How TB-500 Came About

TB-500 has no single inventor or famous launch date. Its story begins with a protein that scientists first mistook for a thymus hormone.

Looking for Thymus Hormones

In the 1960s and 1970s, Allan Goldstein and colleagues prepared extracts of calf thymus, known as thymosin fractions, hoping to find hormones that control the immune system. In a 1982 paper in the Journal of Biological Chemistry, Teresa Low and Goldstein reported the full sequence of one of these peptides. They called it thymosin beta-4: 43 amino acids, starting with an acetylated serine.

A Surprise in Blood Platelets

Later work showed that thymosin beta-4 is not limited to the thymus. In 1991 Daniel Safer and colleagues found that an actin-binding peptide they had isolated from human platelets, called Fx, was in fact identical to thymosin beta-4. The protein turned out to be one of the cell's main tools for managing actin.

Interest in Repair

Around the turn of the century, researchers at the US National Institutes of Health tested thymosin beta-4 on wounds. Their 1999 rat study reported faster skin closure and more new blood vessels. Two papers in Nature, in 2004 and 2007, then linked the protein to heart cell survival and to activation of repair cells in the outer layer of the mouse heart.

Clinical Development of the Full Protein

A US company, RegeneRx Biopharmaceuticals, and later partners developed synthetic thymosin beta-4 under codes such as RGN-259 for eye drops and RGN-137 for skin gels. Trials in chronic skin ulcers, dry eye and a corneal disease called neurotrophic keratopathy followed. None has yet resulted in an approved product.

Enter TB-500

The short acetylated fragment appeared as a veterinary preparation marketed to the horse racing world. In 2012 the Hong Kong Jockey Club's laboratory published the first method able to confirm TB-500 and its breakdown products in horse urine and blood after a single 10 mg dose. Racing authorities and the World Anti-Doping Agency have since treated thymosin beta-4 and its derivatives as prohibited.

Today

TB-500 is widely discussed in fitness forums, often with claims borrowed from studies of the full protein. Recent work in anti-doping science, such as a 2025 population study of thymosin beta-4 levels in racehorses, focuses on telling natural levels apart from misuse.