The endocrine system is comprised of glands that secrete hormones. These glands include the pituitary, thyroid, parathyroid, adrenal and gonads. Hormones circulate through the bloodstream and influence specific target cells. Hormone release is controlled through a system of feedback loops, where more hormone is secreted if blood levels decline below a minimum or production halts if levels exceed a maximum. The menstrual cycle is regulated by hormones like FSH, LH, estrogen and progesterone that are secreted by the hypothalamus, pituitary and ovaries.