{"schemaVersion":1,"id":"en-mechanism-endocrine-system","type":"mechanism","pillar":"body","locale":"en","translationGroupId":"mechanism-endocrine-system","slugPath":"body/systems/the-endocrine-system","title":"The endocrine system","shortTitle":"Endocrine system","summary":"The body's second communication network: slow, diffuse, and addressed to nobody in particular. The hormonal surge following a session does not drive muscle growth, contrary to what common accounts claim.","level":"intermediate","estimatedMinutes":9,"synonyms":["endocrine system","hormonal system","hormones","testosterone","cortisol","growth hormone","insulin"],"blocks":[{"id":"b1","type":"heading","level":2,"text":"The second network","anchor":"the-second-network"},{"id":"b2","type":"paragraph","content":[{"kind":"text","value":"The body has two communication systems, and they are built on opposite principles."}]},{"id":"b3","type":"paragraph","content":[{"kind":"text","value":"The "},{"kind":"link","value":"nervous system","href":"/en/body/systems/the-nervous-system","external":false},{"kind":"text","value":" sends a precise message to a precise recipient, in a fraction of a second, along a cable. The endocrine system does the opposite: it pours a molecule into the blood, where it reaches the whole body, and only the cells equipped with the matching receptor respond."}]},{"id":"b4","type":"comparison","title":"Two ways of sending a message","columns":["Criterion","Nerve message","Hormonal message"],"rows":[{"label":"Medium","cells":["An axon, to its target","The blood, everywhere at once"]},{"label":"Speed","cells":["Fraction of a second","Seconds to hours"]},{"label":"Recipient","cells":["One designated target","Any cell carrying the receptor"]},{"label":"Duration of effect","cells":["Brief","Prolonged"]}]},{"id":"b5","type":"definition","term":"Hormone","definition":"A molecule produced by a gland, released into the blood, which changes how cells able to recognise it work. It does not act on the others, even though it reaches them.","alsoKnownAs":["chemical messenger"]},{"id":"b6","type":"paragraph","content":[{"kind":"text","value":"That is the key to the system: a hormone circulates everywhere but acts only where a receptor is waiting for it. The receptor decides the target, not the molecule."}]},{"id":"b7","type":"heading","level":2,"text":"The main ones, and what they do","anchor":"the-main-ones-and-what-they-do"},{"id":"b8","type":"comparison","title":"Some hormones and their role","columns":["Hormone","Produced by","Main role"],"rows":[{"label":"Insulin","cells":["Pancreas","Moves glucose into cells and favours storage"]},{"label":"Glucagon","cells":["Pancreas","Makes the liver release stored glucose"]},{"label":"Cortisol","cells":["Adrenal glands","Mobilises reserves, modulates inflammation, follows a daily rhythm"]},{"label":"Adrenaline","cells":["Adrenal glands","Prepares for immediate effort: heart, vessels, mobilisation"]},{"label":"Thyroid hormones","cells":["Thyroid","Set the general level of metabolism"]},{"label":"Testosterone, oestrogens","cells":["Gonads, adrenals","Act on many tissues, including bone and muscle"]},{"label":"Growth hormone","cells":["Pituitary","Acts on growth and on the use of fuels"]}]},{"id":"b9","type":"paragraph","content":[{"kind":"text","value":"This table is deliberately coarse. Each of these molecules has several effects depending on the tissue, and none reduces to one function — the same trap the "},{"kind":"link","value":"neuron","href":"/en/body/systems/the-neuron","external":false},{"kind":"text","value":" page describes for neurotransmitters."}]},{"id":"b10","type":"paragraph","content":[{"kind":"strong","value":"Insulin"},{"kind":"text","value":" links this system to the Energy pillar: it commands the entry of glucose and its storage, which the "},{"kind":"link","value":"dietary carbohydrate","href":"/en/energy/what-happens-to-dietary-carbohydrate","external":false},{"kind":"text","value":" page describes as a sorting without naming who conducts it."}]},{"id":"b11","type":"heading","level":2,"text":"What the post-session surge does not do","anchor":"what-the-post-session-surge-does-not-do"},{"id":"b12","type":"paragraph","content":[{"kind":"text","value":"Here is the most useful correction on this page, and it contradicts a very widespread account."