Increasing numbers of transgender and gender-diverse individuals are seeking initiation of gender-affirming hormone therapy. This aligns an individual’s physical characteristics with their gender identity and improves psychological outcomes. Physical changes, including changes to muscle mass and body fat redistribution, can alter sex-specific laboratory reference ranges. This presentation will review the impact of gender affirming hormone therapy on laboratory parameters with sex-specific reference ranges, with a focus on haemoglobin/haematocrit, renal function, cardiac biomarkers, and prostate-specific antigen.