BACKGROUND: The effect of gender affirming hormone therapy (GAHT) on clinical laboratory parameters, including levels of liver enzymes alanine aminotransferase (ALT) and aspartate transaminase (AST), is an area of uncertainty in transgender health. AIM: We sought to estimate the distribution parameters of liver enzyme levels among transmasculine (TM) and transfeminine (TF) persons receiving GAHT relative to the corresponding measures in cisgender reference groups, and to evaluate longitudinal changes in these laboratory measures following GAHT initiation. METHODS: The data for this longitudinal study included 624 TF and 438 transmasculine (TM) people as well as 4,090 cisgender males and 4,797 cisgender females enrolled in 3 integrated health systems. Time under observation was divided into 2 intervals: from the first blood test to the date of the first filled GAHT prescription and from GAHT initiation to the most recent ALT or AST measurement. Linear mixed models were used to compare changes in log-transformed ALT and AST values among transgender cohort members before and after GAHT initiation, and relative to the reference groups. The results were expressed as relative differences (in %) and the ratios of these differences (ratios-of-ratios) along with the 95% confidence intervals (CIs). OUTCOMES: Changes in ALT and AST levels among transgender people over time and relative to the corresponding changes in cisgender referents. RESULTS: Among TM study participants, the post GAHT ratios-of-ratios for AST were 1.61 (95% CI: 1.13, 2.31) and 1.57 (95% CI: 1.06, 2.31) relative to cisgender males and females respectively. For ALT, the corresponding comparisons yielded the ratios-of-ratios (95% CIs) of 2.06 (1.67, 2.54) and 1.90 (1.50, 2.40). No statistically significant changes were observed among TF participants. Other factors associated with higher liver enzyme levels included alcohol use/abuse and obesity. CLINICAL IMPLICATIONS: TM persons may experience modest increases in ALT and AST concentrations following testosterone initiation; however, clinical significance of the observed association remains unclear and requires further investigation. By contrast, feminizing GAHT is unlikely to induce appreciable changes in liver enzyme levels. STRENGTH AND LIMITATIONS: The strengths of this study are the longitudinal design and the ability to assemble an unselected cohort nested within large health systems. The main limitations include the lack of information on hormone levels and the inability to take into account GAHT doses and routes of administration. CONCLUSION: The influence of long-term GAHT on ALT and AST levels appears modest and not likely to reflect clinically meaningful changes in liver function. Hashemi L, Zhang Q, Getahun D, et al. Longitudinal Changes in Liver Enzyme Levels Among Transgender People Receiving Gender Affirming Hormone Therapy. J Sex Med 2021;18:1662-1675.