Endocrine system 1. Describe the adrenal gland: its structure, hormones (cortex vs medulla), and related diseases. 2. Explain the pituitary gland: anatomical divisions, hormones, and disorders. 3. Describe the thyroid gland: structure, hormone synthesis, and common diseases. 4. Explain the parathyroid glands: structure, hormones, and disorders. 5. Describe the pancreas (endocrine part): islets, hormones, and diseases. 6. Explain the hypothalamus: structure, releasing hormones, and dysfunctions. 7. Describe the pineal gland: structure, hormone, and physiological role. 8. Explain the ovaries: structure, hormones, and endocrine disorders. 9. Describe the testes: structure, hormones, and dysfunctions. 10. Explain the thymus: structure, hormones, and role in immunity. 11. Describe the adrenal cortex separately: zones, hormones, and diseases. 12. Explain the adrenal medulla: structure, catecholamines, and related tumors. 13. Describe the anterior pituitary: hormones, regulation, and disorders. 14. Explain the posterior pituitary: hormones, origin, and clinical conditions. 15. Describe thyroid hormone synthesis: steps, regulation, and pathological changes. 16. Explain calcium homeostasis: hormones involved and related diseases. 17. Describe glucose regulation: hormones, mechanisms, and disorders. Give me best answer for viva exam and for better explanation and understanding

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Here is a comprehensive, viva-ready guide to the entire endocrine system. All content is synthesized from Guyton & Hall, Robbins Pathology, Histology (Eroschenko), Harrison's, and Costanzo Physiology.

🧬 ENDOCRINE SYSTEM — Complete Viva Guide

1. Adrenal Gland — Structure, Hormones & Diseases

Structure

The adrenal (suprarenal) glands sit atop each kidney, enclosed in a fibrous capsule. Each gland has two functionally distinct regions:

Cortex — outer, derived from mesoderm (~80–90% of gland)
Medulla — inner, derived from neural crest (neuroectoderm)

Adrenal Cortex — 3 Zones (mnemonic: GFR = Salt, Sugar, Sex)

Zone	% of Cortex	Hormone	Mnemonic
Zona Glomerulosa	15%	Mineralocorticoids (Aldosterone)	Salt
Zona Fasciculata	75%	Glucocorticoids (Cortisol)	Sugar
Zona Reticularis	10%	Sex steroids (DHEA, androgens)	Sex

Cortex hormones are ALL steroids — synthesized from cholesterol.

Adrenal Medulla

Composed of chromaffin cells (modified postganglionic sympathetic neurons)
Synthesizes and secretes catecholamines: Epinephrine (80%) and Norepinephrine (20%)
Stimulated by preganglionic sympathetic fibers → "fight or flight" response

Diseases

Cortex Diseases:

Disease	Cause	Key Features
Cushing Syndrome	Excess cortisol	Moon face, buffalo hump, central obesity, striae, hypertension, hyperglycemia
Cushing Disease	Pituitary ACTH-secreting adenoma (70% of endogenous cases)	Same as above + bilateral adrenal hyperplasia
Conn Syndrome (Primary Hyperaldosteronism)	Adrenal adenoma secreting aldosterone	Hypertension, hypokalemia, metabolic alkalosis
Addison Disease	Adrenocortical insufficiency	Weakness, hyperpigmentation, hypotension, hyponatremia, hyperkalemia
Adrenogenital Syndrome	Excess androgens (CAH)	Virilization, ambiguous genitalia (21-hydroxylase deficiency most common)

Medulla Diseases:

Disease	Feature
Pheochromocytoma	Catecholamine-secreting tumor → episodic hypertension, headache, palpitations, sweating (Rule of 10s)
Neuroblastoma	Malignant tumor of children from neural crest cells

2. Pituitary Gland — Divisions, Hormones & Disorders

Anatomy

Small gland (~1 cm, 0.5–1 g), sits in the sella turcica of the sphenoid bone
Connected to hypothalamus by the pituitary stalk (infundibulum)
Embryology: Anterior from Rathke's pouch (pharyngeal ectoderm); Posterior from neural outgrowth of hypothalamus

Divisions

Division	Also Called	Origin
Anterior pituitary	Adenohypophysis	Rathke's pouch (epithelial)
Posterior pituitary	Neurohypophysis	Neural tissue (hypothalamus)
Pars intermedia	—	Between the two (rudimentary in adults)

