Bromotyrosine is a stable by-product of eosinophil peroxidase activity, a result of eosinophil activation during an inflammatory immune response. The elevated presence of bromotyrosine in tissue, blood and urine in medical conditions involving eosinophil activation has highlighted the potential role of bromotyrosine as a medical biomarker. This is highly beneficial in a paediatric setting as a urinary non-invasive biomarker. However, bromotyrosine and its derivatives may exert biological effects, such as protective effects in the brain and pathogenic effects in the thyroid. Understanding of these pathways may yield therapeutic advancements in medicine. In this review, we summarise the existing evidence present in literature relating to bromotyrosine formation and metabolism, identify the biological actions of bromotyrosine and evaluate the feasibility of bromotyrosine as a medical biomarker.