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The corpus luteum is a temporary endocrine structure formed from the ovarian follicle after ovulation. Its luteinized cells produce substantial progesterone, helping maintain the secretory state of the endometrium. Without sustained hormonal support it regresses; in early pregnancy, human chorionic gonadotropin helps preserve its steroid production.^[ref]

It illustrates that an endocrine gland need not be a permanently separate organ. A local tissue is reorganized into a hormone-secreting structure and later changes again. A measured progesterone concentration reflects both this production and removal from the blood, so it is not a direct count of luteal cells.

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]

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.

Glandular secretion is release of products by a gland. Exocrine routes deliver material through ducts, while an endocrine gland releases signals toward the circulation. Flow rate and composition can change independently, as illustrated by salivary secretion.

The pituitary gland is an endocrine gland at the base of the brain, connected to the hypothalamus. Its anterior pituitary contains hormone-secreting glandular cells; its posterior pituitary contains the terminals of hypothalamic neurons. The two regions consequently communicate with the hypothalamus in different ways.^[ref]

For example, the anterior pituitary secretes prolactin, which promotes milk production, while the posterior pituitary releases oxytocin, which promotes milk ejection. One organ thus participates in distinct, coordinated stages of a physiological response.^[ref]

The thyroid is an endocrine gland in the anterior neck. It produces thyroid hormones involved in metabolism and biological development. Its size and hormone output are different properties: a goitre names enlargement without specifying whether glandular secretion is high, low or normal.

Thyroid-stimulating hormone contributes to control of thyroid hormone secretion. Measuring the gland's dimensions and measuring circulating hormone concentrations therefore answer different physiological questions.