Anterior Pituitary Hormones (6 major)

Hormone	Cell Type	Target	Action
GH (Growth Hormone)	Somatotrophs	Liver/tissues	Growth, IGF-1 release, protein synthesis
ACTH (Corticotropin)	Corticotrophs	Adrenal cortex	Stimulates cortisol
TSH (Thyrotropin)	Thyrotrophs	Thyroid	Stimulates T3/T4
FSH	Gonadotrophs	Gonads	Follicle development, spermatogenesis
LH	Gonadotrophs	Gonads	Ovulation, testosterone production
Prolactin	Lactotrophs	Breast	Milk production

Posterior Pituitary Hormones (2)

ADH (Vasopressin) — synthesized in supraoptic nucleus; controls water reabsorption in collecting duct
Oxytocin — synthesized in paraventricular nucleus; milk ejection, uterine contraction

Disorders

Disorder	Mechanism	Features
Acromegaly	GH excess in adults	Enlarged hands, feet, jaw, coarse features, organomegaly
Gigantism	GH excess before epiphyseal fusion	Tall stature
Hyperprolactinemia	Prolactinoma (most common pituitary tumor)	Galactorrhea, amenorrhea, infertility
Craniopharyngioma	Rathke's pouch remnant	Calcification, visual field defects, bitemporal hemianopia
Diabetes Insipidus	ADH deficiency (central) or resistance (nephrogenic)	Polyuria, polydipsia, dilute urine
SIADH	ADH excess	Hyponatremia, concentrated urine
Sheehan Syndrome	Postpartum pituitary necrosis	Panhypopituitarism
Empty Sella Syndrome	Enlarged sella with CSF	Headache, sometimes hypopituitarism

3. Thyroid Gland — Structure, Hormone Synthesis & Diseases

Structure

Butterfly-shaped, located anterior to the trachea (C5–T1)
Two lobes connected by the isthmus; sometimes a pyramidal lobe
Composed of follicles lined by follicular cells (principal cells), filled with colloid (thyroglobulin)
Parafollicular (C) cells — between follicles, secrete calcitonin

Hormone Synthesis (steps)

Iodide uptake — Na⁺/I⁻ symporter (NIS) on basolateral membrane concentrates iodide
Oxidation — Iodide → oxidized iodine by thyroid peroxidase (TPO) + H₂O₂
Organification — Oxidized iodine binds tyrosine residues on thyroglobulin (in follicular lumen) → MIT (monoiodotyrosine) and DIT (diiodotyrosine)
Coupling — TPO couples:
- DIT + DIT → T4 (thyroxine)
- DIT + MIT → T3 (triiodothyronine)
Storage — As thyroglobulin in colloid (2–3 months supply)
Release — TSH stimulates endocytosis of colloid; lysosomes cleave T3/T4 from thyroglobulin → secreted into blood
Peripheral conversion — Most T4 → T3 by deiodinase (T3 is 3–5× more potent)

Regulation

TRH (hypothalamus) → TSH (anterior pituitary) → T3/T4 (thyroid)
Negative feedback: T3/T4 inhibit TRH and TSH

Common Diseases

Disease	Cause	Features
Graves' Disease	Autoimmune: TSI antibodies stimulate TSH-R	Hyperthyroidism, diffuse goiter, exophthalmos, pretibial myxedema
Hashimoto Thyroiditis	Autoimmune destruction (anti-TPO, anti-thyroglobulin)	Most common hypothyroid cause; Hürthle cell change, lymphocytic infiltrate
Simple/Non-toxic Goiter	Iodine deficiency → ↑TSH → gland hypertrophy	Enlarged gland, euthyroid
Toxic Multinodular Goiter	Autonomously functioning nodules	Hyperthyroidism, irregular enlarged gland
Papillary Carcinoma	Most common thyroid cancer	Orphan Annie nuclei, psammoma bodies, good prognosis
Follicular Carcinoma	Invasion of capsule/vessels	Hematogenous spread to bone/lung
Medullary Carcinoma	C-cell tumor → calcitonin	Amyloid deposits, associated with MEN-2
Anaplastic Carcinoma	Undifferentiated	Most aggressive, poor prognosis
Hypothyroidism	Various	Myxedema, fatigue, cold intolerance, bradycardia

