Hormones are chemical messages carried in the blood. This page explains how hormones act, how the hypothalamus and pituitary run most hormone systems through feedback, what growth hormone does, and how exercising muscle itself acts as an endocrine organ.
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Part 8 · Endocrine and Reproduction · Chapter 34 of 47
Learning objectives
By the end of this page you should be able to:
Describe the main types of hormone and how they act on cells.
Explain hypothalamic–pituitary control and negative feedback.
List the anterior and posterior pituitary hormones and their targets.
Describe the actions and control of growth hormone.
Explain what hormone responses to exercise do and do not achieve.
Describe skeletal muscle as a secretory organ.
Why it matters
Hormones shape tissue healing, bone, muscle, fluid balance and energy. Many patients take hormone-related medicines such as steroids, thyroid replacement and insulin, which affect exercise responses.
Popular ideas about ‘hormone-boosting’ workouts are common in gyms. Understanding the evidence helps physiotherapists give accurate advice.
The hypothalamus links the brain to the endocrine system. It sends releasing and inhibiting hormones through a portal blood system to the anterior pituitary, and sends nerve fibres to the posterior pituitary.
Figure 2 · A hormone axis with feedback
Feedback keeps levels steady. Long-term steroid medicines suppress the axis, which is why they must be tapered, not stopped suddenly.
Most axes use negative feedback: the final hormone suppresses its own release. Stress, sleep, light, illness and exercise feed into the hypothalamus and shift the axes. Long-term steroid tablets suppress the adrenal axis, so stopping them suddenly can be dangerous.
Figure 3 · Pituitary hormones and their targets
The master gland takes orders from the brain. Each anterior hormone controls another gland or tissue; posterior hormones are released directly by hypothalamic nerves.
Growth hormone
Growth hormone is released in pulses, mostly during deep sleep, and also during exercise, fasting and stress. It acts partly through insulin-like growth factor 1 (IGF-1) made by the liver and by muscle.
Tissue
Effect of growth hormone and IGF-1
Bone
Growth at growth plates in children; bone remodelling
Muscle
Protein synthesis; supports repair
Fat
Breakdown of stored fat
Glucose
Raises blood glucose; opposes insulin
Too much in childhood causes gigantism; in adults, acromegaly with enlarged hands, jaw and joint problems. Too little in adults causes reduced muscle and bone mass and fatigue.
Hormones and exercise
Heavy resistance exercise with large muscle groups raises testosterone, growth hormone and IGF-1 for a short time. It was once thought these spikes drive muscle growth. West and colleagues tested this directly [2].
Figure 4 · Hormone spikes do not build extra muscle
Local signals in the trained muscle matter most. Large post-exercise hormone rises did not add to protein synthesis in the arm [2].
Young men exercised one arm alone on one day, and the other arm followed by heavy leg exercise on another. Leg exercise produced large hormone rises, yet protein synthesis in the arm was no higher [2]. Local signals in the trained muscle are likely to be most important [2].
Practical message
Choose exercises for the muscles and functions you want to improve. Loading the target muscle well, with adequate protein and rest, matters more than trying to create hormone spikes [2].
Muscle as an endocrine organ
Pedersen and Febbraio describe skeletal muscle as a secretory organ. Contracting muscle releases hundreds of peptides, called myokines, that act on the muscle itself and on distant organs [1].
Figure 5 · Muscle as an endocrine organ
Exercise is a hormone-like treatment. Many myokines depend on contraction, so inactivity may alter this signalling and contribute to chronic disease [1].
IL-6 released during exercise promotes fat use and has effects on the liver, fat and immune system [1]. Other myokines influence bone, blood vessels and brain. Because many myokines depend on contraction, physical inactivity may change this signalling and contribute to chronic disease [1].
Clinical relevance for the physiotherapist
Steroid users. Expect thin skin, weaker bones and muscles; do not encourage sudden stopping of steroids.
Pituitary disorders. Acromegaly causes joint pain and nerve entrapment; growth hormone deficiency reduces muscle and bone.
Exercise as medicine. Muscle contraction releases signals that benefit many organs [1].
Gym myths. Hormone spikes after exercise are not the main driver of muscle growth [2].
Sleep. Growth hormone peaks in deep sleep; poor sleep may slow recovery.
Assessment and application
Situation
Interpretation
Action
Client wants leg day to ‘boost hormones’ for arm growth
Not supported [2]
Train arms directly with progressive load
Patient on long-term prednisolone wants to stop it
Axis suppression
Advise discussion with doctor; never stop suddenly
Sedentary patient with type 2 diabetes and fatty liver
Low contraction-driven signalling [1]
Regular aerobic and resistance exercise
Adult with large hands, jaw growth and carpal tunnel symptoms
Possible acromegaly
Refer for endocrine assessment
Common mistakes
Designing programmes to maximise hormone spikes [2].
Ignoring muscle’s role as an endocrine organ [1].
Advising patients to stop steroids abruptly.
Confusing anterior and posterior pituitary hormones.
Forgetting that growth hormone opposes insulin.
Exam preparation
Long answer
Describe the hormones of the anterior pituitary and their regulation.
Hormone types and mechanisms (Figure 1).
Hypothalamic control and feedback (Figure 2).
Anterior pituitary hormones and targets (Figure 3).
Growth hormone acts via IGF-1; peaks in deep sleep.
Post-exercise hormone spikes do not boost muscle protein synthesis [2].
Muscle releases myokines that act on many organs [1].
Further reading
Source
Why it earns its place
Pedersen BK, Febbraio MA — Nat Rev Endocrinol, 2012
Muscle as a secretory organ [1]
West DWD et al. — J Physiol, 2009
Hormones and muscle protein synthesis [2]
Greenspan’s Basic and Clinical Endocrinology
Standard endocrine text
References
Pedersen BK, Febbraio MA. Muscles, exercise and obesity: skeletal muscle as a secretory organ.Nature Reviews Endocrinology. 2012;8(8):457–465.
doi:10.1038/nrendo.2012.49PMID 22473333Review
West DWD, Kujbida GW, Moore DR, et al. Resistance exercise-induced increases in putative anabolic hormones do not enhance muscle protein synthesis or intracellular signalling in young men.Journal of Physiology. 2009;587(Pt 21):5239–5247.
doi:10.1113/jphysiol.2009.177220PMID 19736298Experimental study
