Iris de Nie

10 C H A P T E R 1 Figure 2. The hypothalamic–pituitary–gonadal axis in birth-assigned males. E 2 , 17 β -estradiol; DHT, dihydrotestosterone; FSH, follicle-stimulating hor- mone; GnRH, gonadotropin-releasing hormone; LH, luteinizing hormone. Re-used from Dwyer et al 6 , with permission from Springer Nature. Pubertal stages are classified using the Tanner scale, which is an objective classification system that is used to track the development of secondary sex characteristics (Figure 3). Tanner stage 1 corresponds with the pre-pubertal stage. In birth-assigned males, testicular enlargement is the first physical manifestation of puberty and is often accompanied by a thinning of the scrotal skin. When reaching Tanner stage 3, birth-assigned males will have increased muscle growth, more noticeable penile and testicular growth, nocturnal ejaculations and the onset of voice lowering. These characteristics will develop further in Tanner stage 4, and Tanner stage 5 corresponds with the post-pubertal stage.