Endocrine System Overview

Introduction to the Endocrine System

The endocrine system is a network of glands that secrete chemical messengers called hormones directly into the bloodstream. These hormones regulate various physiological processes by acting on specific target cells that possess the appropriate receptors. The endocrine system is essential for maintaining homeostasis, growth, metabolism, and reproduction.

• Endocrine glands: Ductless glands that release hormones into the blood (e.g., pituitary, thyroid, adrenal glands).

• Exocrine glands: Glands with ducts that secrete substances onto epithelial surfaces (e.g., sweat, salivary glands).

• Paracrines: Local hormones that act on nearby cells (e.g., histamine, some pancreatic and GI tract hormones).

Major Endocrine Organs

Hypothalamus

The hypothalamus is a region of the brain that forms the floor and walls of the third ventricle. It plays a central role in regulating the endocrine system by controlling the pituitary gland. The hypothalamus produces releasing and inhibiting hormones that regulate the secretion of anterior pituitary hormones.

Pituitary Gland (Hypophysis)

The pituitary gland is suspended from the hypothalamus by a stalk called the infundibulum. It consists of two main parts:

• Adenohypophysis (anterior pituitary): Arises from the pharynx and produces several key hormones.

• Neurohypophysis (posterior pituitary): Arises from the brain and stores/releases hormones produced by the hypothalamus.

Follicle Stimulating Hormone (FSH)

• In females: Stimulates development of ovarian follicles and egg cells; promotes estrogen production and secondary sex characteristics.

• In males: Stimulates sperm production in the testes.

• Disorders:

  • Hypersecretion: Precocious puberty in children, early menopause in women.

  • Hyposecretion: Failure of sexual maturity in children, infertility in adults.

Luteinizing Hormone (LH)

• In females: Stimulates ovulation and production of estrogen and progesterone.

• In males: Stimulates testosterone production in the testes.

• Disorders:

  • Hypersecretion: Premature sexual maturity.

  • Hyposecretion: Inhibited sexual maturation, infertility.

Thyroid Stimulating Hormone (TSH)

TSH stimulates the thyroid gland, the largest endocrine organ located below the larynx, to release thyroid hormones (T3 and T4), which require iodine for synthesis. Calcitonin is also secreted by the thyroid when blood calcium rises, promoting bone formation.

• Functions of thyroid hormones:

  • Increase basal metabolic rate and oxygen consumption

  • Increase respiration rate and heart rate

  • Maintain body temperature

  • Stimulate appetite and enhance breakdown of lipids, carbohydrates, and proteins

  • Accelerate body growth

TSH Disorders

• Hyposecretion:

  • Congenital hypothyroidism (cretinism): abnormal bone development, brain damage

  • Myxedema (adults): facial swelling, low metabolic rate, lethargy, weight gain, cold sensitivity

• Hypersecretion:

  • Endemic goiter: dietary iodine deficiency, enlarged thyroid

  • Grave’s Disease: autoimmune, exophthalmos (bulging eyes), weight loss, anxiety, goiter

Adrenocorticotropic Hormone (ACTH)

ACTH stimulates the adrenal cortex to produce glucocorticoids, mainly cortisol, which is known as the "stress hormone." The adrenal glands are located atop the kidneys and consist of a cortex (producing steroid hormones) and a medulla (producing catecholamines).

• Cortex hormones:

  • Mineralocorticoids (aldosterone): regulate electrolyte balance

  • Glucocorticoids (cortisol): involved in fat/protein catabolism, anti-inflammatory

  • Sex hormones (androgens, estrogens)

• Medulla hormones:

  • Epinephrine (adrenaline): fight or flight response

  • Norepinephrine

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Prolactin (PRL)

• Females: Stimulates milk production in estrogen-primed mammary glands; associated with maternal behavior.

• Disorders:

  • Hypersecretion: Inappropriate milk production, amenorrhea, impotence in males

  • Hyposecretion: Lack of milk production

Growth Hormone (hGH)

• Promotes growth of bone, muscle, and other tissues; stimulates protein synthesis and lipolysis.

• Stimulates the liver to produce insulin-like growth factors (IGFs).

• Levels fluctuate: higher during deep sleep, after protein meals, and exercise; lower after carbohydrate meals.

Growth Hormone Disorders

• Hypersecretion in childhood: Gigantism (excessive growth)

• Hypersecretion in adulthood: Acromegaly (thickening of bones in jaw, brow, hands, feet)

• Hyposecretion: Pituitary dwarfism (short stature, normal proportions)

Posterior Pituitary Hormones (Neurohypophysis)

Overview

The posterior pituitary stores and releases hormones produced by the hypothalamus: oxytocin (OT) and antidiuretic hormone (ADH). These hormones are transported from the hypothalamus to the posterior lobe, where they are stored until needed.

Oxytocin (OT)

• Target organs: uterus and breasts

• Major actions: stimulates uterine contractions during labor, milk ejection during lactation, may play a role in emotional bonding and sperm transport

• Disorders:

  • Hypersecretion: Early labor, overproduction of milk

  • Hyposecretion: Non-progression of labor, no milk let down (synthetic oxytocin = pitocin)

Antidiuretic Hormone (ADH)

• Major actions: increases water reabsorption in kidney tubules, reduces urine output, constricts arterioles to raise blood pressure, inhibits sweat glands

• Works with aldosterone as part of the Renin-Angiotensin-Aldosterone System (RAAS) to regulate fluid balance

• Target organ: kidney

• Disorders:

  • Hyposecretion: Diabetes insipidus (excessive urine output, dehydration, thirst)

  • Hypersecretion: Water retention, concentrated urine

Additional info: The endocrine system interacts closely with the nervous system to coordinate body functions. Disorders of hormone secretion can have profound effects on growth, metabolism, reproduction, and homeostasis.