Over the years, several studies have shown that many factors are likely to affect the results of forensic hair analyses and complicate their interpretation. Among these factors, one of the major drawbacks in hair analysis is the affectability of deposited xenobiotics by cosmetic treatments which could be eventually used to adulterate the sample. It is well known that some cosmetic treatments containing hydrogen peroxide, such as permanent dyeing or bleaching, lead to the formation of 1-H-pyrrole-2,3,5-tricarboxylic acid (PTCA), a melanin degradation product. Considering that PTCA is also an endogenous compound, spontaneously formed by natural oxidation of melanin, its only detection in hair is not enough to confirm a cosmetic oxidative treatment. For this reason, the aim of the present work was to develop and validate a reliable liquid-liquid extraction method in ultra-high-performance liquid chromatographic-tandem mass spectrometry for the determination of endogenous PTCA in hair from a wide multi-ethnic population (African, Arab, Asian-Pacific, Caucasian, Hispanic, Indian). According to previous studies, untreated hair samples showed a PTCA content of 8.54 ± 5.72 ng/mg (mean ± SD), ranging between 0.44 and 23.7 ng/mg; after in vitro cosmetic bleaching, PTCA increased to 16.8 ± 6.95 ng/mg (range 4.16-32.3 ng/mg). Comparing baseline PTCA levels of each subgroup with the others, we could not observe any statistically significant difference, except for Caucasians (p < 0.05), wherein the concentrations were lower. Further studies and a wider sampling are necessary to elucidate the role of PTCA as diagnostic marker of cosmetic hair treatment in forensic field.