3.4 The Endocrine System

Endocrine vs. Exocrine Glands

Feature Endocrine Glands Exocrine Glands
Ducts Ductless; secrete directly into bloodstream. Have ducts to carry secretions to specific locations.
Secretions Hormones Enzymes, sweat, saliva, etc.
Target Distant target cells/organs. Act locally at site of secretion.

Major Endocrine Glands: Adrenal & Pancreas

  • Adrenal Gland:
    • Adrenal Cortex: Glucocorticoids (Cortisol - metabolism, stress), Mineralocorticoids (Aldosterone - water/salt balance).
    • Adrenal Medulla: Adrenaline/Noradrenaline ("fight or flight" response).
  • Pancreas:
    • Insulin: Lowers blood glucose.
    • Glucagon: Raises blood glucose.

Major Endocrine Glands: Thyroid & Pituitary

  • Thyroid Gland:
    • Thyroxine (T4): Regulates metabolic rate.
  • Pituitary Gland:
    • Anterior Pituitary: Growth Hormone (GH), Tropic Hormones (TSH, ACTH).
    • Posterior Pituitary: Antidiuretic Hormone (ADH), Oxytocin.

Hormonal Imbalances

Gland Hormone Hyposecretion (Too Little) Hypersecretion (Too Much)
Pancreas Insulin Hyperglycemia (Diabetes Mellitus) Hypoglycemia
Thyroid Thyroxine Hypothyroidism Hyperthyroidism
Adrenal Cortisol Addison's Disease Cushing's Syndrome
Pituitary Growth Hormone Dwarfism Gigantism (children); Acromegaly (adults)

Feedback Mechanism: TSH Example

  1. Hypothalamus releases TRH.
  2. TRH stimulates Pituitary to release TSH.
  3. TSH stimulates Thyroid to produce Thyroxine.
  4. High Thyroxine levels inhibit TRH and TSH release (Negative Feedback Loop).

Hormones: Quick Reference (Part 1)

Hormone Origin Hyposecretion Hypersecretion
Insulin Pancreas Diabetes mellitus type 1 Hyperinsulinism
Growth Hormone Pituitary gland Pituitary dwarfism Acromegaly
Thyroxine Thyroid gland Hypothyroidism Hyperthyroidism
Cortisol Adrenal glands Addison's disease Cushing's syndrome
Adrenaline Adrenal glands Adrenal insufficiency Pheochromocytoma
Estrogen Ovaries Primary ovarian insufficiency (POI) Estrogen dominance
Testosterone Testes Hypogonadism Testicular tumors
Progesterone Ovaries Progesterone deficiency Hyperprogesteronism
Parathyroid hormone Parathyroid glands Hypoparathyroidism Hyperparathyroidism
Melatonin Pineal gland SAD Circadian rhythm disorders

Hormones: Quick Reference (Part 2)

Hormone Origin Hyposecretion Hypersecretion
Oxytocin Hypothalamus, posterior pituitary Inadequate uterine contractions Excessive uterine contractions
Aldosterone Adrenal glands Addison's disease Hyperaldosteronism
Prolactin Pituitary gland Hypoprolactinemia Hyperprolactinemia
TSH Pituitary gland Hypothyroidism Hyperthyroidism
FSH Pituitary gland Hypogonadism Gonadotropin-secreting tumors
LH Pituitary gland Hypogonadism Gonadotropin-secreting tumors
ACTH Pituitary gland Adrenal insufficiency Cushing's disease
ADH Hypothalamus, posterior pituitary Diabetes insipidus SIADH
Calcitonin Thyroid gland Hypocalcemia Medullary thyroid cancer
Glucagon Pancreas Hypoglycemia Glucagonoma

Hormones: Quick Reference (Part 3)

Hormone Origin Hyposecretion Hypersecretion
MSH Hypothalamus, pituitary, skin Hypopituitarism Melanosis, hyperpigmentation
Ghrelin Stomach Prader-Willi syndrome Ghrelinoma
Leptin Adipose tissue Leptin deficiency Leptin resistance
Serotonin Enterochromaffin cells Serotonin deficiency syndrome Serotonin syndrome
Vasopressin Hypothalamus, posterior pituitary Diabetes insipidus SIADH
Somatostatin Pancreas, hypothalamus Diabetes mellitus type 2 Somatostatinoma
ANP Heart Sodium retention Atrial myxoma
Relaxin Ovaries, placenta Preterm labor Overactive relaxin production
Thymosin Thymus Reduced T cell development Thymus gland tumors
Erythropoietin Kidneys Anemia Polycythemia

Hormones: Quick Reference (Part 4)

Hormone Origin Hyposecretion Hypersecretion
Cholecystokinin Intestines Decreased appetite Overproduction leading to gallbladder issues
Adiponectin Adipose tissue Insulin resistance Not applicable
Norepinephrine Adrenal glands Low blood pressure, depression Hypertension, anxiety
Glucocorticoids Adrenal glands Adrenal insufficiency Cushing's syndrome
Inhibin Gonads Unknown Granulosa cell tumors
Secretin Duodenum Impaired digestion Not applicable
Thrombopoietin Liver, kidneys Thrombocytopenia Thrombocytosis
FGF23 Bones, kidneys Hypophosphatemic rickets Tumor-induced osteomalacia
Gastrin Stomach, duodenum Hypogastrinemia Zollinger-Ellison syndrome
GLP-1 Intestines Impaired glucose regulation Not applicable
Prostaglandins Various tissues Dysmenorrhea Excessive menstrual bleeding