ABSTRACT
Numerous chemical messages control various functions at the level of cells, organs, and organ systems. Such messages may be autocrine (the chemical message directly affects the cell producing it), paracrine (the message has local effects), or endocrine (the message has distant sites of action). Typically, endocrine effects are caused by hormones that are produced by specialized organs, although several important endocrine functions are performed by nonglandular tissues, most prominently the liver (e.g., insulin-like growth factor I [IGF-I] production and thyroid hormone activation) and the kidney (e.g., renin production and vitamin D activation).
