Endocrine System — Comprehensive Checklist

Hierarchical breakdown: Function → Axis/Pathway → Gland/Tissue → Cells/Hormones/Structures

Organisational principle: Functional first, anatomical second. The endocrine system is organised by regulatory axes and biochemical pathways, not by anatomical location. Glands spanning multiple body regions (hypothalamus → pituitary → adrenal) are grouped by their shared functional circuit. Anatomical detail is nested within each functional grouping.

1. Hypothalamus (Neuroendocrine)

Function: The interface between the nervous system and endocrine system. Converts neural signals into hormonal signals. Receives input from limbic system (emotions), retina (light/dark), visceral afferents (gut, cardiovascular), temperature sensors, and circulating hormones (feedback). Outputs releasing/inhibiting hormones via the portal system and neurohypophyseal hormones via axonal transport.

1.1 Releasing / Inhibiting Hormones (Hypophysiotropic)

1.2 Neurohypophyseal Hormones (synthesised in hypothalamus, stored in posterior pituitary)

1.3 Key Hypothalamic Nuclei (Endocrine-Relevant)

1.4 Hypothalamic-Hypophyseal Portal System

Function: Dedicated vascular link ensuring hypothalamic releasing hormones reach the anterior pituitary at high concentration without systemic dilution. This portal system (two capillary beds connected by veins) is the reason tiny amounts of hypothalamic hormones can control pituitary output.

2. Pituitary Gland (Hypophysis)

Function: The “master gland” — translates hypothalamic commands into circulating hormones that regulate peripheral glands (thyroid, adrenals, gonads) and directly control growth, lactation, and water balance. Anterior pituitary synthesises its own hormones; posterior pituitary stores and releases hypothalamic hormones.

2.1 Anterior Pituitary (Adenohypophysis)

Function: Synthesises 6 major hormones from 5 cell types. Derived from oral ectoderm (Rathke’s pouch). Regulated by hypothalamic releasing/inhibiting hormones via the portal system and by peripheral hormone feedback loops.

Pars Distalis (Main Body)
Cell Type % Hormone Function Regulated by
Somatotrophs 40-50% GH (Growth Hormone / Somatotropin) Linear growth (epiphyseal plates); hepatic IGF-1 secretion; protein synthesis; lipolysis; insulin antagonism GHRH (+), Somatostatin (−), IGF-1 (−), Ghrelin (+)
Lactotrophs 10-25% Prolactin (PRL) Mammary gland development and milk production; immune modulation; reproductive function (inhibits GnRH at high levels → amenorrhoea) Dopamine (−, tonic), TRH (+), Oestrogen (+)
Corticotrophs 15-20% ACTH (from POMC cleavage) Stimulates adrenal cortex (mainly zona fasciculata → cortisol); trophic maintenance of adrenal cortex CRH (+), Vasopressin (+), Cortisol (−)
β-LPH, β-Endorphin, MSH POMC co-products: β-endorphin (analgesia, stress), MSH (melanocyte stimulation → skin pigmentation; elevated in Addison’s disease/ACTH excess)
Thyrotrophs 3-5% TSH (Thyroid-Stimulating Hormone) Stimulates thyroid follicular cells → T4/T3 synthesis and release; trophic maintenance of thyroid TRH (+), T3/T4 (−), Somatostatin (−)
Gonadotrophs 10-15% FSH (Follicle-Stimulating Hormone) Female: follicular development, granulosa cell aromatase (→ oestradiol). Male: Sertoli cell function, spermatogenesis support GnRH (+, pulsatile), Inhibin (−), Oestradiol (−, except mid-cycle surge: +)
LH (Luteinising Hormone) Female: ovulation trigger, corpus luteum formation, theca cell androgens. Male: Leydig cell testosterone production GnRH (+, pulsatile), Testosterone (−), Progesterone (−)
Folliculostellate scattered Paracrine (IL-6, follistatin, VEGF) Local regulation of pituitary cell communication and angiogenesis; not classical endocrine cells
Pars Tuberalis
Pars Intermedia (rudimentary in humans)

2.2 Posterior Pituitary (Neurohypophysis)

Function: Neural extension of the hypothalamus — does not synthesise hormones. Stores and releases ADH and oxytocin produced in hypothalamic nuclei. Derived from neuroectoderm (diencephalon).

