The endocrine system is regulated by feedback in much the same way that a thermostat regulates the temperature in a room. For the hormones that are regulated by the pituitary gland , a signal is sent from the hypothalamus to the pituitary gland in the form of a "releasing hormone," which stimulates the pituitary to secrete a "stimulating hormone" into the circulation. The stimulating hormone then signals the target gland to secrete its hormone. As the level of this hormone rises in the circulation, the hypothalamus and the pituitary gland shut down secretion of the releasing hormone and the stimulating hormone, which in turn slows the secretion by the target gland. This system results in stable blood concentrations of the hormones that are regulated by the pituitary gland.
A girl's gonads, the ovaries (pronounced: OH-vuh-reez), are located in her pelvis. They produce eggs and secrete the female hormones estrogen (pronounced: ESS-truh-jen) and progesterone (pronounced: pro-JESS-tuh-rone). Estrogen is involved when a girl begins to go through puberty. During puberty, a girl will experience breast growth, will begin to accumulate body fat around the hips and thighs, and will have a growth spurt. Estrogen and progesterone are also involved in the regulation of a girl's menstrual cycle. These hormones also play a role in pregnancy.