What is TB-500? The thymosin beta-4 fragment explained

What is TB-500?

TB-500 is a synthetic seven-amino-acid peptide, Ac-LKKTETQ, that matches positions 17 to 23 of thymosin beta-4, a natural protein of 43 amino acids whose sequence was first determined from calf thymus in 1981 (Low et al., Proceedings of the National Academy of Sciences, 1981; Ho et al., 2012). The FDA's briefing document (memo dated 15 May 2026) describes it as an N-acetylated heptapeptide: seven amino acids with an acetyl tag, a small chemical group, on the first. The same document reports that a veterinary preparation called TB-500 appeared in 2011 and was marketed for equine and greyhound sports, according to the FDA's reading of Ho et al.

A 2010 review reports that thymosin beta-4 is found in essentially all cells and body fluids (Sosne et al., FASEB Journal, 2010). TB-500 is a piece of it, not the whole, and the FDA states that the two are not the same substance. The TB-500 identity page sets the two sequences side by side, and the TB-500 research guide collects the published studies.

The FDA's literature search dated the first synthetic version to a 2003 mouse study that used the untagged fragment LKKTETQ (Philp et al., Wound Repair and Regeneration, 2003, as summarised by the FDA). The catalogue lists TB-500 reference material as a laboratory product, and research material is not a medicine.

What is TB-500 also known as?

TB-500 is also written TB500 or TB 500. The FDA's briefing document (memo dated 15 May 2026) gives its chemical name as N-acetyl-L-leucyl-L-lysyl-L-lysyl-L-threonyl-L-alpha-glutamyl-L-threonyl-L-glutamine, or Ac-LKKTETQ in one-letter codes, a shorthand that gives one letter per amino acid; Esposito et al. use the same code (Drug Testing and Analysis, 2012). Papers also call it the N-terminal acetylated 17-23 fragment of thymosin beta 4. The FDA reports that TB-500 is a common name, not a formal drug name, and that it found the name used for different salts and forms, so the name alone does not settle which molecule is meant.

The table below lists each name or code with its source. One conflict is worth noting: Sport Integrity Australia's thymosin beta-4 page (read 14 October 2026) lists TB-500 as another name for thymosin beta-4, while the FDA and the 2012 papers treat them as separate substances, and the Australian Poisons Standard lists them as separate entries.

A CAS number identifies one chemical form. The FDA says several supplier sites use the free base's number for the acetate salt, so a number on a label does not by itself say which form a vial holds.

What is the mechanism of action of TB-500?

The FDA's briefing document (memo dated 15 May 2026) does not report an established mechanism for TB-500 in people. The published idea comes from its parent protein: LKKTETQ contains the central actin-binding stretch of thymosin beta-4, and actin is the protein that builds the cell's internal threads, according to a review of laboratory work (Sosne et al., FASEB Journal, 2010). The FDA says it remains to be determined whether actin binding contributes to the wound-healing activity reported for the untagged fragment. In cultured fibroblasts, TB-500 itself did not close scratch wounds significantly more than vehicle after 8 hours, while a shorter breakdown piece, Ac-LKKTE, produced a small significant closure (number of cultures not stated in the FDA summary), a cell result that does not show an effect in a living body (Rahaman et al., 2024, as summarised by the FDA).

A 2004 Nature paper reported in heart cells in culture that full-length thymosin beta-4 formed a complex with PINCH and integrin-linked kinase and activated the survival kinase Akt, and that treatment after coronary artery ligation in mice improved cardiac function, results for full-length thymosin beta-4 that do not establish effects of TB-500 or effects in people (Bock-Marquette et al., 2004; mouse number not stated in the abstract).

The 2010 review names two sites on the protein by sequence: Ac-SDKP, the four amino acids at the start of the chain, and a short sequence containing LKKTETQ, the central actin-binding domain (positions 17 to 23) plus one more amino acid (Sosne et al., 2010). Sport Integrity Australia says the pathways are 'not fully understood'.

Evidence limits

Who is left out of TB-500 and thymosin beta-4 studies?

The FDA reports that it identified no reproductive or developmental toxicity studies of TB-500, so it identified no data on pregnancy (briefing document, memo dated 15 May 2026). The venous ulcer trial record lists infection on the ulcer, current or former malignancy, diabetes, significant organ disease, pregnancy or lactation among its exclusions (NCT00832091). A healthy-volunteer Phase 1 trial record lists abnormal vital signs or laboratory values, hepatitis B or C, HIV, recent blood donation, pregnancy and breastfeeding among its exclusions (NCT04555824).

Exclusion lists show whom researchers chose not to enrol. They do not show who is harmed. Sport Integrity Australia adds that a therapeutic use exemption, the sport approval route for banned medicines, would not be granted for thymosin beta-4 for any medical condition.

Does TB-500 affect testosterone?

