Athletes with XY DSDs who have testes (usually internal), XY sex chromosomes, male-typical levels of testosterone, and functional androgen receptors are often described as females with "hyperandrogenism," i.e., abnormally high levels of testosterone. They experience physical benefits of this high testosterone during puberty, which translate into athletic advantages over females. The issue for sports is that athletes with the XY DSD 5-alpha reductase deficiency (5-ARD), may be socialized as female, may be legally female, and may live and identify as female; but they are male.
These individuals are usually born with female-appearing genitalia, which can lead to being sexed as female. Here's why. 5-ARD is caused by a mutation in the gene that codes for the enzyme 5-alpha reductase, which converts testosterone into a more potent androgen, DHT. This androgen interacts with the androgen receptor, like testosterone, and is necessary for the typical development of male external genitalia (penis and scrotum) and the prostate. Without DHT, female-typical external genitalia develop.