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Endocrine Glands — Overview
Definition of Hormone
Classical definition: Chemical messengers secreted by ductless (endocrine) glands directly into bloodstream, carried to target organs.
Current scientific definition: Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts.
Endocrine glands = ductless glands — secrete directly into blood (unlike exocrine glands which have ducts).
Human Endocrine System — All Glands
- Primary glands: Hypothalamus, Pituitary, Pineal, Thyroid, Parathyroid, Thymus, Adrenal, Pancreas, Gonads (Testis/Ovary)
- Secondary/accessory: Gastrointestinal tract, Liver, Kidney, Heart also produce hormones
- All hormones of Anterior pituitary are proteinaceous in nature
- Trophic hormones = hormones that control secretion of other endocrine glands (e.g. TSH, ACTH, FSH, LH — all from anterior pituitary; except GH)
Chemical Nature of Hormones
| Type | Examples | Receptor Location | Action Mechanism |
|---|
| Peptide/Protein/Polypeptide | Insulin, Glucagon, GH, ADH, Oxytocin, all hypothalamic hormones, pituitary hormones | Cell surface (membrane receptor) | Second messenger (cAMP, IP3, Ca²⁺) |
| Steroidal | Cortisol, Testosterone, Estradiol, Progesterone, Aldosterone | Intracellular (cytoplasm/nucleus) | Directly enters cell → binds nuclear receptor → alters gene expression |
| Iodothyronines | T₃ (Triiodothyronine), T₄ (Thyroxine) | Intracellular (nucleus) | Directly enters cell → nuclear receptor → alters gene expression |
| Amino acid derivatives | Adrenaline (Epinephrine), Noradrenaline, Melatonin, Dopamine | Cell surface | Second messenger pathway |
Key Facts
- Part of Diencephalon (forebrain); contains the 3rd ventricle (CSF)
- Controls both anterior and posterior pituitary
- Contains secretory neurons (neurosecretory cells) that produce hormones
- Acts as the master of the master gland (controls pituitary)
| Hypothalamic Hormone | Abbrev. | Action on Anterior Pituitary |
|---|
| Growth Hormone Releasing Hormone (Somatocrinin) | GHRH | Stimulates GH/STH release |
| Growth Hormone Inhibiting Hormone (Somatostatin) | GHIH | Inhibits GH release (negative feedback) |
| Thyrotropin Releasing Hormone | TRH | Stimulates TSH release |
| Corticotropin Releasing Hormone | CRH | Stimulates ACTH release |
| Gonadotropin Releasing Hormone | GnRH | Stimulates FSH and LH release |
| Prolactin Releasing Hormone | PRH | Stimulates Prolactin (PRL) release |
| Prolactin Inhibiting Hormone | PIH | Inhibits Prolactin release |
Hormones Synthesized in Hypothalamus but Released by Posterior Pituitary
- Preoptic nucleus → synthesizes ADH (Vasopressin) → stored and released by posterior pituitary
- Paraventricular nucleus → synthesizes Oxytocin → stored and released by posterior pituitary
- These travel as neurosecretory substances (axonal transport) to posterior pituitary
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Pituitary Gland — Master Gland
Basic Facts
- Also called Hypophysis cerebri
- Origin: Ectodermal
- Weight: ~0.5 g
- Position: Sella turcica (bony cavity of sphenoid bone) in diencephalon
- Connected to hypothalamus by Pituitary stalk (infundibulum)
- Called Master gland — controls most other endocrine glands via trophic hormones
Parts of Pituitary Gland
| Part | Origin | % of Total | Hormones |
|---|
| Pars Distalis (Anterior pituitary / Adenohypophysis) | Ectodermal (oral ectoderm — Rathke's pouch) | 75% | GH, TSH, ACTH, FSH, LH, Prolactin |
| Pars Intermedia (part of anterior lobe) | Ectodermal | small | MSH (Melanocyte Stimulating Hormone) |
| Pars Nervosa (Posterior pituitary / Neurohypophysis) | Neural (Neuroectodermal) | 25% | ADH (Vasopressin), Oxytocin (synthesized in hypothalamus, released here) |
Anterior Pituitary Hormones — Complete
| Hormone | Cell Type | Functions | Disorders |
|---|
| GH / STH (Growth Hormone / Somatotropin) | Somatotroph | Controls growth of cartilage, bone, body; stimulates protein synthesis; fat metabolism; diabetogenic effect (↑ blood glucose) | Hypo in child: Dwarfism (mentally normal); Hyper in child: Gigantism; Hyper in adult: Acromegaly (enlargement of extremities) |
| TSH (Thyroid Stimulating Hormone / Thyrotropin) | Thyrotroph | Stimulates thyroid gland to produce T₃ and T₄ | ↑TSH → hyperthyroidism; ↓TSH → hypothyroidism |