No study that measured testosterone after TB-500 appears in the sources opened for this page. The only related finding concerns full-length thymosin beta-4: in 1981 Rebar et al. reported in Science that it stimulated release of luteinizing hormone-releasing factor, a hypothalamic signal in the reproductive hormone chain, from superfused hypothalami taken from female rats (number not stated in the abstract), a laboratory result in rat tissue that is neither TB-500 nor a testosterone measurement and does not establish a result in people. The World Anti-Doping Agency 2026 list names thymosin-ß4 and TB-500 in its growth-factor class, S2.3, and does not name them in S2.2.1, its testosterone-stimulating peptides (read 14 October 2026). That leaves the question open for TB-500 itself.

That experiment used rat tissue, a partly purified thymic extract and synthetic thymosin beta-4, not TB-500, and it measured luteinizing hormone-releasing factor and luteinizing hormone release, not testosterone (Rebar et al., 1981). The FDA's document repeats the history, saying thymosin beta-4 was once proposed as a thymic hormone able to stimulate the hypothalamic-pituitary-gonad axis.

In a PubMed search on 14 October 2026 for thymosin beta 4 with testosterone, Leydig cell or testis terms, none of the first eight titles described a study of testosterone measured after thymosin beta-4 treatment; this is a title screen, not a systematic search, and it does not show that no such study exists.

How does TB-500 make people feel, and can it cause tiredness?

No study in the sources opened for this page asked people how TB-500 feels or whether it causes tiredness. The 2021 Phase 1 paper, covering 84 healthy Chinese adults randomised to recombinant thymosin beta-4 or placebo, lists blood, urine and ECG findings, and its text contains none of the words fatigue, tiredness, headache or dizziness, a word search that does not establish the absence of those symptoms or any TB-500 safety result (Wang et al., Journal of Cellular and Molecular Medicine, 2021). The ARISE-3 trial record, a Phase 3 study of 700 people with dry eye, lists instillation-site pain, reduced visual acuity and nasopharyngitis among its most frequent events, in a different molecule and population from TB-500 (NCT03937882).

The Wang and Ruff trials were small. Wang et al. enrolled 84 healthy adults and Ruff et al. enrolled 40, and neither abstract gives a symptom list. In the two healthy-volunteer parts of Wang et al. (2021; 54 and 30 volunteers), events were reported by 25 of 44 treated and 7 of 10 placebo participants in the first part and by 16 of 24 treated and 5 of 6 placebo participants in the second, in small, short trials of recombinant thymosin beta-4, a different molecule from TB-500.

Online accounts cannot be tied to a product's identity. The FDA reports finding websites that discuss TB-500 but no published human case reports (briefing document, memo dated 15 May 2026). The full adverse-event record, including the 2026 FDA review, is on the side-effects page linked above.

Is TB-500 banned in sport?

Yes. The World Anti-Doping Agency's 2026 Prohibited List, in force from 1 January 2026, names 'Thymosin-ß4 and its derivatives e.g. TB-500' under S2.3, growth factors and growth factor modulators, a class prohibited at all times, in and out of competition (WADA, read 14 October 2026). Sport Integrity Australia says administration of thymosin beta-4 is banned in sport at all times and that a therapeutic use exemption would not be granted for any medical condition (page read 14 October 2026). These are sport rules, not medicine law.

The United States Anti-Doping Agency listed thymosin beta 4 as a prohibited growth factor in a sanction announcement of 2 June 2017, read on 14 October 2026, in which a US weightlifter accepted a four-year sanction for possessing prohibited peptides.

A sport listing is a rule for athletes. It is separate from the medicine law in the next section, and it does not describe what the compound does.

Is TB-500 in the Australian Poisons Standard?

Yes. The Therapeutic Goods (Poisons Standard—October 2026) Instrument 2026, F2026L01327, lists TB-500 and thymosin beta 4 as separate entries in Schedule 4, the schedule for prescription-only medicines, and in Appendix D clause 5, the table of poisons that must not be possessed by a person without authority under the law of the jurisdiction in which the possession occurs: item 38 for TB-500 and item 39 for thymosin beta 4 (read 14 October 2026). A schedule entry classifies a substance. It does not decide any person's legal position. The Federal Register of Legislation shows the instrument as in force on 14 October 2026.

The instrument is dated 28 September 2026 and its commencement table gives 1 October 2026. The Australian peptide law hub reads every catalogue peptide's entry and explains how Appendix D relates to state law, so this page does not repeat that analysis.

Sport Integrity Australia's thymosin beta-4 page adds that the substance is not approved by the TGA.

What do the TGA and Border Force say about importing peptide products?

The TGA said in its safety advisory of 13 April 2026 that a 'research use only' disclaimer alone does not make supply lawful, change a product's regulatory status or permit importation. On 7 May 2026 it said that peptide products that do not meet importation requirements because of poor or unclear labelling will not be released under the Personal Importation Scheme and that it will notify Australian Border Force (ABF) to seize and destroy them when they are intercepted at the border (TGA warning). The April advisory adds that state and territory laws may impose additional controls.

Border Force's role is reported here only as the TGA describes it. This page assesses no import and gives no import pathway.

The scheme's own conditions are on the TGA Personal Importation Scheme page (last updated 8 September 2026, read 14 October 2026) and in the Australian peptide law hub, which this page does not repeat.