Pars Nervosa
Infundibulum (Pituitary Stalk)

3. Pineal Gland (Hypothalamic-Commanded Circadian Effector)

Anatomical note: The pineal is an epithalamic structure (posterior to third ventricle, attached to habenular commissure), not part of the hypothalamus. It is grouped here because it is functionally downstream of the SCN → superior cervical ganglion → pineal sympathetic innervation pathway. It has no portal system connection and no classical axis membership. Its output (melatonin) feeds back to the SCN.

Function: Transduces photoperiod (day length) information into a hormonal signal (melatonin). Darkness stimulates melatonin production; light inhibits it. Melatonin synchronises circadian rhythms, promotes sleep, and in many species regulates seasonal reproduction. In humans, it modulates sleep-wake cycles, core body temperature rhythm, and has immunomodulatory and antioxidant roles.

3.1 Structure

3.2 Hormones

4. Thyroid Gland

Function: Produces thyroid hormones (T3/T4) that set the basal metabolic rate of virtually every cell in the body. Essential for normal growth, brain development (cretinism if deficient in infancy), thermogenesis, cardiac output, and gut motility. Also produces calcitonin (calcium regulation). The thyroid is the only endocrine gland that stores large amounts of pre-formed hormone (as colloid — weeks-months supply).

4.1 Gross Anatomy

4.2 Follicular Cells (Thyrocytes)

Function: Synthesise, store, and release thyroid hormones. Unique among endocrine cells in using iodine and storing hormone extracellularly as colloid.

4.3 Parafollicular Cells (C-cells)

Function: Produce calcitonin in response to hypercalcaemia. Calcitonin lowers serum calcium by inhibiting osteoclast bone resorption. Physiological role in adults is minor (thyroidectomy patients maintain normal calcium). Clinically important as tumour marker (medullary thyroid carcinoma).

4.4 Thyroid Vasculature

4.5 Associated Structures

5. Parathyroid Glands

Function: Maintain serum calcium within a narrow range (2.2-2.6 mmol/L) essential for neuromuscular function, cardiac conduction, coagulation, and bone mineralisation. PTH is the minute-to-minute regulator of calcium — acts on bone (resorption), kidney (reabsorption, phosphate excretion, vitamin D activation), and indirectly on gut (via vitamin D → calcium absorption). Hypoparathyroidism → tetany, seizures; hyperparathyroidism → “bones, stones, abdominal groans, psychic moans.”

5.1 Anatomy

5.2 Chief Cells (Principal Cells)

Function: The calcium sensor and PTH factory. Calcium-sensing receptor (CaSR) on the cell membrane detects extracellular calcium concentration. Low calcium → increased PTH secretion within seconds. High calcium → suppressed PTH.

5.3 Oxyphil Cells

6. Adrenals & Gonads

Organisational note: Adrenal cortex and gonads share the cholesterol → pregnenolone steroidogenic pathway (CYP11A1/StAR). They are grouped together because the same enzyme cascade branches into mineralocorticoids, glucocorticoids, and sex steroids depending on which tissue-specific enzymes are expressed. Cross-talk is constant: cortisol suppresses GnRH (stress amenorrhoea), adrenal DHEA-S is the largest circulating steroid pool and undergoes peripheral conversion to sex hormones. The adrenal medulla and paraganglia are included as the catecholamine-producing chromaffin tissue system. Gonadal reproductive anatomy (follicular apparatus, seminiferous tubules) is detailed in the Sexual Reproduction checklist; this section covers hormonal outputs and regulatory axes.

6.1 Shared Steroidogenesis Pathway

Function: The universal biochemical backbone from which all steroid hormones are built. Every steroid-producing tissue starts with the same substrate (cholesterol) and first enzyme (CYP11A1). What determines the final hormone product is which downstream enzymes each tissue expresses. This shared pathway explains why enzyme deficiencies (e.g., 21-hydroxylase deficiency = congenital adrenal hyperplasia) affect multiple hormone classes simultaneously.

