Master10
Human Body & Medicine Module

Hormones & Endocrine System

The endocrine system comprises ductless glands that secrete chemical messengers known as hormones directly into the circulatory bloodstream to regulate metabolism, growth, and reproduction. The pituitary gland, located at the base of the brain in the sella turcica, functions as the 'master gland' under the control of the hypothalamus, secreting growth hormone, ACTH, TSH, and gonadotropins. The thyroid gland produces thyroxine (T4T_4) and triiodothyronine (T3T_3) to regulate basal metabolic rate. The pancreas functions as both an exocrine and endocrine gland; its Islets of Langerhans contain beta cells that secrete insulin to lower blood glucose and alpha cells that secrete glucagon to raise glucose levels.

Key Concepts & Examination Highlights

  • The pituitary gland is regulated by neurohormones from the hypothalamus and secretes trophic hormones regulating other endocrine glands.
  • Insulin is secreted by the beta cells of the Islets of Langerhans in the pancreas, facilitating cellular glucose uptake and glycogen synthesis.
  • The adrenal medulla secretes catecholamines—epinephrine (adrenaline) and norepinephrine—triggering the body's 'fight-or-flight' stress response.
  • Glucagon is synthesized and secreted by alpha cells of the pancreatic Islets of Langerhans, promoting glycogenolysis and gluconeogenesis in the liver to elevate blood glucose levels.
  • The thyroid gland requires dietary iodine to synthesize thyroxine (T4T_4) and triiodothyronine (T3T_3), which regulate the basal metabolic rate (BMR) across somatic tissues.
  • Calcitonin, secreted by thyroid parafollicular C-cells, lowers serum calcium levels, whereas Parathyroid Hormone (PTH), secreted by parathyroid glands, increases serum calcium concentrations.
  • The anterior pituitary secretes Growth Hormone (GH), Adrenocorticotropic Hormone (ACTH), Thyroid-Stimulating Hormone (TSH), Prolactin, Luteinizing Hormone (LH), and Follicle-Stimulating Hormone (FSH).
  • The posterior pituitary stores and releases two hypothalamic neurohormones: Oxytocin (which stimulates uterine contractions and milk ejection) and Vasopressin / Antidiuretic Hormone (ADH, which promotes water reabsorption in renal collecting ducts).
  • The adrenal cortex secretes glucocorticoids (such as cortisol, which modulates carbohydrate metabolism and inflammation) and mineralocorticoids (such as aldosterone, which regulates sodium and water retention).
  • The pineal gland, situated in the epithalamus of the brain, secretes melatonin to regulate the human circadian rhythm and sleep-wake cycle.
  • Hypersecretion of growth hormone in children leads to gigantism, whereas in adults it results in acromegaly; hyposecretion of growth hormone in childhood causes pituitary dwarfism.
  • Diabetes insipidus is a metabolic disorder caused either by inadequate secretion of Antidiuretic Hormone (ADH) from the posterior pituitary or renal insensitivity to ADH, causing extreme polyuria and polydipsia.
  • Hashimoto's thyroiditis is an autoimmune disease where anti-thyroid antibodies attack thyroid peroxidase and thyroglobulin, leading to chronic hypothyroidism and elevated serum TSH levels.
  • Graves' disease is an autoimmune condition characterized by hyperthyroidism, diffuse goiter, and exophthalmos (protruding eyeballs), caused by thyroid-stimulating immunoglobulins (TSI) activating TSH receptors.
  • Addison's disease is a primary adrenal insufficiency resulting from autoimmune destruction of the adrenal cortex, leading to deficient cortisol and aldosterone production and skin hyperpigmentation.
  • Cushing's syndrome results from prolonged exposure to excess glucocorticoids (cortisol), manifesting with central obesity, moon facies, buffalo hump, and purple abdominal striae.
  • Somatostatin, secreted by delta cells of the pancreatic Islets of Langerhans and the hypothalamus, acts as an inhibitory hormone suppressing the secretion of insulin, glucagon, and growth hormone.
  • Luteinizing Hormone (LH) triggers ovulation and corpus luteum formation in females and stimulates Leydig cells in males to synthesize and secrete testosterone.
  • Follicle-Stimulating Hormone (FSH) promotes ovarian follicle development and estrogen secretion in females and stimulates Sertoli cells to support spermatogenesis in males.
  • Progesterone is synthesized by the corpus luteum in the ovary and by the placenta during pregnancy, preparing the uterine endometrium for blastocyst implantation and maintaining gestation.
  • The thymus gland secretes the peptide hormone thymosin, which directs the differentiation and maturation of T-lymphocytes for cell-mediated immunity.
  • Renin, secreted by the juxtaglomerular apparatus of the kidneys in response to low blood pressure, cleaves angiotensinogen into angiotensin I, initiating the Renin-Angiotensin-Aldosterone System (RAAS).
  • Atrial Natriuretic Peptide (ANP) is a peptide hormone secreted by cardiac atrial myocytes in response to high atrial pressure, promoting natriuresis, diuresis, and systemic vasodilation to lower blood pressure.
  • Erythropoietin (EPO) is a glycoprotein hormone produced primarily by interstitial fibroblasts in the kidneys that stimulates erythropoiesis (red blood cell production) in the bone marrow.
  • Parathyroid Hormone (PTH) maintains calcium homeostasis by stimulating osteoclast-mediated bone resorption, increasing renal calcium reabsorption, and activating Vitamin D into calcitriol.
  • The hypothalamus synthesizes gonadotropin-releasing hormone (GnRH), thyrotropin-releasing hormone (TRH), and corticotropin-releasing hormone (CRH) to regulate anterior pituitary hormone secretion.
