA compounding page with content current as of April 22, 2026, TB-500 appears in the table of bulk substances nominated but withdrawn, listed as thymosin beta-4 fragment LKKTETQ, also known as TB-500. It is therefore not in the current Category 2 table on that page. It also does not appear in the 503A category list updated May 14, 2026 or the 503B category list updated March 21, 2025.

The FDA describes the nominated-but-withdrawn table as substances previously in Category 2 whose nominations were withdrawn by the nominators. These category lists apply to compounding by state-licensed pharmacies and physicians (section 503A) and by outsourcing facilities registered with FDA (section 503B). They do not change the research-use-only basis on which Pepta Labs sells TB-500 as a laboratory reference material. The FDA entry names the fragment, while the Pepta Labs listing is the 43-residue record, which is one more reason to identify the material by CAS number and mass rather than by name. Always check the page's own content-current-as-of date before relying on any summary, including this one.

The status of every peptide named in the September 2023 action is tabulated in FDA Category 2 peptides 2026, and the wider US picture is in peptide legal status 2026. Searches for TB-500 are often paired with Peptide Sciences, a US seller that stopped taking orders in March 2026; its status is covered in Peptide Sciences shutdown. For what a research-use-only listing is, start with what are research peptides.

How is TB-500 listed and stored at Pepta Labs?

Pepta Labs lists TB-500 as a single 10 mg vial of lyophilized powder, with 3-vial and 5-vial packs shown on the product page. The reported RP-HPLC purity and the price per mg are printed on the listing, and cross-listing comparisons of price per mg are on research peptide price per mg. TB-500 10 mg is also one component of the Wolverine 10 bundle; the bundle page lists its contents and price. Sealed vials ship at ambient temperature; store at 2-8 C on arrival or at -20 C for long-term storage, protected from light and moisture. The peptide storage guide covers sealed-vial storage in more detail.

Frequently asked questions

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

On the Pepta Labs listing, yes: CAS 77591-33-4, C212H350N56O78S and 4963.44 Da are the values for full-length N-acetylated thymosin beta-4 (INN timbetasin, 43 residues, PubChem CID 16132341, UNII 2D5MRE3SSY). PubChem and the FDA compounding page apply the name TB-500 to the seven-residue fragment Ac-LKKTETQ instead, so read the CAS number and mass on the report, not the name.

What is the TB-500 fragment (Ac-LKKTETQ)?

The TB-500 fragment is the acetylated heptapeptide Ac-Leu-Lys-Lys-Thr-Glu-Thr-Gln, formula C38H68N10O14, molecular weight 889.0 g/mol (monoisotopic 888.49 Da, calculated). Its seven residues match positions 17 to 23 of the 43-residue thymosin beta-4 sequence. It is the entity PubChem returns for the bare name TB-500 and the wording on the FDA compounding page; it is not the molecule on the Pepta Labs listing.

What is the molecular weight of TB-500?

TB-500 as listed has a molecular weight of 4963.44 Da (average, formula C212H350N56O78S). Calculated from the same formula, the average mass with current atomic weights is 4963.51 Da and the monoisotopic mass is 4960.49 Da. The Ac-LKKTETQ fragment that some databases call TB-500 is 889.0 g/mol.

Which CAS number identifies TB-500?

CAS 77591-33-4 identifies the 43-residue N-acetylated thymosin beta-4 on the Pepta Labs listing, alongside PubChem CID 16132341 and UNII 2D5MRE3SSY. A document that pairs CAS 77591-33-4 with the fragment's formula C38H68N10O14 or a mass near 889 g/mol mixes two different records.

Why can a TB-500 report show a peak 16 Da heavier?

TB-500 contains one methionine, at position 6, and its sulfur can take up one oxygen atom to form methionine sulfoxide, which adds 15.99 Da. The oxidized species deconvolutes to about 4979.4 Da and appears at m/z 1245.87 (4+) and 996.90 (5+), calculated. It is a related substance of the same sequence, not a different compound, and it counts against the area-percent HPLC purity.

All products are supplied for laboratory research use only and are not for human or animal use.