An endocrine gland releases hormones into the surrounding internal fluid for delivery to target tissues, commonly through the blood. Unlike an exocrine gland, it does not use a duct to discharge its hormonal secretion onto an epithelial surface or into an anatomical lumen.[ref]
Anatomy and Physiology 2e by J. Gordon Betts and others. Section 17.1, Structures of the Endocrine System
The thyroid and pituitary are examples. The pancreas shows why classification can be functional: its endocrine cells release hormones such as insulin, while other pancreatic cells send digestive secretion through ducts. Hormone delivery alone does not specify the response. A target cell's receptors and intracellular signaling determine whether and how it reacts, and feedback can regulate further secretion.
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