  • Prolactin is a peptide hormone secreted by the lactotroph cells of the anterior pituitary that stimulates mammary gland development and milk production (lactogenesis).
  • Oxytocin, often called the bonding or milk-ejection hormone, stimulates rhythmic myometrial contractions during parturition and myoepithelial contractions in lactating breasts.
  • Vasopressin (Antidiuretic Hormone, ADH) acts on the V2 receptors of renal collecting duct principal cells to insert aquaporin-2 water channels, concentrating urine and conserving water.
  • Thyroxine (T4T_4) is the primary circulating thyroid hormone, while triiodothyronine (T3T_3) is the physiologically active form produced in peripheral tissues through monodeiodination of T4T_4.
  • Calcitonin acts on bone osteoclasts to inhibit bone resorption and promotes renal calcium excretion, functioning as a physiological antagonist to Parathyroid Hormone.
  • Parathyroid Hormone (PTH) increases serum calcium levels by stimulating osteoclast activity, enhancing intestinal calcium absorption via 1,25-dihydroxyvitamin D3, and reducing urinary calcium excretion.
  • Aldosterone is the primary mineralocorticoid secreted by the zona glomerulosa of the adrenal cortex, stimulating renal Na+Na^+ reabsorption and K+K^+ and H+H^+ excretion in distal tubules.
  • Cortisol is a glucocorticoid synthesized in the zona fasciculata of the adrenal cortex that promotes gluconeogenesis, lipolysis, protein catabolism, and suppresses immune inflammation.
  • The adrenal cortex is anatomically divided into three concentric histological zones: outer Zona Glomerulosa (mineralocorticoids), middle Zona Fasciculata (glucocorticoids), and inner Zona Reticularis (androgens).
  • Epinephrine and norepinephrine, synthesized from the amino acid tyrosine in the adrenal medulla, bind to adrenergic receptors (α\alpha and β\beta) to accelerate heart rate and elevate blood pressure.
  • Insulin is synthesized as preproinsulin, cleaved into proinsulin in the ER, and processed in the Golgi into active insulin and a connecting peptide (C-peptide) in equimolar amounts.
  • Glucagon acts primarily on hepatic cells via G-protein coupled receptors to stimulate glycogenolysis and gluconeogenesis, preventing hypoglycemia during fasting periods.
  • Somatostatin (Growth Hormone Inhibiting Hormone) is secreted by delta cells of the pancreas, D-cells of the stomach, and the hypothalamus to suppress growth hormone, gastrin, and pancreatic secretins.
  • Ghrelin is a peptide hormone produced predominantly by fundic P/D1 cells of the stomach that acts on the hypothalamus to stimulate appetite and growth hormone release.
  • Leptin is an adipokine hormone secreted by white adipose tissue that acts on hypothalamic receptors to promote satiety and regulate long-term energy balance.
  • Melatonin synthesis in the pineal gland is controlled by the suprachiasmatic nucleus (SCN) of the hypothalamus, with secretion peaking during darkness and inhibited by light.
  • Human Chorionic Gonadotropin (hCG) is a glycoprotein hormone produced by the placental syncytiotrophoblast that maintains the ovarian corpus luteum during early gestation, detected in pregnancy tests.
  • Human Placental Lactogen (hPL) is a polypeptide hormone secreted by the placenta that alters maternal carbohydrate and lipid metabolism to ensure adequate glucose delivery to the fetus.
  • Inhibin, secreted by Sertoli cells in males and granulosa cells in females, exerts selective negative feedback on the anterior pituitary to inhibit FSH secretion without affecting LH.
  • Gastrin is a peptide hormone secreted by G-cells in the gastric antrum and duodenum that stimulates parietal cells to secrete hydrochloric acid (HClHCl) and intrinsic factor.
  • Secretin, discovered by William Bayliss and Ernest Starling in 1902 as the first identified hormone, is secreted by S-cells in the duodenum to stimulate bicarbonate-rich pancreatic fluid.
  • Cholecystokinin (CCK), secreted by I-cells in the upper small intestine in response to fat and protein chyme, stimulates gallbladder contraction and pancreatic digestive enzyme release.
  • Gastric Inhibitory Peptide (GIP) and Glucagon-Like Peptide-1 (GLP-1) are incretin hormones secreted by enteroendocrine cells that amplify glucose-dependent insulin secretion from the pancreas.
  • Endocrine hormones travel through the bloodstream to distant target tissues, paracrine hormones act on neighboring local cells, and autocrine hormones act on the same cell that secreted them.
Curriculum & Reference Sources: NCERT Biology Class XI Endocrine System, Endocrine Society, National Institutes of Health (NIH)

Sample Solved Questions & Concept Explanations

8 Verified Concept Questions
Q1.EASY

Which endocrine gland located at the base of the brain is called the 'Master Gland' of the endocrine system?

Q2.EASY

Insulin is a hormone produced by the beta cells of which digestive and endocrine organ?

Q3.EASY

Deficiency of Vitamin C leads to which nutritional deficiency disease characterized by bleeding gums?

Q4.EASY

Deficiency of Vitamin D or calcium in children causes which bone-softening disease?

Q5.MEDIUM

Which hormone secreted by the adrenal medulla during acute stress triggers the 'Fight or Flight' response?

Q6.MEDIUM

Which enzyme present in the human stomach initiates the breakdown of dietary proteins into polypeptides?

Q7.MEDIUM

Night blindness (Nyctalopia) and Xerophthalmia are caused by dietary deficiency of which fat-soluble vitamin?

Q8.MEDIUM

Which hormone regulates the human sleep-wake cycle (circadian rhythm) and is secreted by the pineal gland in darkness?