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What is the endocrine system?
Answer: A network of glands and cells that secrete hormones directly into the bloodstream to regulate distant target tissues.
Contrasts with the exocrine system, which secretes products through ducts (e.g., sweat, digestive enzymes).
Define 'hormone'.
Answer: A chemical messenger secreted by an endocrine gland/cell into the blood that acts on target cells bearing specific receptors.
What are the three chemical classes of hormones?
Answer: Peptide/protein hormones, steroid hormones, and amine hormones.
Chemical class determines solubility, receptor location, and speed/duration of action.
What is a 'tropic' hormone?
Answer: A hormone that stimulates another endocrine gland to secrete its own hormone.
Examples: ACTH (adrenal cortex), TSH (thyroid), FSH/LH (gonads) — all anterior pituitary hormones except GH and prolactin.
How do peptide hormones act on target cells?
Answer: They are water-soluble, cannot cross the cell membrane, so they bind surface receptors and trigger a second-messenger cascade (e.g., cAMP); action is fast but short-lived.
Examples: insulin, glucagon, ADH, growth hormone, all anterior pituitary tropic hormones.
How do steroid hormones act on target cells?
Answer: Lipid-soluble (derived from cholesterol), diffuse through the cell membrane and bind intracellular/nuclear receptors, altering gene transcription; slow onset but long-lasting effect.
Examples: cortisol, aldosterone, estrogen, progesterone, testosterone.
How do amine hormones act, and why are they an exception?
Answer: Catecholamines (epinephrine, norepinephrine) are water-soluble and act like peptides via surface receptors; thyroid hormones (T3/T4) are lipid-soluble and act like steroids via nuclear receptors.
Amines are derived from single amino acids (tyrosine) but split into two functional mechanisms.
What is the role of the hypothalamus in endocrine control?
Answer: It is the master regulator: secretes releasing/inhibiting hormones that control the anterior pituitary, and produces ADH and oxytocin, which are stored/released by the posterior pituitary.
How is the hypothalamus connected to the anterior vs posterior pituitary?
Answer: Anterior pituitary: connected via the hypophyseal portal blood system. Posterior pituitary: connected via direct neural axons (hypothalamic-hypophyseal tract).
This explains why anterior pituitary hormones require releasing hormones, while posterior pituitary hormones (ADH, oxytocin) are made in the hypothalamus and simply transported down axons for release.
Name the six anterior pituitary hormones.
Answer: Growth hormone (GH), Thyroid-stimulating hormone (TSH), Adrenocorticotropic hormone (ACTH), Follicle-stimulating hormone (FSH), Luteinizing hormone (LH), Prolactin.
Mnemonic: 'FLAT PiG' (FSH, LH, ACTH, TSH, Prolactin, GH).
Name the two posterior pituitary hormones and where they are made.
Answer: Antidiuretic hormone (ADH/vasopressin) and oxytocin; both are synthesized in the hypothalamus and released from the posterior pituitary.
Match each hypothalamic releasing/inhibiting hormone to its anterior pituitary target.
Answer: TRH → TSH; CRH → ACTH; GnRH → FSH/LH; GHRH → GH (somatostatin inhibits GH and TSH); Dopamine (PIH) → inhibits prolactin.
Prolactin is unique in being under tonic inhibitory control (dopamine) rather than primarily stimulatory control.
What does ADH (vasopressin) do?
Answer: Increases water reabsorption in the renal collecting ducts, concentrating urine and regulating blood osmolarity/volume.
Released in response to high plasma osmolarity or low blood volume.
What does oxytocin do?
Answer: Stimulates uterine smooth muscle contraction during labor and the milk-ejection (let-down) reflex during breastfeeding.
Classic example of a positive feedback loop (contractions stimulate more oxytocin release).
What is negative feedback in endocrine regulation?
Answer: Rising hormone levels from the target gland suppress further secretion at the hypothalamus and/or pituitary, restoring homeostasis.
Example: rising cortisol inhibits CRH and ACTH release. This is the predominant endocrine regulatory mechanism.
Give an example of positive feedback in the endocrine system.
Answer: Rising estrogen before ovulation triggers a surge in LH (and FSH); also oxytocin release during labor contractions.
Positive feedback amplifies a response rather than dampening it, and is rarer than negative feedback.
Trace the HPA axis cascade.
Answer: CRH (hypothalamus) → ACTH (anterior pituitary) → cortisol (adrenal cortex) → cortisol negatively feeds back on hypothalamus and pituitary.
HPA = hypothalamic-pituitary-adrenal axis; central to the stress response.
Trace the HPT axis cascade.
Answer: TRH (hypothalamus) → TSH (anterior pituitary) → T3/T4 (thyroid) → T3/T4 negatively feed back on hypothalamus and pituitary.
HPT = hypothalamic-pituitary-thyroid axis.
Trace the HPG axis cascade.
Answer: GnRH (hypothalamus) → FSH/LH (anterior pituitary) → gonadal sex steroids (estrogen/progesterone or testosterone) → mostly negative feedback, with a positive feedback surge at ovulation.
HPG = hypothalamic-pituitary-gonadal axis.
What hormones does the thyroid gland produce, and from which cells?
Answer: T3 (triiodothyronine) and T4 (thyroxine) from follicular cells; calcitonin from parafollicular (C) cells.
What are the main functions of T3/T4?
Answer: Increase basal metabolic rate, heat production, growth, and support normal CNS development.
T4 is converted to the more active T3 in peripheral tissues.
What does calcitonin do?
Answer: Lowers blood calcium by inhibiting osteoclast activity and promoting calcium deposition in bone.
