Peptide Basics
Thymosin: The Thymus Peptides, Thymosin Alpha-1 and Beta-4, and What the Records Show
Thymosin is a name used since at least 1966 for a lymphocyte-forming factor prepared from the thymus gland; today it covers two separate families of small proteins, the alpha thymosins (thymosin alpha-1, 28 amino acids) and the beta thymosins (thymosin beta-4, 43 amino acids). This page sets out where the name comes from, which proteins carry it, how thymosin alpha-1 and thymosin beta-4 differ, how TB-500 relates to thymosin beta-4, and what regulators’ records say. Every figure is taken from UniProt, PubChem, MeSH, PubMed or a regulator’s own database.
What thymosin is
MeSH, the U.S. National Library of Medicine’s subject index, defines thymosin as a family of heat-stable polypeptide hormones secreted by the thymus gland, and lists their biological activities as the formation of lymphocytes, restoration of immunological competence and enhancement of the expression of T-cell characteristics and function. The thymus itself, in MeSH’s description, is a primary lymphoid organ in the chest that is necessary for the normal development of immune function early in life; by puberty it begins to shrink, and much of its tissue is replaced by fat. MeSH files the thymosins among the peptide hormones.
The name goes back at least to 1966. PubMed indexes a paper of that year by Goldstein, Slater and White in the Proceedings of the National Academy of Sciences: “Preparation, assay, and partial purification of a thymic lymphocytopoietic factor (thymosin)” (PMID 5230175). PubMed holds that record without an abstract, so we cite it by title only. The later papers worked from a preparation called thymosin fraction 5, which MeSH describes as a crude thymus gland extract.
Single peptides were then isolated from calf thymus. In 1977 Goldstein and colleagues reported the amino acid sequence of thymosin alpha-1, describing it as a heat-stable, highly acidic molecule of 28 amino acids and as one of several peptides in fraction 5 that may take part in the regulation, differentiation and function of T cells; the paper also proposed a naming scheme for the family (PMID 265536). In 1981 the sequence of bovine thymosin beta-4 followed: 43 amino acids, with an acetyl group blocking the start of the chain (PMID 6940133).
Sources: MeSH D013947, D013950 and D000077596; PubMed 5230175 (title only), 265536 and 6940133. Read 11 November 2026.
The thymosin family: alpha and beta thymosins
The human proteins that carry the name today fall into two groups that UniProt files in different families. The table lists the main ones, with the length of the mature chain:
| Protein | Gene; UniProt entry | Amino acids | What the record says |
|---|---|---|---|
| Prothymosin alpha | PTMA; P06454 | 111 | The parent protein of thymosin alpha-1. UniProt places it in the cell nucleus and records that it may mediate immune function by conferring resistance to certain opportunistic infections. |
| Thymosin alpha-1 | PTMA; P06454, positions 2-29 | 28 | SDAAVDTSSEITTKDLKEKKEVVEEAEN, with an acetyl group on the first serine. The synthetic form has the international name thymalfasin. |
| Parathymosin | PTMS; P20962 | 101 | Same family as prothymosin alpha. UniProt records that it may mediate immune function by blocking the effect of prothymosin alpha. |
| Thymosin beta-4 | TMSB4X; P62328 | 43 | Binds single actin molecules and so holds back the assembly of actin filaments. Its first four amino acids are also released as a separate small peptide, AcSDKP. |
| Thymosin beta-10 | TMSB10; P63313 | 43 | A close relative of thymosin beta-4; UniProt assigns it the same actin-binding role by similarity. |
| Thymosin beta-15A | TMSB15A; P0CG34 | 44 | Another actin-binding member of the beta family; UniProt describes its expression as neuroblastoma-specific. |
The lengths need one note. UniProt counts the starting methionine: its entries give 44 amino acids for thymosin beta-4 and thymosin beta-10, 45 for thymosin beta-15A and 102 for parathymosin, and record that the methionine is removed, which leaves the chains in the table. Prothymosin alpha is listed at its full 111; its starting methionine is removed in part of the molecules. The table is not a complete list: UniProt holds further entries in both families.
Sources: UniProt P06454, P20962, P62328, P63313 and P0CG34. Read 11 November 2026.