| ACTH (Adrenocorticotropic Hormone / Corticotropin) | Corticotroph | Stimulates adrenal cortex → cortisol (anti-stress, anti-inflammatory, immunosuppressive) | ↑ACTH → Cushing's syndrome |
| LH (Luteinizing Hormone / ICSH in males) | Gonadotroph | Males: stimulates Leydig cells → Testosterone; Females: triggers ovulation, forms corpus luteum → progesterone | — |
| FSH (Follicle Stimulating Hormone) | Gonadotroph | Males: stimulates Sertoli cells → Inhibin, ABP (Androgen Binding Protein), sperm maturation; Females: stimulates follicle growth, estrogen | — |
| Prolactin (PRL) | Lactotroph | Stimulates mammary gland development; milk production (lactation); inhibits GnRH, FSH, LH → stops menstrual cycle during lactation | — |
| MSH (Melanocyte Stimulating Hormone) | Pars Intermedia | Stimulates melanocytes → melanin production → skin pigmentation | — |
Posterior Pituitary Hormones
| Hormone | Synthesized in | Functions | Disorders |
|---|
| ADH (Antidiuretic Hormone / Vasopressin) | Preoptic nucleus of Hypothalamus | Increases water reabsorption from DCT and Collecting duct → concentrated urine; vasoconstriction → ↑ BP | Diabetes Insipidus — ADH absent → large volume dilute urine, dehydration |
| Oxytocin | Paraventricular nucleus of Hypothalamus | Contraction of smooth muscles of uterus (birth process); milk ejection from mammary glands (milk let-down reflex); also called "love hormone" | — |
Q. What is Acromegaly and how does it differ from Gigantism?
✅ Answer: Both caused by excess GH; Gigantism = excess GH in childhood (before epiphyseal fusion) → excessive height; Acromegaly = excess GH in adults (after growth plates close) → enlargement of hands, feet, jaw
Explanation
Growth Hormone promotes growth of bones by acting on cartilage at epiphyseal plates. In children, the growth plates (epiphyses) are still open, so excess GH causes proportional enlargement of the whole body — resulting in Gigantism (unusually tall individuals, sometimes 7–8 feet). In adults, the epiphyseal plates have fused/closed, so overall height cannot increase. Instead, excess GH causes abnormal enlargement of soft tissues, hands, feet, jaw, and internal organs — this is Acromegaly. Both conditions also cause glucose intolerance (diabetogenic effect of GH). Hypersecretion of GH is usually caused by a pituitary adenoma (benign tumor).
Structure
- Located on either side of trachea (neck region), connected by a thin isthmus
- Composed of follicles — sac-like structures lined by simple cuboidal epithelium (I cells); filled with colloid (thyroglobulin protein)
- Also contains parafollicular cells (C-cells) — secrete Thyrocalcitonin (TCT)
- Parathyroid glands are located on the dorsal side (2 pairs)
Thyroid Hormones — T₃ and T₄
- T₄ (Tetraiodothyronine = Thyroxine) — 80% of secretion; 4 iodine atoms; less active form
- T₃ (Triiodothyronine) — 20% of secretion; 3 iodine atoms; most active form; T₄ is converted to T₃ in liver
- Both require iodine for synthesis; made from Thyroglobulin protein (contains tyrosine residues)
- Both are iodothyronines — can directly enter cells (not water-soluble)
- Secretion controlled by TSH from anterior pituitary (which is controlled by TRH from hypothalamus)
Functions of Thyroxine (T₃ and T₄)
- Regulates Basal Metabolic Rate (BMR) — most important function
- Controls metabolism of carbohydrates, proteins, and fats
- Supports RBC formation (erythropoiesis)
- Maintains normal blood calcium level
- Increases body temperature
- Essential for normal growth, development of nervous system
- Thyrocalcitonin (TCT) — from C-cells → lowers blood Ca²⁺ → stimulates Ca²⁺ deposition into bone matrix; opposes PTH
Thyroid Disorders — All Disorders
- Simple Goiter (Hypothyroid goiter) — iodine deficiency in food → thyroid cannot make T₃/T₄ → pituitary secretes excess TSH → thyroid enlarges (goiter)
- Cretinism — hypothyroidism during pregnancy/in foetus → severe effects: deaf-mutism, thick skin, protruding tongue, very low IQ, sexually retarded (mentally and physically stunted child)
- Myxoedema — hypothyroidism in adult → puffiness of face, fatigue, weight gain, slow heart rate, cold intolerance
- Irregular menstrual cycle — both hypo and hypersecretion of thyroxin cause irregular menstrual cycle in adult females
- Exophthalmic Goiter (Grave's Disease) — hyperthyroidism (autoimmune) → enlarged thyroid, protrusion of eyeballs (exophthalmos), ↑BMR, ↑heart rate, weight loss, nervousness