4. Parathyroid Glands — Structure, Hormones & Disorders

Structure

4 small glands (~40 mg each) on the posterior surface of the thyroid
Chief cells — secrete PTH
Oxyphil cells — function uncertain (possibly involuted chief cells)
Supplied by inferior thyroid arteries

Parathyroid Hormone (PTH)

84 amino acid polypeptide
Secreted in response to ↓ serum Ca²⁺
Actions (raises serum Ca²⁺):
- Bone: Activates osteoclasts (bone resorption) → ↑Ca²⁺ & ↑PO₄³⁻
- Kidney: ↑Ca²⁺ reabsorption (DCT); ↓PO₄³⁻ reabsorption; activates 1α-hydroxylase → ↑Calcitriol (1,25-OH₂D₃)
- GI (indirect): Via calcitriol → ↑Ca²⁺ & PO₄³⁻ absorption

Disorders

Disorder	Cause	Features
Primary Hyperparathyroidism	Adenoma (80%), hyperplasia, carcinoma	Hypercalcemia, hypophosphatemia, "bones, stones, groans, moans" (bone pain, renal stones, GI upset, psychiatric)
Secondary Hyperparathyroidism	Chronic renal failure → ↓Ca²⁺ → ↑PTH	Renal osteodystrophy, hyperphosphatemia
Tertiary Hyperparathyroidism	Autonomous PTH secretion despite CRF correction	Hypercalcemia despite treating secondary cause
Hypoparathyroidism	Post-surgical, autoimmune, DiGeorge syndrome	Hypocalcemia, tetany, Chvostek's sign, Trousseau's sign, prolonged QT
Pseudohypoparathyroidism	End-organ resistance to PTH (Albright Hereditary Osteodystrophy)	Hypocalcemia with ↑PTH

5. Pancreas (Endocrine Part) — Islets, Hormones & Diseases

Structure of Islets of Langerhans

~1–2 million islets scattered throughout the pancreas
Each islet ~0.3 mm diameter, organized around capillaries
Composed of 4 cell types:

Cell	%	Hormone	Action
Beta (B)	60%	Insulin + Amylin	↓Blood glucose (anabolic)
Alpha (A)	25%	Glucagon	↑Blood glucose (catabolic)
Delta (D)	10%	Somatostatin	Inhibits insulin & glucagon
PP (F)	<5%	Pancreatic polypeptide	Inhibits exocrine pancreas

Key Hormones

Insulin:

Synthesized as preproinsulin → proinsulin → insulin + C-peptide (cleaved in Golgi)
Stimulated by: ↑blood glucose, amino acids, GLP-1, GIP, vagal activity
Actions: Glucose uptake (GLUT-4 in muscle/fat), glycogenesis, lipogenesis, protein synthesis, inhibits gluconeogenesis
C-peptide is used to measure endogenous insulin secretion

Glucagon:

Stimulated by: ↓glucose, amino acids, sympathetic activity
Actions: Glycogenolysis, gluconeogenesis, lipolysis, ketogenesis

Diseases

Disease	Mechanism	Features
Type 1 DM	Autoimmune destruction of beta cells	Absolute insulin deficiency; DKA prone; anti-GAD, anti-islet antibodies
Type 2 DM	Insulin resistance + relative deficiency	Most common; associated with obesity, metabolic syndrome
Insulinoma	Beta cell tumor	Fasting hypoglycemia, Whipple's triad
Glucagonoma	Alpha cell tumor	Necrolytic migratory erythema, diabetes, anemia
Zollinger-Ellison	Gastrinoma (delta-cell related)	Peptic ulcers, diarrhea

6. Hypothalamus — Structure, Releasing Hormones & Dysfunctions

Structure

Located in the diencephalon, forms the floor of the 3rd ventricle
Key nuclei:
- Supraoptic nucleus → ADH (vasopressin)
- Paraventricular nucleus → Oxytocin, CRH, TRH
- Arcuate nucleus → GHRH, Dopamine (inhibits prolactin)
- Preoptic area → GnRH, thermoregulation
- Ventromedial nucleus → Satiety center
- Lateral hypothalamus → Hunger/feeding center
- Suprachiasmatic nucleus → Circadian rhythm

Releasing Hormones (Hypophysiotropic Hormones)

Hormone	Action
TRH	↑TSH, ↑Prolactin
CRH	↑ACTH
GHRH	↑GH
Somatostatin (GHIH)	↓GH, ↓TSH
GnRH	↑FSH, ↑LH
Dopamine (PIH)	↓Prolactin

These reach the anterior pituitary via the hypothalamo-hypophyseal portal system.

