Both names point at the thymus. Both appear on peptide catalogs under immune or longevity headings. Both are described in vendor copy with the same vocabulary about restoring immune competence.
One of them is a single sequence of 28 amino acids with 66 registered human trials and a Cochrane systematic review published last week. The other has zero trial registrations anywhere on ClinicalTrials.gov and no primary research indexed since 2024.
But the count is not the most interesting difference, and neither is the evidence gap. The more useful observation is that these two compounds are not the same kind of object, and almost every downstream question — what a mass spectrum can confirm, what a purity percentage refers to, what a study on one says about the other — follows from that.
One is a sequence, the other is a preparation
Thymosin Alpha-1 is a defined molecule. It is the 28-residue peptide SDAAVDTSSEITTKDLKEKKEVVEEAEN, carrying an N-terminal acetyl group, with a mass near 3109 Da for the acetylated form.
It is not a standalone gene product. It corresponds to residues 2–29 of prothymosin alpha (UniProt P06454, annotated at 111 residues — a figure worth checking against, since 109 circulates widely in secondary sources). Under its INN, thymalfasin, it has been manufactured both by chemical synthesis and by cleaving recombinant prothymosin alpha, with the acetylated products reported as mass-identical and co-eluting.
Thymalin is not a sequence at all. Its own originating group — the Saint Petersburg Institute of Bioregulation and Gerontology — describes it as a drug "containing a polypeptide extract of thymus," and their modern mechanistic work (Linkova et al., Int J Mol Sci 2023;24:13377) studies the KE and EW dipeptides they identify as its active substances rather than the preparation itself.
That is the comparison in one line: a molecule with a formula, versus a fraction with a description. It is the same structural distinction this site has drawn between HCG and HMG, and it has the same consequence — a purity percentage on a heterogeneous fraction has no clean referent, because there is no single species for it to be a percentage of.
The shared surname is a filing artifact
Here is the part most compound pages get wrong by omission.
"Thymosin alpha-1" and "thymosin beta-4" — the parent protein behind TB-500 — sound like members of one family. They are not related in sequence, structure, or function. Alpha-1 is a small acidic peptide cleaved from a nuclear protein precursor. Beta-4 is an actin-sequestering polypeptide with an entirely different job.
They share a name because of how they were sorted in the 1970s.
Both were isolated from thymosin fraction 5, a partially purified calf-thymus preparation. The Greek letters were assigned by isoelectric point: peptides with pI below 5.0 were designated alpha, those between 5.0 and 7.0 beta, and those above 7.0 gamma. The subscript number recorded the order in which each was isolated. The nomenclature was proposed in the same paper that reported the alpha-1 sequence (Goldstein et al., PNAS 1977;74:725–729), whose abstract describes the peptide as "heat stable, highly acidic" — which is precisely why it landed in the alpha bin.
So the alpha/beta distinction encodes an electrophoresis result, not a structural relationship. "Thymosin" is a shared address, not a shared biology. This joins a short list of naming traps on this shelf where the label describes history rather than chemistry — see the nomenclature primer.
What is actually in a thymic fraction
Fraction 5 also supplies the most concrete answer available to the question of what an extract-based thymic preparation contains.
Hannappel, Iavarone and Castagnola (Expert Opin Biol Ther 2018;18(sup1):199–203) re-examined a lyophilized fraction 5 sample stored for 37 years using top-down HPLC coupled to high-resolution Orbitrap mass spectrometry. They detected more than 100 monoisotopic masses, including thymosin beta-4, truncated forms of ubiquitin, prothymosin alpha, thymosin beta-4 and thymosin beta-9, plus a previously uncharacterized component they identified as SH3 domain-binding glutamic acid-rich-like protein 3.
Two things are worth separating here. First, this was fraction 5 — a different preparation, from a different lineage, than Thymalin. It is not a measurement of what is in any other extract, and should not be cited as one. Second, the mechanism it demonstrates is general: an extract-derived preparation contains many species, including truncation products of the very peptides it is named for, and modern instrumentation finds components that decades of prior work did not characterize.
That is the analytical position a fraction occupies. The question "is it pure?" cannot be answered, because the material was never defined as one thing. The answerable questions are which components are present and in what proportion — and those require methods no routine release panel runs. More on that framing at /quality/.
