Hormones are chemical messengers in the form of peptides or steroids, produced by nearly every organ system and tissue type in the body or synthetically, transmitting information between cells (or between the components of a cell) and regulating functions such as growth and metabolism in their target cells by attaching to receptor sites, causing a reaction in the affected organ. All multi-cellular organisms (including plants) produce hormones. Hormone molecules are secreted (released) directly into the bloodstream, other body fluids, or into adjacent tissues. Some examples of hormones are estrogen, progesterone, testosterone and insulin. The primary organs involved in hormone production are the pituitary, pancreas, ovary, testes, thyroid, and adrenal glands. Hormones regulate growth, puberty, reproduction, metabolism, the digestive system, thirst, blood glucose and cholesterol levels, fat burning and storage, absorption and excretion, internal clocks such as day and night cycles, menstrual cycles and the sex drive.