}]},{"id":"b13","type":"paragraph","content":[{"kind":"text","value":"Resistance training transiently raises several hormones — testosterone, growth hormone — in the minutes that follow. The idea took hold that this surge drives muscle growth, and that maximising the response would produce more of it."}]},{"id":"b14","type":"evidence","level":"probable","statement":"The systemic hormonal changes following a session predict neither the gain in muscle mass nor the gain in strength obtained from resistance training.","detail":"The trial follows forty-nine trained men split between two very different loading protocols, with hormonal measurements and measurements of muscle cross-section. Neither load nor hormonal responses account for the results. The sample is limited to young, trained, healthy men, and does not transfer as it stands.","referenceIds":["morton2016"]},{"id":"b15","type":"paragraph","content":[{"kind":"text","value":"The "},{"kind":"link","value":"how muscle grows","href":"/en/adaptations/how-muscle-grows","external":false},{"kind":"text","value":" page attributes growth to repeated mechanical tension and to the volume of work. This result points the same way: what counts happens in the muscle being worked, not in a blood concentration measured afterwards."}]},{"id":"b16","type":"heading","level":2,"text":"Cortisol is not a hostile hormone","anchor":"cortisol-is-not-a-hostile-hormone"},{"id":"b17","type":"callout","tone":"caution","title":"Three common shortcuts","content":[[{"kind":"strong","value":"\"Cortisol destroys muscle.\""},{"kind":"text","value":" Cortisol is indispensable: it mobilises reserves, modulates inflammation and follows a daily rhythm needed for waking. It is its prolonged elevation that is a problem, not its existence — the "},{"kind":"link","value":"stress","href":"/en/recovery/stress","external":false},{"kind":"text","value":" page handles that distinction."}],[{"kind":"strong","value":"\"You need to raise testosterone after a session.\""},{"kind":"text","value":" The trial cited above does not support that link. Body Lab moreover mentions no supplement, no substance and no method of raising hormones."}],[{"kind":"strong","value":"\"One hormone, one function.\""},{"kind":"text","value":" Each acts differently depending on the tissue and the receptor. Formulas such as \"the stress hormone\" or \"the happiness hormone\" do not match what physiology describes."}]]},{"id":"b18","type":"heading","level":2,"text":"Why no scene accompanies this page","anchor":"why-no-scene-accompanies-this-page"},{"id":"b19","type":"callout","tone":"limit","title":"Glands absent from the models","content":[[{"kind":"text","value":"The anatomical sources Body Lab uses supply the skeleton, the muscles, the joints and the regions of the body. No endocrine gland appears there, and the visceral system of the atlas used cannot be reused for licence reasons."}]]},{"id":"b20","type":"heading","level":2,"text":"What this page does not do","anchor":"what-this-page-does-not-do"},{"id":"b21","type":"callout","tone":"caution","title":"No tests, no supplements","content":[[{"kind":"text","value":"This page describes a general functioning. It allows no blood test result to be interpreted and addresses no endocrine disease."}],[{"kind":"text","value":"Unusual and lasting fatigue, unexplained weight change, cycle disturbances or intense thirst belongs to a health professional, who has the context and the investigations a page does not."}]]},{"id":"b22","type":"heading","level":2,"text":"Sources","anchor":"sources"},{"id":"b23","type":"sourceList","title":"Main sources","referenceIds":["hall2020","morton2016","mcewen1998"]},{"id":"b24","type":"shapierAction","actionId":"conseils-stress-cortisol","label":"Manage stress and cortisol day to day","description":"Handling stress concretely, day to day, is done in Shapier; Body Lab only explains what cortisol does, and does not do.","webTarget":"https://shapier.app/en/conseils/gestion-stress-cortisol","appTarget":"shapier://conseils/gestion-stress-cortisol"},{"id":"b25","type":"quiz","title":"Check my understanding","questions":[{"id":"q1","prompt":"A hormone poured into the blood reaches the whole body: what determines which cells it acts on?","choices":[{"id":"q1c1","label":"The presence of the matching receptor on the cell","correct":true,"explanation":"The page makes this the key to the system: a hormone circulates everywhere but acts only where a receptor is waiting for it."