The impact of gender affirming hormone therapy on biomarkers of kidney function and glomerular filtration rate in transgender adults

Creatinine and cystatin C are proteins that are made by muscles and cells that are filtered by the kidney. Their blood levels are elevated in the setting of kidney dysfunction. Equations based on creatinine and cystatin C blood levels are used to estimate kidney function in order to identify kidney disease, assess its severity, and guide clinical decision-making. These formulas adjust for age, sex/gender and sometimes race which are non-kidney determinants of creatinine and cystatin C blood levels which need to be considered in order to accurately and precisely estimate kidney function. Estimating kidney function avoids time consuming, impractical and costly direct measurements and is routinely done in laboratories across the world. Transgender men and women are individuals who identify as the opposite gender to that which was assigned to them at birth (based on their biological sex). Transgender men and women are commonly treated with gender affirming hormone therapy (testosterone and estrogen respectively) in order to change their secondary sexual characteristics to that of their identified gender. This hormone therapy changes body composition and muscle mass resulting in changes in blood creatinine levels. How this hormone therapy changes creatinine and cystatin C and the estimation of kidney function by equations that use creatinine and cystatin C (which were developed and validated in non-transgender populations) is unknown. Our study will examine how cystatin C, creatinine and kidney function change over 1 year in 36 transgender men and 36 transgender women initiating hormone therapy across 18 centers in Canada. The study will inform how to properly estimate kidney function in transgender men and women treated with or without hormone therapy which influences their clinical care including referral to kidney doctors, the need for additional laboratory testing, medication dosing, referral for kidney transplantation and the timing of dialysis initiation.