When a trans person starts gender-affirming hormone therapy, routine monitoring usually looks at the liver, lipids, blood pressure or haematocrit. The kidney rarely makes the list. A team of nephrologists and clinicians in the United States has now published in the Clinical Journal of the American Society of Nephrology the largest renal follow-up to date in transgender and gender-diverse people, and the main finding fits in one sentence: testosterone and oestradiol do not behave the same way in the kidney.
The study draws on data from the US Veterans Health Administration between January 2006 and December 2019. Of the 6,616 veterans included, 262 were receiving hormone therapy: 214 transgender women and 48 transgender men. The remaining 6,354 cisgender people not on treatment served as the comparison group: 1,988 women and 4,366 men.
In transgender men, serum creatinine rose early, as soon as three months after starting testosterone, and levelled off around the twelve-month mark. The estimated glomerular filtration rate (eGFR), the figure clinicians use to gauge how well the kidney filters, fell significantly after treatment began. In transgender women starting oestradiol, no relevant change appeared: creatinine held steady and eGFR remained stable.
Does that mean testosterone harms the kidney? Not necessarily, and this is where it gets interesting. Creatinine is a muscle breakdown product, and testosterone increases muscle mass. More muscle means more creatinine, and the equations that estimate eGFR read that as poorer filtration even when the kidney is working just the same. It is a well-known bias in athletes and in people with a lot of lean mass, and a study built on medical records cannot fully separate it out.
There is a second methodological detail the authors themselves highlight: eGFR was calculated with algorithms based on affirmed gender and on sex assigned at birth, and the results are not identical depending on the formula chosen. In other words, the same person can get a different kidney function figure depending on which variable the equation uses. For someone with pre-existing chronic kidney disease, that can change a dosing or monitoring decision.
The study population also calls for caution: these are veterans, mostly adults, with a heavier burden of disease (hypertension, diabetes, smoking) than the general population. The authors stress that this is a retrospective analysis and that the findings should be read as a signal to improve surveillance, not as proven causation.
The practical takeaway is simple and twofold. First, include creatinine and eGFR in the follow-up of trans people on hormone therapy, especially transgender men starting testosterone, and repeat the blood test during the first months. Second, do not panic over a single number: before talking about kidney damage, check muscle mass, hydration and which eGFR formula is being applied. Kidney health is part of trans health, even if until now hardly anyone had put it on the list.
Source: The Effect of Gender-Affirming Hormone Treatment on Kidney Function Trajectories in Transgender and Gender-Diverse Veterans. Jackson N, Buljubasic N, Kalantar-Zadeh K, Wolfe HL, Neugarten J, Hashemi L. Clinical Journal of the American Society of Nephrology 2026;21(10):1710-1718. DOI: 10.2215/CJN.0000001131.
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