Dysfunctions

Condition	Feature
Hypothalamic obesity	Lesions of ventromedial nucleus → hyperphagia
Central DI	ADH deficiency → polyuria
Kallmann Syndrome	GnRH deficiency + anosmia → hypogonadotropic hypogonadism
Froelich Syndrome	Hypothalamic lesion → obesity + hypogonadism
Central precocious puberty	Premature GnRH pulsatility

7. Pineal Gland — Structure, Hormone & Physiological Role

Structure

Small pine-cone shaped gland (~150 mg), sits in the epithalamus (posterior roof of 3rd ventricle)
Composed of pinealocytes (melatonin-producing) and glial cells
Receives input from the retinohypothalamic tract via the suprachiasmatic nucleus
In older adults, calcification occurs ("brain sand" / corpora arenacea) — visible on X-ray/CT

Hormone: Melatonin

Synthesized from tryptophan → serotonin → N-acetylserotonin → Melatonin
Secreted at night (darkness stimulates; light suppresses)
Regulated by: suprachiasmatic nucleus → superior cervical ganglion → pineal gland

Physiological Roles

Circadian rhythm regulation — master clock synchronizer
Sleep induction — acts on MT1/MT2 receptors in SCN
Seasonal reproductive cycles — inhibits GnRH in some mammals (anti-gonadotrophic in long nights)
Antioxidant — free radical scavenger
Immune modulation
In humans: delays puberty onset (pineal tumors in children can cause precocious puberty)

8. Ovaries — Structure, Hormones & Endocrine Disorders

Structure

Paired almond-shaped organs (~3.5 × 2 × 1 cm) in the pelvic cavity
Cortex: Contains follicles (primordial, primary, secondary, Graafian) and corpus luteum
Medulla: Loose connective tissue with blood vessels and nerves
Follicle stages: Primordial → Primary → Secondary (antrum develops) → Graafian (preovulatory) → Corpus luteum → Corpus albicans

Hormones

Hormone	Source	Actions
Estrogens (E2)	Granulosa cells (from androgens via aromatase)	Female sex characteristics, endometrial proliferation, LH surge (positive feedback), bone protection
Progesterone	Corpus luteum (after ovulation)	Endometrial secretory phase, maintains pregnancy, ↑basal body temperature
Inhibin	Granulosa cells	Inhibits FSH (negative feedback)
Activin	Granulosa cells	Stimulates FSH
AMH (anti-Müllerian hormone)	Granulosa cells of small follicles	Ovarian reserve marker

Two-Cell Theory of Estrogen Synthesis

Theca cells (LH receptor) → androgens (androstenedione, testosterone)
Granulosa cells (FSH receptor) → aromatase converts androgens → estradiol

Endocrine Disorders

Disorder	Feature
PCOS (Polycystic Ovarian Syndrome)	↑LH, ↑androgens, insulin resistance; oligomenorrhea, hirsutism, acne, infertility; multiple small cysts
Premature Ovarian Failure	Menopause <40 years; ↑FSH, ↓estrogen
Granulosa Cell Tumor	Estrogen-secreting; precocious puberty in girls, endometrial hyperplasia in adults; Call-Exner bodies
Turner Syndrome (45,X)	Ovarian dysgenesis → streak ovaries → primary amenorrhea, short stature, ↑FSH

9. Testes — Structure, Hormones & Dysfunctions

Structure

Paired ovoid organs in the scrotum
Seminiferous tubules (90% of testicular volume) — contain:
- Sertoli cells (sustentacular cells): Support spermatogenesis, secrete inhibin, ABP, AMH, form blood-testis barrier
- Spermatogenic cells: Spermatogonia → primary spermatocytes → secondary → spermatids → spermatozoa
Interstitial tissue (Leydig cells): Testosterone-producing; stimulated by LH