Q. What is Cretinism and what causes it?
✅ Answer: Cretinism = hypothyroidism during foetal development or early childhood → stunted physical & mental development
Explanation
Thyroid hormones (T₃ and T₄) are absolutely essential for normal development of the nervous system, brain, and body during foetal life and early childhood. If a pregnant woman is iodine-deficient, her foetus cannot make enough T₃/T₄. This causes Cretinism in the baby — characterized by deaf-mutism (cannot hear or speak properly), thick coarse skin, protruding tongue, very low IQ (intellectual disability), sexual retardation, and stunted growth (short stature). Unlike dwarfism (caused by GH deficiency), cretin children are mentally affected. Early detection and thyroid hormone supplementation can prevent or reduce damage if started right after birth.
Basic Facts & Functions
- 2 pairs (4 glands) — located on dorsal surface of thyroid gland (1 pair per lobe)
- Secrete Parathyroid Hormone (PTH) — also called Parathormone
- PTH is released when blood Ca²⁺ falls
- PTH actions — all aimed at ↑ blood Ca²⁺:
- Stimulates osteoclasts (macrophages in bone) → bone resorption → Ca²⁺ released from bone matrix into blood
- Stimulates kidney tubules → ↑ Ca²⁺ reabsorption (↓ Ca²⁺ in urine)
- Stimulates JG cells of kidney → release Calcitriol (Vitamin D₃ active form) → ↑ Ca²⁺ absorption from gut
- PTH is a hypercalcemic hormone (raises blood Ca²⁺); TCT (from thyroid) is hypocalcemic (lowers blood Ca²⁺) → antagonistic pair
Parathyroid Disorders
- Hyperparathyroidism (↑PTH) → Hypercalcemia (↑ blood Ca²⁺) → bones become weak, fractures, kidney stones
- Hypoparathyroidism (↓PTH) → Hypocalcemia (↓ blood Ca²⁺) → Tetany (muscle spasms, cramps) — similar to tetany seen in excretory chapter
Pineal Gland
- Located in Diencephalon (dorsal part of forebrain)
- Secretes Melatonin
- Light inhibits melatonin; darkness stimulates melatonin secretion
- Functions of Melatonin:
- Regulates 24-hour (circadian/diurnal) rhythm of body (sleep-wake cycle)
- Controls body temperature regulation
- Influences skin pigmentation (in lower vertebrates)
- Influences menstrual cycle regulation
- Acts on anterior pituitary to modulate reproductive functions
Thymus Gland
- Lobular structure, located on ventral side of aorta, behind sternum
- Secretes peptide hormone — Thymosin
- Thymus is well developed in children; degenerates (atrophies) in old age → less thymosin → weaker immunity in elderly
- Functions of Thymosin:
- Stimulates T-lymphocyte maturation and differentiation in thymus
- Promotes Cell-Mediated Immunity (CMI)
- Also promotes Humoral / Antibody-mediated Immunity (BMI) via T-helper cells
- Stimulates TH, TC, TS cells and T-memory cells
Basic Structure
- One pair — one on top of each kidney (suprarenal glands)
- Two distinct regions: Adrenal Cortex (outer) + Adrenal Medulla (inner)
- Cortex: Origin = Mesodermal; hormones = Steroidal; vitally essential for life
- Medulla: Origin = Ectodermal (neural crest); hormones = Proteinaceous (Catecholamines); not essential for life
Adrenal Cortex — 3 Zones (Outer → Inner)
| Zone | Relative Size | Hormones | Key Functions | Regulation |
|---|
| Zona Glomerulosa (Outermost) | Narrow | Mineralocorticoids (mainly Aldosterone) | ↑ Na⁺ reabsorption in DCT; water retention; ↑ blood volume; osmoregulation; blood pressure regulation | RAAS (Renin-Angiotensin) |
| Zona Fasciculata (Middle) | Widest | Glucocorticoids (mainly Cortisol / Hydrocortisone) | Anti-stress; stimulates gluconeogenesis (↑ blood glucose); anti-inflammatory; immunosuppressive; stimulates erythropoiesis; anti-allergic; used in asthma, organ transplant, arthritis treatment | ACTH from anterior pituitary |
