consice notes of endocrine system for mbbs 1st year
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I now have excellent content from Guyton and Costanzo to compile comprehensive MBBS 1st year endocrine notes. Let me now produce the full notes:
🧬 Endocrine System - Concise Notes for MBBS 1st Year
Sources: Guyton & Hall Textbook of Medical Physiology | Costanzo Physiology 7e | Ganong's Review 26e
1. INTRODUCTION TO THE ENDOCRINE SYSTEM
What is a Hormone?
A chemical messenger secreted by an endocrine gland directly into the bloodstream, which acts on distant target cells.
Chemical Classes of Hormones
| Class | Examples | Solubility | Receptor Location |
|---|---|---|---|
| Peptide/Protein | Insulin, GH, PTH, ADH, Oxytocin, TSH, ACTH, FSH, LH | Water-soluble | Cell surface (membrane) |
| Steroid | Cortisol, Aldosterone, Estrogen, Testosterone, Vit D | Lipid-soluble | Intracellular (nuclear) |
| Amine | T3, T4 (thyroid), Epinephrine, Norepinephrine, Dopamine | T3/T4 = lipid-soluble; Catecholamines = water-soluble | T3/T4 = intracellular; Catecholamines = membrane |
Mechanisms of Hormone Action
- Membrane receptors (peptide hormones): Use second messengers - cAMP, cGMP, IP3/DAG, or tyrosine kinase (insulin)
- Intracellular receptors (steroid/thyroid hormones): Enter cell, bind nuclear receptor, alter gene transcription
Feedback Control (KEY CONCEPT)
- Negative feedback - most common; hormone product inhibits its own release (e.g., cortisol inhibits CRH and ACTH)
- Positive feedback - rare; LH surge before ovulation is the classic example
2. HYPOTHALAMUS & PITUITARY GLAND
Hypothalamus - The Master Regulator
Controls the anterior pituitary via hypothalamic-hypophysial portal vessels (blood-borne signaling).
| Hypothalamic Hormone | Action |
|---|---|
| TRH (thyrotropin-releasing hormone) | Stimulates TSH & Prolactin release |
| CRH (corticotropin-releasing hormone) | Stimulates ACTH release |
| GHRH (growth hormone-releasing hormone) | Stimulates GH release |
| Somatostatin (GHIH) | Inhibits GH (and TSH) release |
| GnRH (gonadotropin-releasing hormone) | Stimulates LH & FSH release |
| Dopamine (prolactin-inhibiting factor) | Inhibits Prolactin release |
Pituitary Gland - Two Lobes, Two Origins
Anterior Pituitary (adenohypophysis) - derived from oral ectoderm (Rathke's pouch)
- Linked to hypothalamus by portal blood vessels
- Hormones: GH, TSH, ACTH, Prolactin, FSH, LH (mnemonic: FLAT PiG)
Posterior Pituitary (neurohypophysis) - derived from neural tissue (downgrowth of hypothalamus)
- Linked to hypothalamus by nerve axons (neurosecretory neurons)
- Cell bodies in supraoptic nucleus (ADH) and paraventricular nucleus (Oxytocin)
- Hormones released here are made in the hypothalamus, transported down axons, stored in posterior pituitary
| Posterior Pituitary Hormone | Target | Action |
|---|---|---|
| ADH (Vasopressin) | Kidney collecting duct, arterioles | Increases water reabsorption (V2 receptor); vasoconstriction (V1 receptor) |
| Oxytocin | Uterus, breast | Uterine contractions during labor; milk ejection (let-down reflex) |
3. GROWTH HORMONE (GH)
- Secreted by somatotrophs of anterior pituitary
- A 191 amino acid polypeptide
- Structurally similar to prolactin and human placental lactogen
Regulation of GH Secretion
| Stimulates GH | Inhibits GH |
|---|---|
| Hypoglycemia | Hyperglycemia |
| Fasting/starvation | Obesity |
| Exercise, stress | Somatostatin |
| Deep sleep (Stage III-IV) | IGF-1 (somatomedin C) - negative feedback |
| Estrogen, testosterone (puberty) | Free fatty acids |