},{"id":"q1c2","label":"The gland that produces the hormone chooses its destination","correct":false,"explanation":"Tempting by analogy with the nerve message and its cable, but the endocrine system does the opposite: it pours the molecule into the blood with no designated recipient."},{"id":"q1c3","label":"The hormone acts on every cell it reaches","correct":false,"explanation":"This reverses the principle described: it does not act on cells unable to recognise it, even though it reaches them."}]},{"id":"q2","prompt":"What does the cited trial, run on forty-nine trained men, show about the post-session hormonal surge?","choices":[{"id":"q2c1","label":"It predicts neither the gain in muscle mass nor the gain in strength","correct":true,"explanation":"Neither load nor hormonal responses account for the results; growth comes down to repeated mechanical tension and work volume."},{"id":"q2c2","label":"The bigger the testosterone surge, the more the muscle grows","correct":false,"explanation":"That is the received idea the page corrects: the trial does not support that link, and maximising the hormonal response produces no extra growth."},{"id":"q2c3","label":"Only growth hormone matters, not testosterone","correct":false,"explanation":"None of the measured hormonal responses explains the results, and assigning a single function to a hormone is the trap the page points out."}]},{"id":"q3","prompt":"According to the page, when does cortisol become a problem?","choices":[{"id":"q3c1","label":"When its elevation is prolonged","correct":true,"explanation":"Cortisol is indispensable — reserves, inflammation, the daily waking rhythm; it is its prolonged elevation that is a problem, not its existence."},{"id":"q3c2","label":"Whenever it is present, because it destroys muscle","correct":false,"explanation":"A common shortcut the page corrects: cortisol mobilises reserves, modulates inflammation and follows a rhythm needed for waking."},{"id":"q3c3","label":"Only when training is completely absent","correct":false,"explanation":"The page does not tie the problem to training: it distinguishes the existence of the hormone, which is needed, from its prolonged elevation."}]}]},{"id":"b26","type":"relatedContent","title":"Read next","targetIds":["en-mechanism-stress","en-mechanism-how-muscle-grows","en-mechanism-nervous-system"]}],"relations":[{"type":"prerequisite","targetId":"en-mechanism-nervous-system"},{"type":"explains","targetId":"en-mechanism-how-muscle-grows"},{"type":"explains","targetId":"en-mechanism-dietary-carbohydrate"},{"type":"next-step","targetId":"en-mechanism-stress"}],"authors":["equipe-editoriale-shapier"],"reviewers":["Thanh Chau"],"references":[{"id":"hall2020","authors":"Hall JE, Hall ME","year":2020,"title":"Guyton and Hall Textbook of Medical Physiology, 14th edition","source":"Elsevier","kind":"book","url":"https://shop.elsevier.com/books/guyton-and-hall-textbook-of-medical-physiology/hall/978-0-323-59712-8"},{"id":"morton2016","authors":"Morton RW, Oikawa SY, Wavell CG, Mazara N, McGlory C, Quadrilatero J, Baechler BL, Baker SK, Phillips SM","year":2016,"title":"Neither load nor systemic hormones determine resistance training-mediated hypertrophy or strength gains in resistance-trained young men","source":"Journal of Applied Physiology","kind":"journal-article","doi":"10.1152/japplphysiol.00154.2016"},{"id":"mcewen1998","authors":"McEwen BS","year":1998,"title":"Protective and damaging effects of stress mediators","source":"The New England Journal of Medicine","kind":"review","doi":"10.1056/nejm199801153380307"}],"review":{"publishedAt":"2026-08-02","reviewedAt":"2026-08-02","reviewDueAt":"2027-08-02","evidenceLevel":"established"},"limitations":["No scene accompanies this page: the endocrine glands appear in none of the anatomical sources Body Lab uses.","The trial cited on hormones and hypertrophy covers forty-nine trained, young, healthy men. It transfers neither to women, nor to beginners, nor to situations of illness.","This page allows no test result to be interpreted and addresses no endocrine disease.","No supplement, no substance and no method of raising hormones is mentioned: that would be outside Body Lab's field."],"seo":{"title":"The endocrine system — what hormones do, and what they do not","description":"A slow, diffuse network opposed to the nerve: the major hormones, their real role, and why the post-session surge does not drive muscle growth.","canonicalPath":"/en/body/systems/the-endocrine-system","image":"/og/en/mechanism-endocrine-system.png"},"app":{"offlineEligible":true},"version":1}