Endocrine system Study Guide

📖 Core Concepts Endocrine system – Hormone‑based messenger network; hormones travel in the blood to distant targets. Neuroendocrine integration – Hypothalamus (brain) controls the pituitary; together they link nervous signals to peripheral endocrine glands. Hormone classes – Steroid (lipid‑soluble, intracellular receptors, gene transcription); Peptide/Protein (water‑soluble, cell‑surface receptors, second‑messenger cascades); Amine (derived from single amino acids, e.g., catecholamines). Feedback loops – Negative feedback maintains homeostasis (e.g., high T₃/T₄ ↓ TRH/TSH). Endocrine axes – Hypothalamic‑pituitary‑target organ circuits (Thyroid, Gonadal, Adrenal) that operate via releasing‑hormone → tropic‑hormone → effector‑hormone cascades. --- 📌 Must Remember Thyroid hormones: T₄ (thyroxine) → peripheral conversion to T₃ (triiodothyronine); increase basal metabolic rate. Parathyroid hormone (PTH): ↑ blood Ca²⁺ (bone resorption, kidney reabsorption, gut absorption via active vitamin D). Pituitary tropic hormones: TSH, ACTH, GH, LH, FSH (anterior); ADH & oxytocin (posterior). Adrenal cortex hormones: Cortisol (glucocorticoid), Aldosterone (mineralocorticoid), Androgens. Adrenal medulla hormones: Epinephrine, Norepinephrine – released during sympathetic activation. Pancreatic islet hormones: Insulin (↓ glucose), Glucagon (↑ glucose), Somatostatin (inhibits both). RAAS cascade: Renin → Ang I → ACE → Ang II → Aldosterone (Na⁺ retention, K⁺ excretion). Key feedback: High cortisol ↓ CRH & ACTH; high T₃/T₄ ↓ TRH & TSH; high Ca²⁺ ↓ PTH. --- 🔄 Key Processes Hypothalamic–Pituitary–Thyroid Axis Hypothalamus releases TRH → Anterior pituitary releases TSH → Thyroid secretes T₃/T₄ → Negative feedback on TRH/TSH. Hypothalamic–Pituitary–Adrenal Axis CRH → ACTH → Cortisol → Inhibits CRH & ACTH. Renin–Angiotensin–Aldosterone System (RAAS) Kidney → Renin → cleaves angiotensinogen → Ang I → ACE converts to Ang II → stimulates aldosterone release → Na⁺ reabsorption, K⁺ excretion. Insulin‑Glucagon Regulation of Glucose High blood glucose → β‑cells release insulin → ↑ cellular uptake, ↓ hepatic output. Low blood glucose → α‑cells release glucagon → hepatic glycogenolysis ↑ glucose. Steroid Hormone Signaling Hormone diffuses across membrane → binds intracellular receptor → hormone‑receptor complex → translocates to nucleus → binds DNA → alters transcription. Peptide Hormone Signaling (cAMP pathway) Hormone binds G‑protein‑coupled receptor → activates adenylate cyclase → ↑ cAMP → activates PKA → phosphorylates target enzymes. --- 🔍 Key Comparisons Endocrine vs. Exocrine glands Endocrine: duct‑less, highly vascular, release hormones into blood. Exocrine: have ducts, secrete onto surfaces (e.g., sweat, saliva). Paracrine vs. Autocrine signaling Paracrine: hormone acts on nearby cells (e.g., somatostatin on neighboring pancreatic cells). Autocrine: hormone acts on the same cell that secreted it. Steroid vs. Peptide hormone action Steroid: lipid‑soluble, intracellular receptors, long‑term genomic effects. Peptide: water‑soluble, cell‑surface receptors, fast second‑messenger cascades. --- ⚠️ Common Misunderstandings “All hormones act quickly.” Steroid hormones often have delayed (hours‑to‑days) effects via gene transcription, whereas peptide hormones act within seconds‑minutes. “ADH is produced in the pituitary.” ADH is synthesized in hypothalamic magnocellular neurons and stored/released by the posterior pituitary. “Calcitonin raises calcium.” Calcitonin lowers blood Ca²⁺ (opposes PTH). “Renin is a hormone.” Renin is an enzyme released by the kidney that initiates the RAAS cascade; it is not a circulating hormone. --- 🧠 Mental Models / Intuition “Hormone ladder” – Think of each axis as a three‑step ladder: Releasing hormone (hypothalamus) → Tropic hormone (pituitary) → Effector hormone (target gland). “Lock‑and‑key vs. messenger” – Steroid hormones are keys that unlock nuclear receptors; peptide hormones are messengers that ring the doorbell (cell‑surface receptor) and trigger an internal cascade. “RAAS = pressure‑salt controller” – Low blood pressure → renin → Ang II → vasoconstriction + aldosterone → salt & water retention → pressure restored. --- 🚩 Exceptions & Edge Cases Cortisol’s dual role: While primarily a glucocorticoid, high cortisol also potentiates epinephrine synthesis by up‑regulating phenylethanolamine‑N‑methyltransferase. Pituitary hormone storage: Only ADH and oxytocin are stored in the posterior pituitary; anterior pituitary hormones are synthesized and secreted on demand. Thyroid hormone feedback: Both T₃ and T₄ provide negative feedback, but T₃ is the more biologically active form. --- 📍 When to Use Which Diagnosing hyper‑/hypocalcemia: Measure PTH → high PTH = secondary (e.g., vitamin D deficiency); low PTH = primary (e.g., hypoparathyroidism). Evaluating adrenal disorders: High ACTH + high cortisol → Cushing’s disease (pituitary source). Low ACTH + low cortisol → Addison’s disease (primary adrenal insufficiency). Choosing a therapeutic target for hypertension: If renin activity is elevated → consider renin inhibitors or ACE inhibitors. If aldosterone excess (e.g., primary hyperaldosteronism) → use mineralocorticoid receptor antagonists (spironolactone). --- 👀 Patterns to Recognize “Elevated hormone → suppressed upstream releasing hormone.” Look for negative feedback loops in labs (e.g., high T₃/T₄ with low TSH). “Symptoms of hormone excess/deficiency often mirror feedback status.” Cushing’s (hyper‑cortisol) → muscle wasting, hyperglycemia; Addison’s (hypo‑cortisol) → hypotension, hyperpigmentation. “RAAS activation = low perfusion + sodium loss.” Hypertension with high Ang II suggests overactive RAAS. --- 🗂️ Exam Traps Distractor: “Calcitonin raises blood calcium.” – Wrong; it lowers Ca²⁺. Distractor: “ADH is synthesized in the posterior pituitary.” – Wrong; synthesized in hypothalamic magnocellular neurons. Distractor: “All endocrine disorders are primary.” – Wrong; secondary (pituitary) and tertiary (hypothalamic) diseases exist. Distractor: “Renin is a hormone secreted by the adrenal gland.” – Wrong; renin is released by the kidneys and is an enzyme. Distractor: “Steroid hormones act via cAMP.” – Wrong; they act through intracellular receptors and gene transcription, not second messengers. ---