| Arginine, alpha-adrenergic agonists | Beta-adrenergic agonists |
Actions of GH
GH acts via IGF-1 (Insulin-like Growth Factor-1), produced mainly by the liver:
- Protein metabolism: Increases amino acid uptake and protein synthesis (anabolic)
- Carbohydrate metabolism: Anti-insulin effect - decreases glucose uptake (diabetogenic)
- Fat metabolism: Increases lipolysis (mobilizes fat for energy)
- Bone/cartilage: Stimulates chondrocyte proliferation and linear bone growth (epiphyseal plates)
GH Disorders
| Disorder | Cause | Features |
|---|---|---|
| Gigantism | Excess GH before epiphyseal fusion | Abnormally tall stature |
| Acromegaly | Excess GH after epiphyseal fusion | Large hands, feet, jaw (coarse features) |
| Dwarfism (pituitary) | GH deficiency in childhood | Short stature, proportionate |
| Laron dwarfism | GH receptor defect (no IGF-1 response) | Short stature, high GH, low IGF-1 |
4. THYROID HORMONES
Synthesis Steps (HIGH YIELD)
- Trapping - I⁻ actively transported into follicular cell (Na/I symporter)
- Oxidation - I⁻ oxidized to I₂ by thyroid peroxidase (TPO) (blocked by PTU, methimazole)
- Organification - I₂ attached to tyrosine residues on thyroglobulin (TG) → MIT (monoiodotyrosine) and DIT (diiodotyrosine)
- Coupling - MIT + DIT → T3 | DIT + DIT → T4 | (both reactions catalyzed by TPO)
- Storage - T3 & T4 stored in follicular lumen as colloid (largest hormone store in the body)
- Secretion - TSH stimulates endocytosis of colloid, lysosomal cleavage, release of T3 & T4 into blood
Key Points on T3 vs T4
| T3 | T4 | |
|---|---|---|
| Iodine atoms | 3 | 4 |
| Activity | More active (3-5x) | Less active (prohormone) |
| % secreted by thyroid | ~20% | ~80% |
| Plasma binding | Less | More (to TBG, albumin) |
| Conversion | T4 → T3 in peripheral tissues (deiodinase) |
Actions of Thyroid Hormones
- Increase basal metabolic rate (BMR) - increases O2 consumption and heat production
- Cardiovascular: Increases heart rate and cardiac output; upregulates beta-adrenergic receptors
- Growth and development: Essential for normal brain development (fetal/neonatal period) and skeletal maturation
- Protein: At physiologic levels = anabolic; at excess = catabolic
- Carbohydrate: Increases glucose absorption from gut and gluconeogenesis
- Fat: Increases lipolysis; lowers cholesterol (increases LDL receptors)
Thyroid Disorders (High Yield)
| Hypothyroidism | Hyperthyroidism (Graves') | |
|---|---|---|
| TSH | High (primary) | Low |
| T3/T4 | Low | High |
| Metabolic rate | Low | High |
| Heart rate | Bradycardia | Tachycardia |
| Weight | Gain | Loss |
| Skin/Hair | Dry, coarse; hair loss | Warm, moist; fine hair |
| Reflexes | Slow (delayed relaxation) | Brisk |
| Special features | Myxedema, cretinism (congenital) | Exophthalmos, goiter (Graves') |
Cretinism = congenital hypothyroidism → mental retardation, growth failure (iodine deficiency commonest cause worldwide)
5. ADRENAL GLAND
Structure - "GFR = Salt, Sugar, Sex" (outer to inner)
| Zone | Layer | Hormone Secreted | Mnemonic |
|---|---|---|---|
| Zona Glomerulosa | Outermost | Mineralocorticoids (Aldosterone) | Salt |
| Zona Fasciculata | Middle (widest, 75%) | Glucocorticoids (Cortisol) | Sugar |
| Zona Reticularis | Innermost cortex | Androgens (DHEA, Androstenedione) | Sex |
| Adrenal Medulla | Core (20%) | Catecholamines (Epi 80%, Norepi 20%) | - |
All cortical hormones are steroids derived from cholesterol. The medulla is of neuroectodermal origin (modified sympathetic ganglia).
