Hormones are produced in small organs called glands, which are found inside your body and your mind. When your body is ready to enter puberty, a gland called the pituitary releases hormones that set the process in motion. This tiny, pea-shaped organ is located at the base of your brain, in an area called the hypothalamus. The beginning of puberty is a bit like a series of falling dominoes. First, the hypothalamus starts to produce a hormone called gonadotropin-releasing hormone (GnRH), which develops in your blood. When its levels are high enough, GnRH signals the pituitary to release two other hormones called follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH and LH travel through the blood and reach your testicles (see page 36), telling them to start making the hormone called testosterone.

Testosterone is the hormone that causes most of the changes that will happen inside and outside your body during puberty. Physical sensations Hormones are powerful chemicals. Although they are not responsible for everything you feel during puberty, they can affect your mood in unexpected ways. If you feel irritable, upset, or anxious and don’t know why, take a break and do something that relaxes you to give yourself a chance to calm down. See page 68 for more tips on relaxation.