This study investigates how sex hormones affect kidney function in people undergoing gender-affirming hormone therapy (GAHT). We know men have a faster progression of kidney disease. Earlier studies suggest that the female sex hormone estradiol may have a protective effect on kidney function while the male sex hormone testosterone may have the opposite effect. But the reasons why this happens remain unclear. By studying participants undergoing (GAHT) we gain insight into the mechanisms by which testosterone and estradiol influence the kidneys. People undergoing GAHT provide a unique chance to study how sex hormones interact with the kidneys. The results may help us to understand why men and women exhibit differences in kidney disease development. This study will include 30 men and 30 women, aged 18 to 40, who start GAHT. Participants will have three study visits, two of which will happen during their scheduled healthcare appointments. During the first visit, a screening will take place to check if patients can take part in the study. At study visits before and after one year of therapy, kidney function is measured, kidney MRI is performed, urine is collected and a small sample of fat tissue. Taking part in the study does not delay the start of GAHT.