Cortisol (Glucocorticoid)
- Regulation: CRH (hypothalamus) → ACTH (anterior pituitary) → Cortisol (zona fasciculata); negative feedback
- Diurnal variation: Peak at 8 AM (morning), nadir at midnight
- Actions:
- Carbohydrate: Increases gluconeogenesis → hyperglycemia
- Protein: Catabolic (muscle wasting, thin skin, poor wound healing)
- Fat: Redistributes fat (central obesity, buffalo hump, moon face - in excess)
- Anti-inflammatory and immunosuppressive
- Maintains vascular responsiveness to catecholamines
- Permissive effect on other hormones
Aldosterone (Mineralocorticoid)
- Regulation: Primarily by renin-angiotensin-aldosterone system (RAAS) and high K⁺
- Actions: Acts on principal cells of collecting duct
- Increases Na⁺ reabsorption (via ENaC) → water retention → ECF volume expansion
- Increases K⁺ secretion → hypokalemia (excess)
- Increases H⁺ secretion → metabolic alkalosis (excess)
Adrenal Disorders (IMPORTANT)
| Disorder | Problem | Key Features |
|---|---|---|
| Cushing's Syndrome | Cortisol excess | Central obesity, moon face, buffalo hump, striae, hyperglycemia, hypertension, osteoporosis |
| Addison's Disease | Adrenocortical insufficiency (all zones) | Hypoglycemia, hypotension, hyperkalemia, weight loss, hyperpigmentation (high ACTH/MSH) |
| Conn's Syndrome | Primary hyperaldosteronism (adenoma) | Hypertension, hypokalemia, metabolic alkalosis |
| Congenital Adrenal Hyperplasia (CAH) | 21-hydroxylase deficiency (most common) | Can't make cortisol/aldosterone; shunts to androgens → virilization in females |
| Pheochromocytoma | Catecholamine-secreting tumor (medulla) | Episodic hypertension, palpitations, sweating, headache |
6. INSULIN & GLUCAGON (Pancreatic Hormones)
Islets of Langerhans
| Cell Type | % | Hormone |
|---|---|---|
| Beta (β) cells | 60% | Insulin + Amylin |
| Alpha (α) cells | 25% | Glucagon |
| Delta (δ) cells | 10% | Somatostatin |
| PP cells | Small | Pancreatic polypeptide |
Note: Insulin inhibits glucagon; somatostatin inhibits both insulin and glucagon
Insulin
- Stimulus for secretion: High blood glucose (primary), amino acids, GIP, GLP-1, parasympathetic activity
- Inhibitors: Hypoglycemia, somatostatin, sympathetic activity (alpha-2)
- Mechanism: Tyrosine kinase receptor → GLUT-4 translocation to cell membrane (muscle, adipose)
- Actions: "Anabolic hormone of fed state"
- Increases glucose uptake by muscle and fat
- Increases glycogen synthesis (liver, muscle)
- Increases protein synthesis
- Increases fat synthesis (lipogenesis); inhibits lipolysis
- Inhibits gluconeogenesis and glycogenolysis in liver
Glucagon
- Stimulus: Hypoglycemia, amino acids (protein meal), fasting, stress
- Actions: "Hormone of fasting"
- Increases glycogenolysis and gluconeogenesis in liver → raises blood glucose
- Increases lipolysis
- Increases ketone body formation
Diabetes Mellitus
| Type 1 | Type 2 | |
|---|---|---|
| Cause | Autoimmune destruction of beta cells | Insulin resistance + relative insulin deficiency |
| Age | Usually young (<30 yrs) | Usually older (>40 yrs), but increasing in young |
| Insulin | Absent (absolute deficiency) | Normal or reduced |
| Body type | Often lean | Often obese |
| Ketoacidosis | Common (DKA) | Rare |
| Treatment | Insulin required | Diet, oral agents, +/- insulin |
DKA mechanism: No insulin → lipolysis → free fatty acids → hepatic ketogenesis → ketonemia, metabolic acidosis, ketonuria, fruity breath
7. PARATHYROID HORMONE (PTH) & CALCIUM REGULATION
PTH
- Secreted by chief cells of parathyroid glands (4 glands posterior to thyroid)
- Stimulus: Low plasma Ca²⁺ (detected by calcium-sensing receptor)
- Actions (raises Ca²⁺, lowers PO₄³⁻):
- Bone: Activates osteoclasts (via osteoblasts as intermediary) → bone resorption → releases Ca²⁺ and PO₄³⁻
- Kidney: Increases Ca²⁺ reabsorption; increases PO₄³⁻ excretion (phosphaturic); activates 1-alpha-hydroxylase → active Vit D (1,25-(OH)₂D₃)
- Intestine: Indirect - via Vit D → increases Ca²⁺ and PO₄³⁻ absorption
Calcitonin
