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Mechanism
Intermediate
9 min read

The endocrine system

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.

The second network

The body has two communication systems, and they are built on opposite principles.
The nervous system 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.

Two ways of sending a message

Criterion
Nerve message
Hormonal message
Medium
An axon, to its target
The blood, everywhere at once
Speed
Fraction of a second
Seconds to hours
Recipient
One designated target
Any cell carrying the receptor
Duration of effect
Brief
Prolonged
Hormone
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.
chemical messenger
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.

The main ones, and what they do

Some hormones and their role

Hormone
Produced by
Main role
Insulin
Pancreas
Moves glucose into cells and favours storage
Glucagon
Pancreas
Makes the liver release stored glucose
Cortisol
Adrenal glands
Mobilises reserves, modulates inflammation, follows a daily rhythm
Adrenaline
Adrenal glands
Prepares for immediate effort: heart, vessels, mobilisation
Thyroid hormones
Thyroid
Set the general level of metabolism
Testosterone, oestrogens
Gonads, adrenals
Act on many tissues, including bone and muscle
Growth hormone
Pituitary
Acts on growth and on the use of fuels
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 neuron page describes for neurotransmitters.
Insulin links this system to the Energy pillar: it commands the entry of glucose and its storage, which the dietary carbohydrate page describes as a sorting without naming who conducts it.

What the post-session surge does not do

Here is the most useful correction on this page, and it contradicts a very widespread account.
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.
Certainty level · Probable
The systemic hormonal changes following a session predict neither the gain in muscle mass nor the gain in strength obtained from resistance training.
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.
Morton RW, Oikawa SY, Wavell CG, Mazara N, McGlory C, Quadrilatero J, Baechler BL, Baker SK, Phillips SM (2016)
The how muscle grows 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.

Cortisol is not a hostile hormone

Caution
Three common shortcuts
"Cortisol destroys muscle." 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 stress page handles that distinction.
"You need to raise testosterone after a session." The trial cited above does not support that link. Body Lab moreover mentions no supplement, no substance and no method of raising hormones.
"One hormone, one function." 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.

Why no scene accompanies this page

Limit
Glands absent from the models
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.

What this page does not do

Caution
No tests, no supplements
This page describes a general functioning. It allows no blood test result to be interpreted and addresses no endocrine disease.
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.

Sources

Main sources

  • Hall JE, Hall ME (2020). Guyton and Hall Textbook of Medical Physiology, 14th edition. Elsevier.
  • Morton RW, Oikawa SY, Wavell CG, Mazara N, McGlory C, Quadrilatero J, Baechler BL, Baker SK, Phillips SM (2016). Neither load nor systemic hormones determine resistance training-mediated hypertrophy or strength gains in resistance-trained young men. Journal of Applied Physiology.
  • McEwen BS (1998). Protective and damaging effects of stress mediators. The New England Journal of Medicine.
Put it into practice in Shapier

Manage stress and cortisol day to day

Handling stress concretely, day to day, is done in Shapier; Body Lab only explains what cortisol does, and does not do.
Manage stress and cortisol day to day
Body Lab explains; Shapier lets you act and track.

Check my understanding

A hormone poured into the blood reaches the whole body: what determines which cells it acts on?
What does the cited trial, run on forty-nine trained men, show about the post-session hormonal surge?
According to the page, when does cortisol become a problem?
Choose an answer

Read next

  • Stress
    A mobilisation response, not an illness. What wears you down is not triggering it — it is never switching it off, and training is itself one of the triggers.
    Described without a scene
  • How muscle grows
    What actually makes a muscle grow? This page follows the chain of events that links a set of exercise to a thicker muscle fibre, and separates what is established from what is still debated by research.
    With a 3D scene
  • The nervous system
    The network that gathers, decides and commands. It works in two directions at once, and a whole part of it escapes the will — otherwise you would have to think about every heartbeat.
    Described without a scene

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Trust and method

Author
equipe-editoriale-shapier
Editorial review
Thanh Chau
Scientific review
Pending
Published on
August 2, 2026
Reviewed on August 2, 2026
Next review due August 2, 2027
Limits of this page
  • 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.
Sources
  • Hall JE, Hall ME (2020). Guyton and Hall Textbook of Medical Physiology, 14th edition. Elsevier.
  • Morton RW, Oikawa SY, Wavell CG, Mazara N, McGlory C, Quadrilatero J, Baechler BL, Baker SK, Phillips SM (2016). Neither load nor systemic hormones determine resistance training-mediated hypertrophy or strength gains in resistance-trained young men. Journal of Applied Physiology.
  • McEwen BS (1998). Protective and damaging effects of stress mediators. The New England Journal of Medicine.
Educational content. Body Lab does not diagnose and does not replace professional advice.
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