Endocrine glands are glands of the endocrine system that secrete hormones (their products) directly into the blood rather than through a duct (exocrine organs use ducts). The major glands of the endocrine system include: adrenal glands, hypothalamus, parathyroid glands, thyroid gland, testes, ovaries, pancreas, pituitary gland, and pineal gland. The hypothalamus and pituitary gland are considered neuroendocrine organs. The pineal gland is located in the brain and secretes the hormone melatonin, which helps regulate the circadian (sleep-wake) cycle, especially before sleep.

The adrenal cortex has three zones. The first zone releases aldosterone, the second releases cortisol, and the third releases androgens, which includes testosterone. Vasopressin (antidiuretic hormone) is synthesized by the hypothalamus and released by the posterior pituitary.

Endocrine glands only secrete their products into the blood. Sweat glands secrete sweat outside the body and never touch the blood. Similarly, anything inside the gastrointestinal tract is technically outside the body! For something (broken-down food molecules, water, salts etc.) to enter the body, it must be absorbed across the walls of the gastrointestinal tract. Remember, humans are like hollow cylinders in the sense that we have a tube from mouth to anus, which is considered outside the body.

The adrenal medulla is very different from the adrenal cortex and has a different origin as well. It is made of chromaffin cells, which are neuroendocrine cells, and release catecholamines (epinephrine and norepinephrine), in response to sympathetic stimulation.

Adrenocorticotropic hormone (ACTH) helps stimulate the release of cortisol from the adrenal glands. Vasopressin and oxytocin are produced by the hypothalamus and secreted by the posterior pituitary. Growth hormone and prolactin have no direct effect on the adrenal glands.

When thinking of hormones, it helps to remember that they are generally slow acting, affect all types of tissues at once, and can last for long periods of time in the body. As a result, hormones would not be described as fast-acting in the body.

In contrast, neurotransmitters are generally considered fast-acting, as they are released to a small, targeted area and elicit an immediate response.

The pituitary gland is composed of an anterior and a posterior pituitary lobe, both of which are responsible for the secretion of various hormones.

The anterior pituitary secretes thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), prolactin, luteinizing hormone (LH), adrenocorticotropic hormone (ACTH), and growth hormone (GH). It can help to remember these hormones with the pneumonic "FLAT PEG."

The posterior pituitary secretes oxytocin and antidiuretic hormone (ADH). Antidiuretic hormone is also known as vasopressin.

Calcitonin is secreted by the thyroid gland, not the pituitary.