Secretion of Hormones

The secretion of most hormones (except steroid) is by the process of exocytosis. Figure summarises the formation, transport, release and reconstitution of secretory vesicles. The release of hormones from the endocrine glands is controlled by nervous, hormonal or metabolic stimuli which also control the rate of release into the bloodstream. As with neurotransmitters, ca²⁺ seems to play an important role in the release of hormones. The secretion is not random but follows a certain pattern. This may be circadian (i.e. approximately a 24 hour cycle) or seasonal or may be periodical (e.g. the human menstrual cycle). Some hormones like the thyroxine or triiodothyronine are secreted continuously, by the thyroid gland).