Hormones

Hormone	Source	Action
Testosterone	Leydig cells (LH stimulated)	Spermatogenesis (local), male secondary sex characteristics, anabolic effects, negative feedback on LH/FSH
DHT (dihydrotestosterone)	5α-reductase converts T → DHT	External genitalia development, prostate growth, male pattern baldness
Inhibin	Sertoli cells	Inhibits FSH (selective feedback)
Estradiol	Sertoli cells (aromatase)	Bone development, spermatogenesis
AMH	Sertoli cells (fetal)	Regression of Müllerian (female) ducts

Dysfunctions

Disorder	Cause	Features
Klinefelter Syndrome (47, XXY)	Extra X chromosome	Hypogonadism, gynecomastia, tall stature, azoospermia, ↑FSH/LH, ↓testosterone, small testes
Cryptorchidism	Undescended testis	↑Risk of seminoma, infertility
Hypogonadotropic Hypogonadism	Hypothalamic/pituitary failure	Low LH/FSH + low testosterone
Primary Hypogonadism	Testicular failure	Low testosterone, high LH/FSH
5α-Reductase Deficiency	↓DHT	46,XY with ambiguous genitalia; virilization at puberty
Androgen Insensitivity Syndrome	AR mutation	46,XY phenotypic female; absent uterus, testes in labia

10. Thymus — Structure, Hormones & Role in Immunity

Structure

Bilobed lymphoid organ in the anterior superior mediastinum (behind sternum)
Largest and most active in childhood; involutes (replaced by fat) after puberty
Divided into:
- Cortex: Dense, darkly staining — immature T lymphocytes (thymocytes)
- Medulla: Pale staining — mature T cells + Hassall's corpuscles (whorls of epithelial cells, hallmark of thymic medulla)
Thymic epithelial cells (nurse cells, cortical, medullary) orchestrate T-cell education

Key Processes

Positive selection (cortex): T cells that can recognize self-MHC survive
Negative selection (medulla): T cells with too-strong self-reactivity are deleted (central tolerance)
AIRE gene in medullary epithelium presents self-antigens to eliminate autoreactive T cells

Thymic Hormones

Hormone	Action
Thymosin	Promotes T-cell differentiation and maturation
Thymulin	T-cell maturation, immunological functions
Thymopoietin	Promotes early T-cell development

Immunological Role

Generates naïve T cells (both CD4+ helper and CD8+ cytotoxic)
Critical for cell-mediated immunity
Absence (DiGeorge syndrome / 22q11 deletion) → T-cell deficiency → recurrent infections, absent thymic shadow on CXR

11. Adrenal Cortex — Zones, Hormones & Diseases (Detailed)

Zones in Detail

Zona Glomerulosa (outermost)

Cells: Small, round clusters
Hormone: Aldosterone (mineralocorticoid)
Regulation: Renin-angiotensin-aldosterone system (RAAS) + hyperkalemia (NOT ACTH primarily)
Action: Na⁺ retention (↑ENaC, Na⁺/K⁺-ATPase in collecting duct) → ↑K⁺ and H⁺ excretion → ↑blood volume and BP

Zona Fasciculata (middle, largest 75%)

Cells: Large, pale, lipid-laden "spongiocytes" in cords
Hormone: Cortisol (glucocorticoid)
Regulation: ACTH (diurnal rhythm — peaks at 8 AM)
Actions: Gluconeogenesis, anti-inflammatory, immunosuppression, ↑protein catabolism, ↑lipolysis, ↑blood pressure (sensitizes vasculature to catecholamines)

Zona Reticularis (innermost)

Cells: Anastomosing cords, lipofuscin pigment
Hormone: DHEA, androstenedione (weak androgens / sex steroids)
Regulation: ACTH
Actions: Converted peripherally to estrogens/androgens

Key Diseases (see also Topic 1)

Waterhouse-Friderichsen Syndrome: Bilateral adrenal hemorrhage (usually meningococcal septicemia) → acute adrenal insufficiency
Congenital Adrenal Hyperplasia (CAH): Most common: 21-hydroxylase deficiency → cortisol ↓ → ACTH ↑ → adrenal hyperplasia + shunting to androgens → virilization; also salt-wasting form (↓aldosterone) and hypertensive form (11β-hydroxylase deficiency → ↑11-deoxycorticosterone)