6.1.1 Rate-Limiting Entry Step

6.1.2 Branch Enzymes (Determine Final Product)

6.1.3 Pathway Branches

6.2 Adrenal Cortex (Mesodermal Origin)

Function: Three concentric zones producing three classes of steroid hormones. Mnemonic: “GFR — salt, sugar, sex” (Glomerulosa = mineralocorticoids, Fasciculata = glucocorticoids, Reticularis = androgens). The cortex is essential for life — complete cortical failure (Addisonian crisis) is fatal without replacement.

6.2.1 Zona Glomerulosa (Outer)

Function: Mineralocorticoid production — regulates sodium/potassium balance and blood volume. Primarily controlled by the Renin-Angiotensin-Aldosterone System (RAAS) and serum potassium, NOT by ACTH (though ACTH has permissive role).

6.2.2 Zona Fasciculata (Middle — Widest)

Function: Glucocorticoid production — the body’s primary stress response hormone and metabolic regulator. Directly controlled by ACTH from the anterior pituitary (HPA axis). Zone accounts for ~80% of cortical mass.

6.2.3 Zona Reticularis (Inner)

Function: Adrenal androgen production. Produces weak androgens (DHEA, DHEA-S, androstenedione) that serve as precursors for peripheral conversion to testosterone and oestradiol. DHEA-S is the most abundant circulating steroid. Primary source of androgens in women and pre-pubertal children. Adrenarche (onset ~6-8 years) precedes gonadarche by ~2 years.

6.3 Gonadal Endocrine Function

Cross-reference: Gonadal reproductive anatomy (follicular apparatus, seminiferous tubule structure, corpus luteum lifecycle) is in the Sexual Reproduction checklist. This section covers hormonal outputs and their regulatory axes only.

Function: Sex steroid production for reproductive function, secondary sex characteristics, and anabolic effects. Controlled by HPG axis (GnRH → FSH/LH → gonadal steroids → feedback). The gonads are steroidogenic tissues that express the same core enzymes as adrenal cortex (§6.1) but with different enzyme profiles yielding different end products (predominantly sex steroids rather than corticosteroids).

6.3.1 Testicular Endocrine Cells

6.3.2 Ovarian Endocrine Cells

6.3.3 Placental Endocrine Function (Temporary)

Function: Temporary endocrine organ of pregnancy. Takes over steroid production from corpus luteum (luteal-placental shift ~week 8-12). Unique: lacks CYP17 → cannot produce androgens de novo; depends on fetal and maternal adrenal DHEA-S as androgen substrates for placental aromatase to produce oestrogens (feto-placental unit).

6.4 Adrenal Medulla & Paraganglia (Neural Crest-Derived Chromaffin Tissue)

Function: Rapid stress response system (“fight-or-flight”). The adrenal medulla is a modified sympathetic ganglion — preganglionic sympathetic neurons synapse directly on chromaffin cells, which release catecholamines into the blood (endocrine, not neural). This is faster than hypothalamic-pituitary-adrenal cortisol (seconds vs minutes) and provides the immediate physiological response to acute stress: increased heart rate, blood pressure, bronchodilation, glucose mobilisation.

6.4.1 Adrenal Medulla (Modified Sympathetic Ganglion)

6.4.2 Extra-Adrenal Paraganglia

Function: Scattered clusters of chromaffin-like cells outside the adrenal medulla. Sympathetic paraganglia produce catecholamines; parasympathetic paraganglia function primarily as chemoreceptors (O₂, CO₂, pH sensing). Clinically relevant as sites of paraganglioma tumours.

Sympathetic Paraganglia (Catecholamine-Producing)
Parasympathetic Paraganglia (Chemoreceptors)

6.5 Adrenal Vasculature

Function: The adrenal’s unique vascular arrangement (cortical-medullary portal system) ensures that medullary chromaffin cells are bathed in cortisol-rich blood from the cortex. This is functionally critical: cortisol induces PNMT, the enzyme that converts noradrenaline to adrenaline. Without this vascular arrangement, the adrenal medulla would produce only noradrenaline.

7. Pancreatic Islets (Glucose Homeostasis)

Function: Moment-to-moment regulation of blood glucose within a narrow range (4-6 mmol/L fasting). The islet cells collectively sense blood glucose and release opposing hormones: insulin (lowers glucose) and glucagon (raises glucose). This dual control prevents both hypoglycaemia (seizures, coma, death) and hyperglycaemia (osmotic damage, ketoacidosis, vascular disease). The islets also regulate appetite, gut motility, and exocrine pancreatic function via paracrine signalling.