Released when blood calcium is high; its physiological role in humans is relatively minor compared to PTH.
What does parathyroid hormone (PTH) do?
Answer: Raises blood calcium by stimulating osteoclast-mediated bone resorption, increasing renal calcium reabsorption and phosphate excretion, and activating vitamin D to boost intestinal calcium absorption.
Secreted by the parathyroid glands in response to low serum calcium.
Summarize calcium regulation by PTH vs calcitonin.
Answer: PTH raises blood Ca2+ (released when Ca2+ is low); calcitonin lowers blood Ca2+ (released when Ca2+ is high) — they act as opposing regulators.
What are the three zones of the adrenal cortex and what does each secrete?
Answer: Zona glomerulosa → mineralocorticoids (aldosterone); zona fasciculata → glucocorticoids (cortisol); zona reticularis → androgens (DHEA).
Mnemonic: 'GFR' zones correspond to 'salt, sugar, sex' (aldosterone, cortisol, androgens).
What does aldosterone do?
Answer: Part of the renin-angiotensin-aldosterone system (RAAS); increases Na+ reabsorption and K+ excretion in the kidney, raising blood pressure and volume.
What are the main functions of cortisol?
Answer: Increases blood glucose via gluconeogenesis, suppresses the immune system, has anti-inflammatory effects, and mediates the stress response; follows a circadian rhythm (highest in the morning).
Chronic excess cortisol produces Cushing's features.
What is the typical pattern of cortisol secretion over 24 hours?
Answer: Cortisol peaks in the early morning (around waking) and declines to its lowest level around midnight, following a circadian rhythm driven by the HPA axis.
This diurnal rhythm is why morning cortisol is used clinically to screen for adrenal disorders.
What hormones does the adrenal medulla release, and under what control?
Answer: Epinephrine and norepinephrine (catecholamines), released upon direct sympathetic nervous system stimulation (fight-or-flight response).
Unlike the cortex, the medulla is essentially a modified sympathetic ganglion and is not under hypothalamic releasing-hormone/tropic-hormone control.
What hormones do the pancreatic islet cells secrete?
Answer: Alpha cells secrete glucagon; beta cells secrete insulin; delta cells secrete somatostatin.
What does insulin do?
Answer: Lowers blood glucose by promoting glucose uptake into muscle/fat cells, glycogenesis, and lipogenesis; secreted when blood glucose is high.
Insulin deficiency or resistance underlies diabetes mellitus.
What does glucagon do?
Answer: Raises blood glucose by promoting glycogenolysis and gluconeogenesis in the liver; secreted when blood glucose is low.
Insulin and glucagon act as an opposing pair maintaining glucose homeostasis.
What hormones do the gonads secrete?
Answer: Testes (Leydig cells): testosterone. Ovaries (follicle/corpus luteum): estrogen and progesterone.
Both are steroid hormones under HPG axis control.
What hormone does the pineal gland secrete, and what does it do?
Answer: Melatonin; regulates the circadian sleep-wake cycle, with secretion increasing in darkness.
What are the two pathways of the physiological stress response?
Answer: Fast pathway: sympathetic-adrenal-medullary (SAM) axis releasing epinephrine/norepinephrine within seconds. Slow pathway: HPA axis releasing cortisol over minutes to hours for sustained response.
Together they mediate the 'fight-or-flight' (acute) and prolonged stress adaptation responses.
Cushing syndrome: cause and features?
Answer: Caused by excess cortisol (e.g., pituitary adenoma secreting ACTH, adrenal tumor, or exogenous steroids). Features: moon face, buffalo hump, central obesity, hyperglycemia, hypertension, muscle wasting, purple striae.
Cushing 'disease' specifically refers to a pituitary ACTH-secreting adenoma causing the syndrome.
Addison's disease: cause and features?
Answer: Primary adrenal insufficiency — low cortisol and aldosterone (often autoimmune destruction of the adrenal cortex). Features: hypotension, hyperpigmentation, weight loss, hyponatremia, hyperkalemia, fatigue.
Hyperpigmentation occurs because low cortisol removes negative feedback, causing elevated ACTH (and its precursor POMC, which also stimulates melanocytes).
Graves' disease: cause and features?
Answer: Autoimmune hyperthyroidism caused by antibodies that stimulate the TSH receptor. Features: weight loss, tachycardia, heat intolerance, exophthalmos (bulging eyes), goiter.
Contrast with Hashimoto's thyroiditis, an autoimmune cause of hypothyroidism (weight gain, cold intolerance, bradycardia, fatigue, goiter, low T3/T4, high TSH).
Diabetes insipidus: cause and features?
Answer: Caused by deficient ADH secretion (central) or renal insensitivity to ADH (nephrogenic). Features: excessive dilute urine output (polyuria), excessive thirst (polydipsia), risk of dehydration and hypernatremia.
Distinct from diabetes mellitus, which involves glucose dysregulation rather than water balance; distinguished clinically by normal blood glucose but very dilute urine.
Acromegaly: cause and features?
Answer: Caused by excess growth hormone in adults (after growth plates have fused), usually from a pituitary adenoma. Features: enlarged hands, feet, and jaw, organomegaly, coarsened facial features.
If GH excess occurs before growth plate fusion (in children), it causes gigantism (excessive linear growth) instead.
SIADH: cause and features?
Answer: Syndrome of Inappropriate ADH secretion — excess ADH causes water retention and dilutional hyponatremia despite normal/expanded blood volume.
Contrasts with diabetes insipidus, which is due to ADH deficiency/resistance and causes the opposite problem (excess water loss).
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