12. Adrenal Medulla — Structure, Catecholamines & Tumors

Structure

Innermost portion of adrenal gland
Composed of chromaffin cells (named for brown-black color with potassium dichromate stain = chromaffin reaction)
Richly vascularized; functionally = modified sympathetic ganglion (no axons, directly secretes into blood)
Supported by sustentacular (sustaining) cells

Catecholamine Synthesis Pathway

Tyrosine → DOPA → Dopamine → Norepinephrine → Epinephrine

Key enzyme: PNMT (phenylethanolamine N-methyltransferase) converts NE → Epi; induced by cortisol from adrenal cortex
Epinephrine: 80% of adrenal medullary output
Norepinephrine: 20%

Actions

Catecholamine	α1	β1	β2	Key Effect
Epinephrine	++	++	+++	↑HR, ↑CO, bronchodilation, metabolic (↑glucose, lipolysis)
Norepinephrine	+++	++	+	Mainly vasoconstriction, ↑BP

Related Tumors

Pheochromocytoma:

Catecholamine-secreting tumor of chromaffin cells
Rule of 10s: 10% bilateral, 10% extra-adrenal (paraganglioma), 10% malignant, 10% familial (MEN-2, VHL, NF-1)
Presentation: Episodic hypertension, headache, palpitations, diaphoresis (5 H's: Hypertension, Headache, Hyperhidrosis, Hyperglycemia, Hypermetabolism)
Diagnosis: ↑urinary catecholamines/metanephrines, plasma metanephrines
Treatment: Alpha-blockade first (phenoxybenzamine), then beta-blockade → surgical resection

Neuroblastoma:

Malignant tumor of neural crest (children < 5 yrs); most common extracranial solid tumor of childhood
Secretes VMA (vanillylmandelic acid) and HVA (homovanillic acid)
Presents as abdominal mass; N-Myc amplification = poor prognosis

13. Anterior Pituitary — Hormones, Regulation & Disorders (Detailed)

Cell Types and Hormones

Cell Type	%	Hormone	Staining
Somatotrophs	50%	GH	Acidophilic
Lactotrophs	15-20%	Prolactin	Acidophilic
Corticotrophs	15-20%	ACTH	Basophilic
Thyrotrophs	5%	TSH	Basophilic
Gonadotrophs	10%	FSH, LH	Basophilic

Mnemonic for acidophils vs basophils: "FLAT PiG" — FSH, LH, ACTH, TSH = basophils; ProlActin, GH = acidophils

Regulation

Each axis follows the pattern: Hypothalamus → Anterior Pituitary → Target gland → Negative feedback
Prolactin unique: tonically inhibited by dopamine (PIH) from hypothalamus — therefore hypothalamic damage/stalk section causes ↑prolactin but ↓all other anterior pituitary hormones

Key Disorders

Disorder	Cause/Mechanism	Features
Acromegaly	GH-secreting adenoma (adults)	Frontal bossing, prognathism, ↑shoe/ring size, carpal tunnel, DM, hypertension; ↑IGF-1 is diagnostic test
Gigantism	GH excess in children (before epiphyseal closure)	Tall stature, can exceed 7 feet
Prolactinoma	Most common pituitary adenoma	Women: amenorrhea-galactorrhea; Men: impotence, gynecomastia. Treatment: Dopamine agonists (cabergoline, bromocriptine)
Nelson Syndrome	ACTH-secreting pituitary adenoma after bilateral adrenalectomy	Hyperpigmentation (ACTH/MSH), enlarging pituitary mass
Hypopituitarism	Mass effect, infarction, radiation	Deficiency of multiple hormones — GH first affected, then LH/FSH, then TSH/ACTH

14. Posterior Pituitary — Hormones, Origin & Clinical Conditions

Origin & Structure

Neurohypophysis — true neural tissue extension of the hypothalamus
Contains pituicytes (modified glial cells) and axon terminals of hypothalamic neurons
Hormones synthesized in hypothalamic nuclei, transported via axons, stored and released from posterior pituitary:
- ADH/Vasopressin → from supraoptic nucleus (SON)
- Oxytocin → from paraventricular nucleus (PVN)

ADH (Vasopressin)

9 amino acid peptide
Stimuli for release: ↑plasma osmolality, ↓blood volume, pain, stress, nausea
Action: V2 receptors in collecting duct → inserts AQP2 (aquaporin-2) → water reabsorption → concentrated urine

