Classically, endocrinology has consisted of the study of hormone-secreting organs and the evaluation of hormonal activity. Hormones were considered chemical substances that, when secreted into the circulation, would affect the metabolic pathways of cells with appropriate receptors. In recent years it has been appreciated that many hormones can also act as neurotransmitters, and in addition to being secreted in the systemic circulation, hormones may also have a local effect where they are secreted. Compounds such as vitamin D are now called hormones, even though they are not produced by any one gland. Hormone action is dependent on the functional status of specific hormone receptors as well as the absolute blood level.