Thymosin alpha-1 and thymosin beta-4 compared
The two best-known thymosins share a name and a starting material, not a structure:
| Feature | Thymosin alpha-1 | Thymosin beta-4 |
|---|---|---|
| Length | 28 amino acids | 43 amino acids |
| Formula and molecular weight (PubChem) | C129H215N33O55; 3,108.3 | C212H350N56O78S; 4,963 |
| Where it comes from | The first 28 amino acids of prothymosin alpha, a 111-amino-acid protein of the cell nucleus | Its own gene, TMSB4X; only the starting methionine is removed |
| Protein family in UniProt | Pro/parathymosin family | Thymosin beta family |
| What the records say it does | MeSH: used as an immunomodulator. The 1977 paper: one of several peptides in thymosin fraction 5 that may take part in regulating T cells | UniProt: organises the cytoskeleton by binding and sequestering actin monomers |
| International name of the medicine form | Thymalfasin | Timbetasin |
| Research compound that borrows from it | Thymosin alpha-1 itself | TB-500, a seven-amino-acid fragment, not the whole protein |
Two later papers changed how each is understood. In 1984 Haritos, Goodall and Horecker isolated from rat thymus a protein of about 112 amino acids with the thymosin alpha-1 sequence at its start, named it prothymosin alpha, and reported that peptides corresponding to thymosin alpha-1 itself were not detected in their extracts; they concluded that prothymosin alpha appears to be the native protein from which thymosin alpha-1 and other fragments are generated while thymosin fraction 5 is being prepared (PMID 6583693). In 1991 Safer, Elzinga and Nachmias showed that a peptide which holds much of the unassembled actin in resting human platelets is indistinguishable from thymosin beta-4, which they described as first isolated from calf thymus and thought to be a thymic hormone, and concluded from its widespread distribution and high concentration inside cells that it plays a significant role in regulating actin assembly in many cell types (PMID 1999398).
Thymosin beta-4 also carries a second, smaller peptide. UniProt records that its first four amino acids are released as AcSDKP, which it describes as a potent inhibitor of the differentiation of bone-marrow-derived stem cells, and that this fragment is inactivated by angiotensin-converting enzyme (ACE). PubChem lists AcSDKP under the name goralatide, with a molecular weight of 487.5 (CID 65938).
Sources: UniProt P06454 and P62328; MeSH D000077596 and D000099201; PubChem CID 16130571, 16132341 and 65938; PubMed 265536, 6583693 and 1999398. Read 11 November 2026.
Thymosin beta-4 and TB-500 are not the same thing
TB-500 is the name used for a synthetic seven-amino-acid peptide, LKKTETQ, carrying an acetyl group. On the UniProt sequence of thymosin beta-4 that stretch sits at positions 17 to 23 of the 43-amino-acid chain. The protein has a molecular weight of 4,963 in PubChem; a seven-amino-acid fragment is a small fraction of that. The difference matters when reading the literature. PubMed indexes clinical trials under thymosin beta-4, for example a phase III trial of a thymosin beta-4 eye-drop solution in neurotrophic keratopathy reported in 2022 (PMID 36613994, cited by its title); that trial studied the protein, not the fragment. Our TB-500 guide follows that distinction through the record.
Sources: UniProt P62328 (sequence); PubChem CID 16132341; PubMed 36613994 (title only). Read 11 November 2026.
Thymosin in regulators’ records
No thymosin is an FDA-approved medicine. A search of Drugs@FDA by active ingredient on 11 November 2026 returned no product for thymalfasin, timbetasin or any ingredient containing the word thymosin, and none under the brand name Zadaxin; the same form of query returns the record of pentagastrin, which we used as a check. MeSH records that thymalfasin, the synthetic form of thymosin alpha-1, is used alone or with interferon as an immunomodulator for chronic hepatitis B and hepatitis C; that description does not correspond to any FDA-approved product. The table summarises the records our compound guides hold, each with the date it was read:
| Substance | United States | Elsewhere |
|---|---|---|
| Thymosin alpha-1 (thymalfasin) | No approved product: Drugs@FDA returned no match for thymalfasin, thymosin or the brand name Zadaxin (search of 11 November 2026). FDA's compounding advisory committee voted 4 to 17 against adding it to the list of substances pharmacies may compound from (4 December 2024). | Italy's medicines agency AIFA lists Zadaxin (thymosin alpha-1) as authorised and prescription-only, with a pack authorised in May 1996. EMA's medicines data holds no EU-wide authorisation (records read 20 October 2026). |
| Thymosin beta-4 (timbetasin) | No approved product: Drugs@FDA returned no match for timbetasin or thymosin (search of 11 November 2026). | The 2026 WADA Prohibited List names thymosin beta-4 and its derivatives, TB-500 among them, in section S2.3 (list read 20 October 2026). |
| TB-500 (the fragment) | No approved product. It is named in Appendix A of the HHS draft interim policy on compounding of 30 October 2026, which is not an FDA approval. | Covered by the same WADA entry as a derivative of thymosin beta-4. |
The full, dated tables for the United States, the European Union, the United Kingdom and Australia are on the thymosin alpha-1 guide and the TB-500 guide; the rules behind them are explained on Are Peptides Legal?
