Chemical produced and secreted by endocrine (ductless) glands that travels through the bloodstream and controls or regulates the activity of another organ or group of cells - its target organ. (For example, growth hormone released by the pituitary gland controls the growth of long bones of the body.) There are two main types of hormones - steroidal (e.g., oestrogen, testosterone, aldosterone, cortisol) and non-steroidal. Secretion of hormones is regulated by feedback mechanisms and neurotransmitters.