The term 'hormone' was initially applied to molecules which were synthesized in endocrine glands, secreted into the bloodstream, and then transported to distant target sites. Originally, only three classes of hormones were recognized: proteins and peptides (e.g. gonadotropinreleasing hormone, luteinizing hormone), steroids (e.g. testosterone, oestradiol) and amino acid-related hormones (e.g. noradrenaline (norepinephrine), adrenaline (epinephrine).