Sources: Drugs@FDA via openFDA, searched by active ingredient and brand name (data updated 10 November 2026); MeSH D000077596; FDA Pharmacy Compounding Advisory Committee minutes of 4 December 2024; AIFA medicines database; EMA medicines data; WADA 2026 Prohibited List; HHS draft interim policy of 30 October 2026. Doses are not reproduced here.
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Where this sits in the rest of the reference
FAQ
What is thymosin?
Thymosin is a name used since at least 1966 for a lymphocyte-forming factor prepared from the thymus gland. MeSH defines thymosin as a family of heat-stable polypeptide hormones secreted by the thymus gland. Today the name covers two separate protein families: the alpha thymosins, such as the 28-amino-acid thymosin alpha-1, and the beta thymosins, such as the 43-amino-acid thymosin beta-4.
What is the function of thymosin?
It depends on which thymosin. MeSH lists the activities of the family as the formation of lymphocytes, restoration of immunological competence and enhancement of T-cell characteristics and function. For thymosin beta-4, UniProt records a different job: it organises the cell's cytoskeleton by binding single actin molecules and holding back the assembly of actin filaments.
Where is thymosin produced?
The first thymosins were isolated from calf thymus, and MeSH describes them as secreted by the thymus gland. Later work found them elsewhere too: a 1991 paper showed that an actin-binding peptide of human blood platelets is identical to thymosin beta-4 and described its distribution as widespread, and UniProt places prothymosin alpha, the parent of thymosin alpha-1, in the cell nucleus.
What is the difference between thymosin alpha-1 and thymosin beta-4?
They share a name and a history, not a structure. Thymosin alpha-1 has 28 amino acids and is the first part of a larger nuclear protein, prothymosin alpha; MeSH describes its use as an immunomodulator. Thymosin beta-4 has 43 amino acids, comes from its own gene and binds actin inside cells. UniProt files them in different protein families.
What is thymosin fraction 5?
A partly purified extract of calf thymus. MeSH calls thymosin fraction 5 a crude thymus gland extract, and the 1977 paper that reported the sequence of thymosin alpha-1 describes that peptide as one of several present in fraction 5. Thymosin beta-4 was sequenced from the same starting material in 1981.
Is thymosin alpha-1 FDA approved?
No. A search of Drugs@FDA by active ingredient and brand name on 11 November 2026 returned no approved product for thymalfasin, the international name of thymosin alpha-1. It does hold a national authorisation in Italy, where the medicines agency AIFA lists Zadaxin as authorised and prescription-only. Our thymosin alpha-1 guide sets out the record region by region.
Is TB-500 the same as thymosin beta-4?
No. Thymosin beta-4 is a 43-amino-acid protein with a molecular weight of 4,963 in PubChem. TB-500 is the name used for a synthetic seven-amino-acid fragment, LKKTETQ with an acetyl group, which matches positions 17 to 23 of the protein. The 2022 eye-drop trial cited on this page studied the protein, not the fragment; our TB-500 guide follows that distinction.
Is thymosin a hormone?
MeSH files the thymosins as polypeptide hormones of the thymus gland, and that is how they were first described. Later records complicate the label: a 1984 paper found that thymosin alpha-1 is the opening stretch of a larger protein, prothymosin alpha, and concluded that it appears to be generated from that protein while the thymus extract is prepared, and a 1991 paper identified thymosin beta-4 as an actin-binding peptide inside cells.
Is thymosin prohibited in sport?
Thymosin beta-4 is. The 2026 World Anti-Doping Agency Prohibited List names thymosin beta-4 and its derivatives, with TB-500 as the example, in section S2.3. The list does not name thymosin alpha-1. This reflects the list as read on 20 October 2026; athletes should check the current list and their own federation.
Is this medical advice?
No. This page explains what the thymosins are, from public scientific and regulatory sources. It gives no dose, schedule or recommendation and makes no claim of benefit; anyone considering a medicine should follow their own clinician and the product label. Peptides sold for research are labelled research use only, not for human use. Medibact supplies USP-grade bacteriostatic water, made in the USA, and is not affiliated with any manufacturer of the products named here.
References: UniProt P06454, P20962, P62328, P63313, P0CG34; MeSH D013947, D000077596, D000099201, D013950; PubChem CID 16130571, 16132341 and 65938; PubMed 5230175, 265536, 6940133, 6583693, 1999398, 36613994; Drugs@FDA via openFDA; the regulator records named above. Read 11 November 2026 unless another date is given.
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