- Secreted by parafollicular C cells of thyroid
- Stimulus: High plasma Ca²⁺
- Action: Inhibits osteoclasts → decreases bone resorption → lowers Ca²⁺ (opposes PTH)
- Physiological importance in humans is minimal (contrast to fish/birds)
Vitamin D
- Sources: Skin (UV radiation converts 7-dehydrocholesterol → Vit D₃), diet
- Activation: Liver → 25-OH-D₃ → Kidney (1α-hydroxylase, stimulated by PTH/low PO₄) → 1,25-(OH)₂D₃ (Calcitriol) = active form
- Actions: Increases Ca²⁺ and PO₄³⁻ absorption from intestine; promotes bone mineralization
Ca²⁺ Disorders
| Disorder | Ca²⁺ | PTH | Common Cause |
|---|---|---|---|
| Hyperparathyroidism (1°) | High | High | Parathyroid adenoma |
| Hypoparathyroidism | Low | Low | Post-thyroid surgery |
| Hypocalcemia signs | Low | Tetany, Chvostek sign, Trousseau sign, perioral tingling |
8. OTHER IMPORTANT HORMONES
Prolactin
- Secreted by lactotrophs of anterior pituitary
- Tonically inhibited by dopamine from hypothalamus
- Actions: Breast development, milk production (lactation)
- Hyperprolactinemia causes: galactorrhea, amenorrhea (inhibits GnRH), infertility
Antidiuretic Hormone (ADH/Vasopressin) - Summary
- Made in supraoptic nucleus, released from posterior pituitary
- Stimulus: Increased plasma osmolality (main), decreased blood volume, pain, nausea
- V2 receptor in collecting duct → inserts aquaporin-2 → water reabsorption
- Diabetes Insipidus: ADH deficiency (central) or resistance (nephrogenic) → dilute polyuria
Mineralocorticoids - Renin-Angiotensin-Aldosterone System (RAAS)
Low BP/volume → Kidney JGA → Renin → Angiotensinogen → Angiotensin I → ACE (lung) → Angiotensin II → Aldosterone (from adrenal) + vasoconstriction → restore BP and Na⁺
9. QUICK REVISION TABLE - All Major Endocrine Glands
| Gland | Hormone | Chemical Type | Main Action |
|---|---|---|---|
| Hypothalamus | TRH, CRH, GHRH, GnRH, Somatostatin, Dopamine | Peptide/Amine | Controls anterior pituitary |
| Ant. Pituitary | GH, TSH, ACTH, FSH, LH, Prolactin | Peptide | Tropins + direct effects |
| Post. Pituitary | ADH, Oxytocin | Peptide | Water retention; uterus/breast |
| Thyroid | T3, T4, Calcitonin | Amine / Peptide | Metabolism; Ca²⁺ lowering |
| Parathyroid | PTH | Peptide | Raises Ca²⁺ |
| Adrenal Cortex | Cortisol, Aldosterone, DHEA | Steroid | Stress; Na⁺ retention; sex hormones |
| Adrenal Medulla | Epinephrine, Norepinephrine | Amine | "Fight or flight" |
| Pancreas (β) | Insulin | Peptide | Lowers blood glucose |
| Pancreas (α) | Glucagon | Peptide | Raises blood glucose |
| Gonads | Testosterone, Estrogen, Progesterone | Steroid | Sex characteristics, reproduction |
10. HIGH-YIELD MNEMONICS
- Anterior pituitary hormones: FLAT PiG = FSH, LH, ACTH, TSH, Prolactin, (i) GH
- Adrenal zones (outer→inner): GFR = Glomerulosa, Fasciculata, Reticularis → Salt, Sugar, Sex
- PTH effects on kidney: "Calcium In, Phosphate Out" (reabsorbs Ca²⁺, excretes PO₄³⁻)
- Cushing's features: "Cushing's is BIG" - Buffalo hump, central obesity, Insulin resistance (hyperglycemia), Gonadal dysfunction
- Insulin vs Glucagon: Insulin = fed state anabolic; Glucagon = fasting catabolic
11. IMPORTANT CLINICAL CORRELATIONS
| Clinical Finding | Endocrine Cause |
|---|---|
| Hyperpigmentation | Addison's disease (high ACTH stimulates MSH receptors) |
| Goiter (enlarged thyroid) | Iodine deficiency, Graves', Hashimoto's |
| Tetany + Chvostek sign | Hypocalcemia (hypoparathyroidism) |
| Moon face + buffalo hump | Cushing's syndrome (cortisol excess) |
| Polyuria + polydipsia + polyphagia | Diabetes mellitus |
| Fruity breath + Kussmaul breathing | Diabetic ketoacidosis |
| Galactorrhea + amenorrhea | Hyperprolactinemia |
| Delayed deep tendon reflex relaxation | Hypothyroidism |
| Exophthalmos + pretibial myxedema | Graves' disease |
| Episodic hypertension + sweating | Pheochromocytoma |
Sources: Guyton and Hall Textbook of Medical Physiology (14e), Costanzo Physiology (7e), Ganong's Review of Medical Physiology (26e)
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