| Zona Reticularis (Innermost) | Narrow | Sex corticoids / Androgens (mainly DHEA — Dehydroepiandrosterone) | In both male and female — responsible for axillary hair, body hair, pubic hair at puberty; in females → contributes to female secondary sexual characters (Estradiol) | ACTH |
Adrenal Cortex Disorders
- Addison's Disease — hyposecretion of glucocorticoids + mineralocorticoids → extreme weakness, weight loss, low BP, hypoglycaemia, hyperpigmentation of skin
- Cushing's Syndrome — hypersecretion of glucocorticoids → obesity (central), moon face, buffalo hump, high BP, high blood glucose, easy bruising
- Conn's Syndrome — hypersecretion of mineralocorticoids (aldosterone) → hypertension, low blood K⁺, excessive Na⁺ retention
Adrenal Medulla — Emergency Gland (Fight or Flight)
- Secretes Catecholamines — derived from Tyrosine amino acid
- Adrenaline (Epinephrine) — major hormone (~80%)
- Noradrenaline (Norepinephrine) — ~20%
- Released during emergency, excitement, stress, danger
- Functions of Adrenaline/Noradrenaline (Fight or Flight response):
- ↑ Heart beat rate
- ↑ Rate of breathing
- ↑ Blood glucose level (glycogenolysis)
- Pupillary dilation
- ↑ Blood pressure (vasoconstriction)
- Breakdown of fat for energy
- Inhibit smooth muscle activity of gut
Q. Why is cortisol called the anti-stress hormone and what are its key effects?
✅ Answer: Cortisol is released during stress and has anti-inflammatory, immunosuppressive, and gluconeogenic effects — it prepares the body to survive stress
Explanation
Cortisol (hydrocortisone) from Zona Fasciculata is the primary glucocorticoid hormone. During stress, ACTH from pituitary → stimulates cortisol release. Cortisol: (1) raises blood glucose by breaking down glycogen and promoting gluconeogenesis (needed for energy during stress); (2) breaks down proteins and fats for energy; (3) suppresses immune system (reduces inflammation — used therapeutically in asthma, arthritis, allergies, organ transplant rejection); (4) stimulates RBC production; (5) maintains blood pressure. Its anti-inflammatory and immunosuppressive properties make synthetic glucocorticoids (like dexamethasone, prednisone) among the most widely used drugs in medicine.
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Pancreas — Islets of Langerhans
Structure
- Pancreas is a composite (heterocrine) gland — both exocrine (Acini → digestive enzymes) and endocrine functions
- Endocrine part = Islets of Langerhans — 1–2% of pancreatic tissue; ~2 million islets in humans
- α-cells (A cells) — secrete Glucagon
- β-cells (B cells) — secrete Insulin (most abundant cells)
- δ-cells (D cells) — secrete Somatostatin → inhibits both α and β cell secretion (local regulation)
| Feature | Glucagon | Insulin |
|---|
| Cell | α-cells | β-cells |
| Trigger | Low blood glucose (hypoglycaemia) | High blood glucose (hyperglycaemia) |
| Effect on blood glucose | ↑ blood glucose (hyperglycaemic) | ↓ blood glucose (hypoglycaemic) |
| Main mechanism | Glycogenolysis (glycogen→glucose), Gluconeogenesis (fat/protein→glucose); Inhibits cellular uptake of glucose | ↑ permeability of cell membrane for glucose; ↑ cellular uptake; glycogenesis (glucose→glycogen); inhibits gluconeogenesis |
| Disorder | Excess glucagon → hyperglycaemia | Deficiency → Diabetes Mellitus |
Diabetes Mellitus — Complete (Insulin Deficiency/Resistance)
- Hyposecretion or ineffectiveness of Insulin → high blood glucose (hyperglycaemia)
- 4 Polys: Polyuria (↑ urine volume), Polydipsia (↑ thirst), Polyphagia (↑ hunger), Glycosuria (glucose in urine)
- Insulin deficiency → body breaks down fat completely → Ketone bodies formed (β-hydroxybutyric acid, acetoacetic acid, acetone)