7.1 Islet Cell Types

7.1.1 Beta Cells (β-cells) — 60-80%

Function: Glucose sensor and insulin factory. Detect blood glucose via GLUT2 transporter and glucokinase (the “glucose sensor”). Glucose metabolism → ATP → closes KATP channels → depolarisation → Ca²+ influx → insulin granule exocytosis. First-phase (preformed granules, 5-10 min) and second-phase (newly synthesised, sustained) insulin release.

7.1.2 Alpha Cells (α-cells) — 15-20%

Function: Counter-regulatory to β-cells. Activated by hypoglycaemia, amino acids, sympathetic stimulation. Suppressed by insulin (paracrine), somatostatin (paracrine), and hyperglycaemia. Glucagon acts primarily on the liver.

7.1.3 Delta Cells (δ-cells) — 5-10%

Function: Local brake on both insulin and glucagon. Somatostatin acts as a paracrine “off switch” preventing excessive hormone swings. Also inhibits GI secretion and motility (same somatostatin as hypothalamic and GI forms).

7.1.4 PP Cells (F-cells) — 1-2%

Function: Post-prandial regulation. Released after meals (especially protein-rich). Reduces appetite and inhibits exocrine pancreatic secretion. Concentrated in the head of the pancreas (embryological ventral pancreas origin).

7.1.5 Epsilon Cells (ε-cells) — <1%

Function: Appetite stimulation. Produce ghrelin within the islet, though the stomach is the primary ghrelin source. Role in islet development may be more significant than endocrine output in adults.

7.2 Islet Architecture

Function: The spatial arrangement is not random — it optimises paracrine signalling. β-cells in the core are exposed to incoming arterial blood first, then blood flows outward past α and δ cells. This means insulin reaches α-cells at high concentration (paracrine suppression of glucagon). Disruption of islet architecture (as in type 2 diabetes, islet amyloidosis) impairs glucose regulation beyond what cell loss alone would explain.

7.3 Pancreas Anatomy (Endocrine Context)

8. Diffuse Neuroendocrine System (DNES)

Organisational note: These are hormone-producing cells embedded within non-endocrine organs. They synthesise and secrete into the bloodstream or paracrine space. The gut is arguably the largest endocrine organ by cell count. All entries below are producers, not receivers.

8.1 Gastrointestinal Enteroendocrine Cells

Function: The GI tract contains more endocrine cells than any other organ. They sense luminal contents (nutrients, pH, microbiota metabolites) and release hormones that coordinate digestion, absorption, metabolism, appetite, and gut motility. Many GI hormones also act as incretins (augment insulin release in response to oral glucose — the “incretin effect” explains why oral glucose provokes more insulin than IV glucose).

8.1.1 Stomach

8.1.2 Small Intestine

8.1.3 Large Intestine

8.2 Pulmonary Neuroendocrine Cells

Function: Oxygen sensing and airway regulation. Rare cells scattered in airway epithelium. Produce local mediators affecting bronchial tone, mucosal blood flow, and immune responses. Clinically relevant as the cell of origin for small cell lung carcinoma.

8.3 Cardiac Endocrine Cells

Function: Volume and pressure regulation. Cardiac myocytes sense atrial stretch (volume overload) and release natriuretic peptides that promote sodium and water excretion, vasodilation, and oppose the RAAS system. Net effect: reduce blood volume and blood pressure. BNP is a cornerstone biomarker in heart failure diagnosis.

8.4 Renal Endocrine Function

Function: The kidney is both an endocrine target (aldosterone, ADH, PTH, ANP all act on it) and an endocrine organ. It produces renin (initiates RAAS for blood pressure control), EPO (drives red cell production), and active vitamin D (calcitriol, for calcium absorption). These three products regulate blood pressure, oxygen-carrying capacity, and mineral balance respectively.

8.5 Hepatic Endocrine Function

Function: The liver is the body’s metabolic command centre and a major endocrine organ. It produces growth mediators (IGF-1, IGF-2), iron regulators (hepcidin), blood pressure substrates (angiotensinogen), platelet regulators (TPO), and metabolic hormones (FGF21). Most of these are “endocrine at a distance” — hepatocytes release them into systemic circulation to act on distant organs.