The evidence files are not the same size, and the larger one is still uncertain
Verified against the ClinicalTrials.gov API today: a search on thymalfasin returns 66 study records; a search on Thymalin returns none. Not zero results posted — zero registrations. Nothing on file, including intentions.
The thymalfasin records span decades, multiple countries, industry and academic sponsors, and include trials registered as recently as February 2026. Against that, note a separate verified fact: there is no thymalfasin record in Drugs@FDA and no US label in openFDA. A large international trial file and US approval are different things.
The freshest item is a Cochrane review published on 9 September 2026 (Naing et al., Cochrane Database Syst Rev 2026;9:CD014610) assessing thymosin alpha-1 in chronic hepatitis B. It included 10 randomized trials with 1,349 participants, published between 1991 and 2018, conducted across six countries.
The pooled estimates pointed toward reduced all-cause mortality, serious adverse events and non-serious adverse events among trial participants, with little to no effect on quality of life or histological improvement. But the certainty ratings are the result. The reviewers graded the evidence low for serious adverse events and very low for everything else, downgrading for risk of bias, imprecision and heterogeneity, and stated plainly that they "are not sure whether" thymosin alpha-1 reduces those outcomes. They identified no ongoing trials, and 16 further studies remain awaiting assessment because of incomplete reporting.
This is what a comparatively large file looks like once a formal certainty framework is applied to it: not a verdict, but a measured statement of how much the existing trials can support. It sits alongside — not against — the negative Phase 3 sepsis result covered in our immune-modulating roundup. Different indication, different question, both part of the same compound's record.
The mechanism is still moving
A week before the Cochrane review, Cancer Research published a finding that adds a genuinely new layer (Wei et al., 2026;86:4238–4254, 2 September 2026).
Working in mice and patient samples, the authors report that thymosin alpha-1 binds tumor apoptotic bodies and interacts with apoptotic-body-borne microRNAs including miR-146a-5p after those bodies are phagocytosed into the endolysosomal compartment of dendritic cells. The interaction is described as protecting miR-146a-5p from lysosomal RNase A degradation, permitting TLR7 activation and downstream dendritic cell maturation.
Note what kind of mechanism that is. It is not receptor agonism — it is a chaperone role for a nucleic acid ligand, inside a vesicle. Receptor-pharmacology vocabulary of affinity, efficacy and selectivity does not describe it, a recurring situation on this shelf covered in the receptor pharmacology primer, and the compartment involved is exactly the one that makes access a separate question from binding. This is animal and cell work — one paper, held at that level.
Thymalin has nothing comparable. Its most recent indexed primary research dates to 2024.
What each one's paperwork can and cannot say
For thymosin alpha-1, the documentation questions are ordinary but specific. The N-terminal acetyl group is part of the molecule, so a theoretical mass calculated from bare sequence letters will disagree by 42 Da — a paperwork error indistinguishable from a product error on a certificate. Des-acetyl material is the obvious near-mass impurity. The sequence carries no tryptophan and no tyrosine, so UV quantitation at 280 nm is unavailable, and it is strongly anionic — unusual on a shelf dominated by cationic sequences. Manufacturing route also matters: a recombinant route makes host-derived impurities and endotoxin relevant questions in a way a fully synthetic route does not.
For Thymalin, the first question is not purity but what the document is claiming to be a document about. A percentage attached to a heterogeneous fraction needs a stated referent before it means anything, and the originating group's own current work targets defined dipeptides said to be present in the preparation rather than the preparation as a whole.
FAQ
Is Thymalin a Russian version of thymosin alpha-1? No. They are different kinds of material from different research lineages. Thymosin alpha-1 is one defined sequence; Thymalin is a thymic polypeptide extract. Treating a study of one as evidence about the other requires an assumption nobody has tested.
Does the Cochrane review mean thymosin alpha-1 works? It means the opposite of a clean answer. The reviewers found point estimates favoring the compound but rated the certainty very low for nearly every outcome and stated they are not sure. Certainty ratings exist precisely so that a directionally favorable pooled number is not read as a demonstrated effect.
Why does the fraction-5 naming history matter for reading a catalog? Because it explains why two unrelated compounds carry one surname, and it is a working example of a general habit: a name records how something was found, not what it does. The same reading applies to fragment numbering, catalog indices and category labels across the library.
This article is educational and for the laboratory research community. Trulogic Labs products are sold for laboratory and research use only and are not for human consumption.