Oxytocin

Action: Milk ejection (let-down reflex via myoepithelial cells); uterine contractions during labor (Ferguson reflex — positive feedback)
Synthetic oxytocin (Pitocin) used for labor induction

Clinical Conditions

Condition	Mechanism	Features
Central DI	↓ADH synthesis/secretion	Polyuria (>3L/day), polydipsia, dilute urine (osmolality <200), responds to desmopressin
Nephrogenic DI	ADH resistance (V2 receptor mutation or lithium)	Same symptoms, does NOT respond to desmopressin
SIADH	Excess ADH	Euvolemic hyponatremia, concentrated urine, urine Na >20, causes: lung cancer (small cell), CNS disorders, drugs

15. Thyroid Hormone Synthesis — Steps, Regulation & Pathological Changes

Detailed Steps

Step	Process	Key Enzyme/Protein
1	Dietary iodide absorption → blood	—
2	Iodide uptake into follicular cell	NIS (Na⁺/I⁻ symporter) — inhibited by thiocyanate, perchlorate
3	Iodide transport to apical surface	Pendrin (apical transporter) — mutations → Pendred syndrome (goiter + deafness)
4	Oxidation of I⁻ to I⁰ or I₃⁻	Thyroid Peroxidase (TPO) + H₂O₂ — blocked by PTU, methimazole
5	Organification of thyroglobulin (iodination of tyrosines → MIT, DIT)	TPO
6	Coupling: DIT+DIT → T4; DIT+MIT → T3	TPO
7	Storage as colloid	Thyroglobulin in follicular lumen
8	Endocytosis of colloid (TSH-stimulated)	TSH → cAMP → endocytosis
9	Lysosomal hydrolysis → T3, T4 released	Proteases
10	MIT/DIT deiodinated (iodine recycled)	Dehalogenase (DEHAL1)
11	T4 → T3 peripherally (liver, kidney)	5'-deiodinase (type 1, 2)

Regulation Summary

TSH → activates every step of synthesis and release
Negative feedback: T3 (at nucleus) → ↓TRH sensitivity and ↓TSH release
Wolff-Chaikoff effect: Excess iodide transiently inhibits thyroid hormone synthesis (exploited therapeutically in thyroid storm — Lugol's iodine)

Pathological Changes

Pathology	Mechanism
Hypothyroidism	Hashimoto's, iodine deficiency, post-thyroidectomy, anti-thyroid drugs
Hyperthyroidism	Graves' (TSI), toxic nodule, excess iodine (Jod-Basedow), exogenous T4
Goiter	↑TSH drive due to iodine deficiency → gland hyperplasia
Thyroid Storm	Life-threatening thyrotoxicosis; fever, tachycardia, confusion; treat with PTU + beta-blockers + steroids + Lugol's

16. Calcium Homeostasis — Hormones Involved & Related Diseases

Normal Serum Ca²⁺: 8.5–10.5 mg/dL (ionized: 4.5–5.5 mg/dL)

Three Hormones Controlling Ca²⁺

Hormone	Source	Effect on Ca²⁺	Effect on PO₄³⁻
PTH	Chief cells, parathyroid	↑	↓ (phosphaturic)
Calcitriol (1,25-OH₂D₃)	Kidney (1α-hydroxylase, activated by PTH)	↑	↑
Calcitonin	C-cells, thyroid	↓	↓

PTH Actions (recap)

Bone: ↑osteoclast activity → ↑Ca²⁺ + ↑PO₄³⁻ release
Kidney: ↑Ca²⁺ reabsorption (DCT), ↓PO₄³⁻ reabsorption (PCT), ↑1α-hydroxylase → ↑calcitriol
Gut (indirect): Calcitriol → ↑Ca²⁺ absorption via calbindin

Vitamin D Pathway

Skin: UV light → cholecalciferol (D3) from 7-dehydrocholesterol
Liver: 25-hydroxylation → 25-OH-D3 (calcidiol) — storage form
Kidney: 1α-hydroxylation → 1,25-(OH)₂D3 (calcitriol) — active form; stimulated by PTH, hypophosphatemia

