Abstract BACKGROUND For transgender individuals taking hormone therapy (HT), data on laboratory values are limited, and the effects on laboratory values cannot be easily predicted. We evaluated the impact on common laboratory analytes in transgender individuals before and after initiation of HT. METHODS We conducted a retrospective chart review of transgender patients identified at transgender-specific clinics at an urban county hospital and community clinic. Laboratory data were collected on hormone concentrations, hematologic parameters, electrolytes, lipids, and liver and renal markers before and after initiation of HT. RESULTS We identified 183 transgender women (TW) and 119 transgender men (TM) for whom laboratory data were available. In all, 87 TW and 62 TM had baseline laboratory data, and data were also available for 133 TW and 89 TM on HT for >6 months. The most significant changes were seen in red blood cell count, hemoglobin concentration, hematocrit, and creatinine levels after >6 months of HT, which increased in TM and decreased in TW after HT (P < 0.005; d index > 0.6). Alkaline phosphatase, aspartate aminotransferase, and alanine aminotransferase levels increased in TM; however, the effect size was small (d index < 0.5). Calcium, albumin, and alkaline phosphatase levels significantly decreased in TW (P < 0.001; d > 0.6). Additionally, TM were found to have increased triglycerides and decreased HDL levels (P < 0.005; d > 0.6). CONCLUSIONS Changes occur in several common laboratory parameters for patients on HT. Some laboratory values changed to match the gender identity, whereas others remained unchanged or were intermediate from the baseline values. These findings will help guide interpretation of laboratory test results in transgender patients taking HT.