The neuroendocrine system and major endocrine glands

Endocrinology is the healthcare specialty that deals with endocrine disorders. These disorders relate to endocrine glands, hormones, and hormonal effects on the body. An endocrinologist is a specialist involved in the diagnosis, treatment, and prevention of endocrine disorders.

Besides being seen in the endocrine department, patients with endocrine concerns are typically seen in primary and urgent care departments. Medical assistants should have a strong understanding of common endocrinology diseases, diagnostic tests, and treatments.

Neuroendocrine system

The endocrine system is composed of ductless glands throughout the body. Endocrine glands release hormones directly into the bloodstream, and the blood transports them to target cells. The hormones act as messengers to the target cells, telling the cells to alter their functions to help maintain homeostasis in the body. Hormones function as the body’s chemical messengers, transferring information from one group of cells to another. They control metabolism, growth, mood, sexual maturity, reproduction, and water and electrolyte balance. Hormone levels vary and can be affected by outside factors, such as illness and stress.

Maintaining homeostasis

The nervous system and the endocrine system can work alone or together. Working jointly, as a neuroendocrine system, they perform communication functions and maintain homeostasis. The brain sends out signals to and continually receives feedback from the endocrine system. The nervous system communicates quickly through nerve impulses and delivers rapid responses to maintain homeostasis. The endocrine system communicates slowly through hormones, which maintain homeostasis for a longer period of time. The body needs both systems working together through communication to maintain homeostasis.

Hypothalamus

The hypothalamus, located in the middle of the brain, is the major connection for the neuroendocrine system. It plays an important role in controlling the endocrine system. When the hypothalamus detects rising levels of a target organ’s hormones, it sends a signal to the pituitary gland to release or prevent pituitary hormone production.

The hypothalamus is also responsible for the production of antidiuretic hormone (ADH) and oxytocin. These two hormones are produced by the hypothalamus and stored and secreted by the posterior lobe of the pituitary gland. They will be discussed in more detail in the next section.

Anatomy of the pituitary gland

The pituitary gland is also known as the hypophysis. The pituitary gland is a pea-sized gland connected to the hypothalamus by the infundibulum, a small stalk of tissue. The hormones from the pituitary control the other endocrine glands; thus, the pituitary gland is called the “master gland.” The pituitary gland is composed of two lobes. The anterior and posterior lobes act as separate glands, each having its own function.

Anterior lobe of the pituitary gland

The anterior lobe, also known as the adenohypophysis, produces and secretes these hormones:

• Adrenocorticotropic hormone (ACTH): Causes the adrenal cortex to produce and release steroids (e.g., cortisol).
• Follicle-stimulating hormone (FSH): Stimulates the development of ova (eggs) through ovulation in females and stimulates the seminiferous tubules to produce sperm in males.
• Growth hormone (GH): Stimulates growth of the long bones and muscles in children and teens. Growth hormone is also involved with glucose metabolism in the body. Growth hormone is also called somatotropin or somatotropic hormone (STH).
• Luteinizing hormone (LH): In females, it stimulates the ovaries to produce estrogen, ova to mature, and the production of progesterone. LH also initiates ovulation and signals the corpus luteum to develop. In men, it stimulates interstitial cells in the testes to develop and secrete testosterone. Thus, in men, LH is also called interstitial cell–stimulating hormone (ICSH).
• Prolactin (PRL): Stimulates breast tissue development and milk production toward the end of pregnancy and after childbirth.
• Thyroid-stimulating hormone (TSH): Stimulates the thyroid gland to release T3 and T4. (More information about the thyroid will be presented later in the chapter.)

Posterior lobe of the pituitary gland

The posterior lobe, also known as the neurohypophysis, is composed of nervous tissue. It does not produce hormones but stores hormones produced by the hypothalamus. The hormones are transported from the hypothalamus to the posterior lobe directly through the infundibulum. The posterior lobe stores the hormones until it gets a signal from the hypothalamus to release them into the bloodstream. The blood then carries the hormones to the target organ. Two hormones are released by the posterior lobe:

• Antidiuretic hormone (ADH): Also called vasopressin. Stimulates contraction of the blood vessels, raising the blood pressure. It also stimulates the kidney tubules to reabsorb water, which concentrates the urine.
• Oxytocin (OT): During the delivery of a child, OT is released and stimulates the uterine muscles to contract. This is an example of a positive feedback loop. OT also helps release breast milk by stimulating the contraction of the muscles surrounding the mammary ducts.

Anatomy of the thyroid and parathyroid glands

The thyroid gland and the parathyroid glands are located in the neck. Both glands have very unique functions. The following sections discuss these glands.

Thyroid gland

The thyroid gland is a butterfly-shaped gland in the neck above the collarbone. The thyroid produces, stores, and secretes the following:

• Triiodothyronine (T3): Regulates metabolism and increases the basal metabolic rate.
• Thyroxine (T4): Regulates metabolism and increases the basal metabolic rate. It also supports the activities of growth hormones.
• Calcitonin: Regulates calcium and phosphate levels in the blood. It works against the parathyroid hormone action. Calcitonin helps lower blood calcium levels by inhibiting osteoclast activity. By inhibiting the breakdown of bone, it helps to retain calcium in the bones. Calcitonin also works in the kidney by reducing the resorption of calcium, thus also lowering the blood calcium level.

When thyroid hormone levels decrease in the body, the hypothalamus secretes TSH-releasing hormone. This hormone “tells” the anterior lobe of the pituitary gland to produce TSH, which stimulates the thyroid to produce hormones. Iodine from our diet is absorbed in the blood and carried to the thyroid gland. Iodine is required to produce T3 and T4.

Increasing the basal metabolic rate

T3 and T4 hormones increase the basal metabolic rate, which has the following functions:

• Causes an increase in the body temperature and pulse rate; it also creates a stronger heartbeat.
• Helps the brain mature in children and promotes growth.
• Improves concentration and faster