SIADH occurs when ADH is secreted inappropriately (despite low plasma osmolality) causing water retention and dilutional hyponatraemia (Na+ <135 mmol/L). Features: Hyponatraemia (low blood sodium), Low plasma osmolality (100 mOsm/kg), Normal blood volume (euvolaemia - no oedema), and Urine Na+ >20 mmol/L. Causes: Malignancy (small cell lung cancer - most common), CNS disorders (meningitis, head injury, stroke), Pulmonary disease (pneumonia, TB), and Drugs (SSRIs, carbamazepine, cyclophosphamide, vincristine). Treatment: Fluid restriction (first-line), hypertonic saline (severe symptomatic hyponatraemia), and Tolvaptan (vasopressin V2 receptor antagonist).

The Anterior Pituitary (Adenohypophysis) produces 6 major hormones - mnemonic FLAT PiG: F = FSH (Follicle Stimulating Hormone) stimulates follicle development in ovaries and sperm production in testes. L = LH (Luteinising Hormone) triggers ovulation and stimulates testosterone production in testes. A = ACTH (Adrenocorticotrophic Hormone) stimulates adrenal cortex to produce cortisol. T = TSH (Thyroid Stimulating Hormone) stimulates thyroid to produce thyroid hormones. Pi = Prolactin stimulates milk production (lactation). G = GH (Growth Hormone) stimulates growth of bones and muscles.

A Phaeochromocytoma is a catecholamine-secreting tumour arising from chromaffin cells of the adrenal medulla (or extra-adrenal sympathetic ganglia - paraganglioma). Classic presentation: Triad of paroxysmal hypertension (episodic severe high blood pressure) + headache + sweating + palpitations (5 Ps: Pressure, Pain-headache, Perspiration, Palpitations, Pallor). Rule of 10s: 10% bilateral, 10% extra-adrenal, 10% malignant, 10% familial. Associated with MEN 2A and 2B (Multiple Endocrine Neoplasia), VHL syndrome, and Neurofibromatosis type 1. Diagnosis: 24-hr urine/plasma metanephrines. Treatment: Alpha-blockade first (phenoxybenzamine), then beta-blockade, then surgery.

Acromegaly results from chronic excess GH in adults (after epiphyseal growth plates have closed). Most common cause: GH-secreting pituitary adenoma (somatotrophinoma). Features: Enlarged hands and feet (increased ring/shoe size - key symptom), Prognathism (protruding jaw), macroglossia (large tongue), coarse facial features, Frontal bossing, widely spaced teeth, Carpal tunnel syndrome (nerve compression), Sweating, oily skin, hypertension, diabetes, Visual field defects (bitemporal hemianopia - tumour compresses optic chiasm), and Arthritis (joint damage from overgrowth). Compare with Gigantism: Same cause (GH excess) but occurs before epiphyseal closure causing excessive height. Diagnosis: IGF-1 level + Oral Glucose Tolerance Test (GH not suppressed by glucose). Treatment: Transsphenoidal surgery, Somatostatin analogues (octreotide, lanreotide), Pegvisomant (GH receptor antagonist).

The Posterior Pituitary (Neurohypophysis) does NOT produce hormones - it stores and releases two hormones that are produced by the hypothalamus (in the supraoptic and paraventricular nuclei): ADH (Antidiuretic Hormone / Vasopressin) = Increases water reabsorption in kidney collecting ducts causing concentrated urine and raises blood pressure. Released when blood osmolarity rises (dehydration). Deficiency causes Diabetes Insipidus. Oxytocin = Stimulates uterine contractions during labour and milk ejection (let-down reflex) during breastfeeding. Also called the love hormone - promotes bonding, trust, and social behaviour.

The Pancreas is a mixed gland with both exocrine and endocrine functions: Exocrine (99%) = Acinar cells produce digestive enzymes (amylase, lipase, proteases) secreted into the duodenum. Endocrine (1%) = Islets of Langerhans - scattered clusters of cells: Alpha cells (~25%) produce Glucagon (raises blood glucose), Beta cells (~65%) produce Insulin (lowers blood glucose - most numerous), Delta cells (~5%) produce Somatostatin (inhibits insulin and glucagon), and PP cells produce Pancreatic Polypeptide (inhibits pancreatic secretion). Discovered by Paul Langerhans (1869). Insulin was isolated by Banting and Best (1921).

Graves' Disease is the most common cause of hyperthyroidism (~70-80% of cases). It is an autoimmune disease where Thyroid Stimulating Immunoglobulins (TSI) - also called TRAb (TSH Receptor Antibodies) - bind to and activate the TSH receptor causing continuous stimulation of thyroid and excess T3/T4. Features of hyperthyroidism PLUS unique Graves' features: Exophthalmos (Proptosis) = bulging eyes due to inflammation and GAG deposition in orbital tissues (NOT just thyroid hormone), Pretibial myxoedema = firm, non-pitting swelling over shins (despite the name - it's hyperthyroidism), Thyroid acropachy = periosteal new bone formation + finger clubbing, and Diffuse goitre with bruit. Diagnosis: TFTs (low TSH, high T3/T4), TRAb positive. Treatment: Antithyroid drugs, radioiodine, surgery.

Growth Hormone (GH / Somatotropin) is secreted by somatotroph cells of the anterior pituitary. It is the most abundant anterior pituitary hormone. Actions: Stimulates bone growth (lengthening at epiphyseal plates) and muscle growth, Promotes protein synthesis and fat breakdown (lipolysis), Raises blood glucose (anti-insulin effect - diabetogenic), Acts via IGF-1 (Insulin-like Growth Factor 1) from the liver (indirect effects). GH excess (before puberty): Gigantism (excessive height). GH excess (after puberty): Acromegaly (enlargement of hands, feet, jaw, tongue - due to closed epiphyses). GH deficiency (children): Dwarfism (short stature). Regulated by: GHRH (stimulates) and Somatostatin (inhibits) from hypothalamus.

Insulin is a peptide hormone produced by beta cells of the islets of Langerhans in the pancreas. Released when blood glucose rises (after meals). Actions - all ANABOLIC (building) and hypoglycaemic (glucose-lowering): Increases glucose uptake by cells (especially muscle and fat) via GLUT-4 transporters, Promotes glycogen synthesis (glycogenesis) in liver and muscle, Promotes fat synthesis (lipogenesis) and inhibits fat breakdown, Promotes protein synthesis, and Inhibits gluconeogenesis and glycogenolysis in liver. Discovered by Frederick Banting and Charles Best (1921, University of Toronto) - Nobel Prize 1923. Insulin deficiency/resistance causes Diabetes Mellitus.

Hashimoto's Thyroiditis (Chronic Autoimmune Thyroiditis) is the most common cause of hypothyroidism in developed (iodine-sufficient) countries. Named after Japanese physician Hakaru Hashimoto (1912). It is an autoimmune condition where the immune system attacks the thyroid: Anti-TPO antibodies (Thyroid Peroxidase - most common and sensitive marker) and Anti-Tg antibodies (Anti-Thyroglobulin). Lymphocytic infiltration causes destruction of thyroid follicles causing reduced T3/T4 production. Features: Hypothyroidism (fatigue, weight gain, cold intolerance, dry skin, constipation) and goitre (early - may have small/no goitre later as gland is destroyed). More common in females (7:1 female:male ratio). Associated with other autoimmune diseases (Type 1 DM, coeliac disease, Addison's, rheumatoid arthritis, pernicious anaemia). Treatment: Levothyroxine (T4 replacement).