- Ketoacidosis — accumulation of ketone bodies → blood pH falls → life-threatening
- Ketones in blood = Ketonaemia; in urine = Ketonuria; ketone breath (fruity smell)
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Gonads — Testis and Ovary
Testis — Male Gonad
Cells and Hormones
- Leydig cells (Interstitial cells) — stimulated by LH (ICSH) → secrete Testosterone (androgen)
- Sertoli cells (in seminiferous tubules) — stimulated by FSH → produce Inhibin (inhibits FSH — negative feedback) and ABP (Androgen Binding Protein) → concentrates testosterone near developing sperm
- FSH + Testosterone from Sertoli cells → Sperm maturation factor
Functions of Androgens (Testosterone)
- Stimulate development, growth and maturation of male accessory sex organs (prostate, seminal vesicles, vas deferens, epididymis)
- Stimulate muscular growth; voice deepening (low pitch)
- Development of axillary hair, pubic hair, facial hair
- Aggressiveness and libido (sexual behaviour) via central neural effects
- Anabolic effect (protein synthesis)
- Stimulate spermatogenesis
Ovary — Female Gonad
Cells and Hormones
- Growing ovarian follicles → secrete Estrogen (Estradiol)
- Corpus Luteum (formed after ovulation) → secretes Progesterone (mainly) + Estrogen + Relaxin + Inhibin
- Ovary also secretes Inhibin → negative feedback on FSH
Functions of Estrogen
- Development and maturation of female accessory sex organs (uterus, fallopian tubes, vagina)
- Development of female secondary sexual characters: high-pitched voice, breast development, wide pelvis, pubic hair, feminine fat distribution
- Stimulates growth of ovarian follicles
- Regulates female sexual behaviour (libido)
Functions of Progesterone
- Supports and maintains pregnancy — prevents uterine contractions; prepares endometrium for implantation
- Acts on mammary glands → stimulates formation of alveoli (milk-producing tissue)
- Stimulates milk secretion
- Maintains corpus luteum
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Hormones of Other Organs
| Organ | Hormone | Function |
|---|
| Heart (Atria) | ANF (Atrial Natriuretic Factor) | Decreases blood pressure (vasodilation + inhibits RAAS) |
| Kidney (JG cells) | Erythropoietin, Renin, Calcitriol | Erythropoietin → ↑ RBC production; Renin → RAAS → ↑ BP; Calcitriol → ↑ Ca²⁺ absorption from gut |
| GI Tract — Stomach | Gastrin | Acts on gastric glands → stimulates HCl and Pepsinogen secretion |
| GI Tract — Duodenum | Secretin | Acts on exocrine pancreas → stimulates secretion of water and bicarbonate ions (neutralize acid) |
| GI Tract — Duodenum/Small Intestine | CCK (Cholecystokinin) | Acts on gallbladder + pancreas → bile juice and pancreatic enzymes secretion |
| GI Tract — Duodenum | GIP (Gastric Inhibitory Peptide) | Inhibits gastric secretion and motility |
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Quick Revision — Must-Know Points
Master gland → Pituitary
Master of master → Hypothalamus
GH hypo in child → Dwarfism
GH hyper in adult → Acromegaly
ADH absent → Diabetes Insipidus
Oxytocin → Milk ejection + Birth
T₃ most active form → Triiodothyronine
Iodine deficiency → Simple Goiter
Foetal hypothyroidism → Cretinism
Grave's Disease → Hyperthyroidism + Exophthalmos
PTH → ↑ Blood Ca²⁺
TCT → ↓ Blood Ca²⁺
Melatonin → Circadian rhythm, Pineal gland
Thymosin → T-cell maturation, Thymus
Aldosterone → Na⁺ reabsorption, Zona Glomerulosa
Cortisol → Anti-stress, Zona Fasciculata
DHEA → Sex corticoid, Zona Reticularis
Addison's → Hypo cortex hormones
Cushing's → Hyper glucocorticoids
α-cells → Glucagon; β-cells → Insulin
Insulin deficiency → Diabetes Mellitus
Leydig cells → Testosterone (stimulated by LH)