9. Endocrine Tissues (Non-Glandular Hormone Producers)

Organisational note: These are mesenchymal, connective, and epithelial tissues recognised (largely in the last 20 years) as having significant endocrine function. They are distinct from the DNES (§8) because they are not neuroendocrine cells — they are structural/metabolic tissues that also secrete hormones.

9.1 Adipose Tissue

Function: Far more than energy storage. Adipose tissue is now recognised as the body’s largest endocrine organ by mass. Adipokines regulate appetite, insulin sensitivity, inflammation, vascular function, and reproduction. Excess adipose tissue (obesity) creates a pro-inflammatory, insulin-resistant endocrine environment — many metabolic diseases are fundamentally diseases of adipose endocrine dysfunction.

9.1.1 White Adipose Tissue (WAT) — Adipokines

9.1.2 Brown Adipose Tissue (BAT) — Batokines

Function: Thermogenesis — BAT burns fatty acids to generate heat (non-shivering thermogenesis) via UCP1 uncoupling of the mitochondrial proton gradient. Active in infants (thermoregulation), rediscovered in adults (2009, PET-CT studies). BAT activation improves glucose and lipid metabolism. BAT secretes its own endocrine products (batokines).

9.1.3 Beige/Brite Adipocytes

Function: Inducible thermogenic cells within WAT depots. “Browning” of WAT — white adipocytes can be converted to beige cells that express UCP1 and generate heat. This is a therapeutic target for obesity (increase energy expenditure). Induced by cold, exercise (irisin), catecholamines (β3), thyroid hormones, and certain drugs.

9.2 Bone — Osteokines

Function: Bone is a metabolically active endocrine organ, not an inert scaffold. Osteoblasts and osteocytes secrete hormones that regulate phosphate balance (FGF23), glucose metabolism (osteocalcin), and bone remodelling (sclerostin, RANKL/OPG). The skeleton communicates bidirectionally with pancreas, kidney, muscle, and brain.

9.3 Skeletal Muscle — Myokines

Function: Exercising muscle is an endocrine organ. Myokines are cytokines and peptides released by contracting muscle fibres that mediate systemic effects of exercise: anti-inflammatory action, fat browning, insulin sensitivity, neuroprotection, and bone-muscle cross-talk. This explains many “exercise-as-medicine” benefits at a molecular level.

9.4 Skin

Function: The skin is the body’s primary vitamin D synthesis site. UVB radiation initiates the first step of vitamin D activation. The skin also produces antimicrobial peptides that function as local endocrine/paracrine effectors in innate immunity.

9.5 Thymus (Endocrine Function)

Function: Beyond its lymphoid role (T-cell maturation, see Lymphatic System §1.1), the thymus produces peptide hormones that promote T-cell differentiation, immune competence, and tissue repair. Thymic endocrine function declines with age (involution) but persists at reduced levels throughout life.

9.5.1 Thymic Hormones / Peptides

9.5.2 Thymic Involution

10. Endocrine Cross-Talk (Inter-System Regulatory Interfaces)

Organisational note: This section maps the bidirectional signalling between the endocrine system and other major systems (immune, nervous). These are not discrete glands or tissues but regulatory interactions that modulate multiple systems simultaneously.

10.1 Immune-Endocrine Interface

Function: The immune and endocrine systems communicate bidirectionally. Cytokines from immune cells activate endocrine axes (especially HPA → cortisol), and hormones modulate immune cell function, proliferation, and cytokine production. This interface explains why stress suppresses immunity, why autoimmune diseases have sex differences, and why chronic inflammation causes metabolic disease.

10.1.1 Cytokines with Endocrine Effects (Immune → Endocrine)

10.1.2 Hormonal Immunomodulation (Endocrine → Immune)

10.2 Neuroendocrine Signalling Molecules

Function: Molecules produced by neurons that have systemic hormonal effects, blurring the line between nervous and endocrine systems. Many serve dual roles: neurotransmitter within the CNS and hormone/paracrine factor in the periphery. They regulate pain, appetite, reproduction, vascular tone, and stress responses.

10.2.1 Neuropeptides with Endocrine Relevance

10.2.2 Reproductive Neuroendocrine Regulators

10.2.3 Endogenous Opioids & Cannabinoids