Related Diseases

Disease	Abnormality	Ca²⁺	PTH	PO₄³⁻
Primary Hyperparathyroidism	PTH adenoma	↑	↑	↓
Hypoparathyroidism	↓PTH	↓	↓	↑
Pseudo-hypoparathyroidism	PTH resistance	↓	↑	↑
Vitamin D deficiency (Rickets/Osteomalacia)	↓Calcitriol → ↓Ca²⁺ → 2° hyperPTH	↓/normal	↑(2°)	↓
Malignancy-associated hypercalcemia	PTHrP or osteolysis	↑	↓	↑/normal
CRF (Secondary hyperPTH)	↓Calcitriol, ↓Ca²⁺, ↑PO₄³⁻	↓	↑	↑

17. Glucose Regulation — Hormones, Mechanisms & Disorders

Normal Fasting Glucose: 70–99 mg/dL

Hormones Involved

Hormone	Effect on Glucose	Source	Stimulated By
Insulin	↓ (hypoglycemic)	Beta cells	↑Glucose, amino acids, GLP-1, GIP, vagus
Glucagon	↑ (hyperglycemic)	Alpha cells	↓Glucose, amino acids, epinephrine
Epinephrine	↑	Adrenal medulla	Stress, hypoglycemia
Cortisol	↑	Adrenal cortex (ZF)	ACTH
GH	↑ (anti-insulin)	Anterior pituitary	GHRH, sleep, hypoglycemia
Glucocorticoids	↑	Adrenal cortex	ACTH
T3/T4	Modulates	Thyroid	TSH
GLP-1 / GIP	↓ (incretin effect)	L-cells/K-cells of gut	Meals

Post-Meal State (Fed/Anabolic)

↑Glucose → ↑Insulin → GLUT-4 translocation (muscle, fat) → glucose uptake
Glycogenesis (liver, muscle), protein synthesis, lipogenesis
↓Glucagon

Fasting/Hypoglycemia State (Catabolic — Counter-regulatory response)

↓Insulin; ↑Glucagon, ↑Epinephrine, ↑Cortisol, ↑GH
Glycogenolysis (liver) → glucose release
Gluconeogenesis (liver, kidney) from alanine, lactate, glycerol
Lipolysis → FFA → ketogenesis (acetoacetate, β-hydroxybutyrate)

Disorders

Disorder	Mechanism	Key Features
Type 1 DM	Autoimmune beta cell destruction	Absolute insulin deficiency; DKA (fruity breath, ↑anion gap, ↑ketones, ↑glucose)
Type 2 DM	Insulin resistance → relative deficiency	HHS (hyperosmolar hyperglycemic state); no ketones; ↑osmolality
Insulinoma	Excess insulin (tumor)	Fasting hypoglycemia; Whipple's triad: symptoms + low glucose + resolution with glucose
Glucagonoma	Excess glucagon	Hyperglycemia, necrolytic migratory erythema, weight loss
Cushing Syndrome	Excess cortisol → gluconeogenesis	Secondary/steroid DM
Acromegaly	Excess GH → insulin resistance	DM/glucose intolerance
Reactive Hypoglycemia	Exaggerated insulin response post-meal	Symptoms 2–4 hrs after eating

Quick Viva Summary Table — Endocrine Glands at a Glance

Gland	Key Hormone(s)	Key Disease
Adrenal Cortex	Cortisol, Aldosterone, DHEA	Cushing's, Addison's, Conn's
Adrenal Medulla	Epi, NE	Pheochromocytoma
Anterior Pituitary	GH, ACTH, TSH, FSH, LH, PRL	Acromegaly, Prolactinoma
Posterior Pituitary	ADH, Oxytocin	DI, SIADH
Thyroid	T3, T4, Calcitonin	Graves', Hashimoto's, CA
Parathyroid	PTH	Hyperparathyroidism, Tetany
Pancreas	Insulin, Glucagon	DM Type 1 & 2, Insulinoma
Hypothalamus	Releasing hormones	Kallmann, DI
Pineal	Melatonin	Circadian disorders
Ovaries	Estrogen, Progesterone	PCOS, Turner Syndrome
Testes	Testosterone, Inhibin	Klinefelter, Hypogonadism
Thymus	Thymosin	DiGeorge Syndrome

Sources: Guyton & Hall Medical Physiology | Robbins & Kumar Basic Pathology | Costanzo Physiology | Harrison's Principles of Internal Medicine | Histology (Eroschenko)

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