ENDOCRINE SYSTEM
1. Introduction
The endocrine system consists of glands that secrete hormones directly into the blood, regulating growth, metabolism, reproduction, and homeostasis.
Major glands: Hypothalamus, Pituitary, Thyroid, Parathyroid, Adrenal, Pancreas, Pineal, Gonads.
Hormone action: Endocrine (blood), Paracrine (local), Autocrine (self)
2. Hypothalamus and Pituitary
2.1 Hypothalamus Anatomy
Located in diencephalon, floor of 3rd ventricle
Divisions: Supraoptic, Paraventricular, Arcuate, Preoptic nuclei
Neurosecretory cells produce releasing/inhibiting hormones (TRH, CRH, GnRH, GHRH, Somatostatin, Dopamine)
2.2 Pituitary Anatomy
Anterior pituitary (Adenohypophysis): Pars distalis, pars tuberalis, pars intermedia
Posterior pituitary (Neurohypophysis): Pars nervosa; stores ADH & Oxytocin
Portal system: Hypophyseal portal veins connect hypothalamus → anterior pituitary
Clinical Correlation:
GH deficiency → dwarfism; excess → gigantism/acromegaly
ADH deficiency → diabetes insipidus; excess → SIADH
3. Thyroid and Parathyroid
3.1 Thyroid Anatomy
Butterfly-shaped, located anterior neck, C5–T1
Lobes connected by isthmus; follicles lined by follicular cells, parafollicular (C cells) produce calcitonin
3.3 Parathyroid
4 glands on posterior thyroid
Hormone: PTH
Action: ↑ Blood Ca²⁺ via bone resorption, ↑ kidney reabsorption, ↑ calcitriol synthesis
Clinical correlation:
Hyperparathyroidism → Hypercalcemia, bone weakness
Hypoparathyroidism → Hypocalcemia, tetany
4. Adrenal Glands
4.1 Anatomy
Suprarenal glands: Cortex (3 zones), Medulla
Zones:
Zona glomerulosa → Aldosterone
Zona fasciculata → Cortisol
Zona reticularis → Androgens
Medulla → Epinephrine, Norepinephrine
Clinical correlation:
Addison’s disease → adrenal insufficiency
Cushing’s syndrome → cortisol excess
Pheochromocytoma → catecholamine excess
5. Pancreas
5.1 Anatomy
Mixed gland: Exocrine acini + endocrine Islets of Langerhans (α, β, δ, PP cells)
Clinical correlation:
Diabetes mellitus: Type 1 → β cell destruction; Type 2 → insulin resistance
6. Pineal Gland
Located in epithalamus, secretes melatonin
Regulates circadian rhythm and reproductive hormone cycles
Dysfunction → sleep disorders, precocious puberty
7. Gonads
Ovary
Hormones: Estrogen, Progesterone
Actions: Female sexual characteristics, menstrual cycle, pregnancy maintenance
Testis
Hormones: Testosterone
Actions: Male sexual characteristics, spermatogenesis
9. Clinical Correlations
Hypothyroidism → Cretinism, Myxedema
Hyperthyroidism → Graves’ disease, Thyrotoxicosis
Hyperaldosteronism → Hypertension
Hypoparathyroidism → Tetany
Diabetes mellitus → Polyuria, polydipsia, polyphagia
Addison’s disease → Hypotension, hyperpigmentation
10. MCQs
Hormone increasing basal metabolic rate? → T3/T4
Hormone increasing blood glucose? → Glucagon, Cortisol, Catecholamines
Site of aldosterone synthesis? → Zona glomerulosa
Hormone inhibited by dopamine? → Prolactin
Endocrine gland derived from diencephalon? → Pineal
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