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[Physiology](https://durgical.com/en/physiology/) · [Endocrine](https://durgical.com/en/physiology/#s-endocrine)

# Hormones: types, receptors and feedback

- Concept
- Exam priority: medium

Hormones are chemical messages in the blood. Protein hormones act at the membrane; steroid and thyroid hormones act inside the cell.

Updated 2026-11-01

Also called: Hormone classes, Negative and positive feedback

## Key points

- Peptides and catecholamines: water-soluble, stored in vesicles, travel free, short half-life, membrane receptor, effect in seconds to minutes.
- Steroids and thyroid hormone: lipid-soluble, carried on proteins, long half-life, intracellular receptor, effect in hours via transcription.
- Steroids are not stored: their secretion equals their synthesis.
- Only the free fraction is active and it is the one that drives feedback.
- Positive feedback: the LH surge before ovulation (sustained estrogen) and oxytocin in labor.

## What it is

Hormones are grouped by their chemistry, and the chemistry predicts nearly everything else: how they are stored and carried, where their receptor sits, how fast they act and whether they work by mouth. Almost every axis is run by negative feedback, which holds a set point; positive feedback is the exception and always has an end. Signal pathway details are in Biochemistry.

## Key facts

- Membrane receptors: Gs–cAMP (glucagon, ACTH, TSH, PTH, ADH at V2), Gq–IP₃/Ca²⁺ (ADH at V1, oxytocin, GnRH, TRH), tyrosine kinase (insulin, IGF-1), JAK-STAT (GH, prolactin).
- Many hormones are secreted in pulses and with a circadian rhythm: one value may not represent the average.
- Down-regulation (fewer receptors) explains why continuous GnRH shuts the gonadal axis down while pulsatile GnRH drives it.
- Feedback loops: long (final hormone → hypothalamus and pituitary), short (pituitary hormone → hypothalamus).

### Hormone axes and feedback

Hypothalamic–pituitary–thyroid axis **Lesion** None (normal)

Tap a box to see the hormone, or choose a lesion to see the lab pattern.

- Stimulates (+)
- Inhibits / negative feedback (−)
- Level ↑
- Level ↓

Illustrative schematic drawn by Durgical: simplified axes and classic lab patterns; not for clinical decisions.

## Hormone classes

| Feature | Peptides and proteins | Steroids | Amines |
| --- | --- | --- | --- |
| Examples | Insulin, glucagon, PTH, ADH, pituitary hormones | Cortisol, aldosterone, estradiol, testosterone, calcitriol | Catecholamines (like peptides); T3/T4 (like steroids) |
| Storage | Secretory granules | Not stored | Catecholamines in granules; T4 in thyroid colloid |
| Transport in blood | Mostly free | Protein-bound | Catecholamines free; T4/T3 bound |
| Receptor | Membrane (Gs, Gq, tyrosine kinase, JAK-STAT) | Intracellular (nuclear) | Catecholamines: membrane; T3: nuclear |
| Speed and duration | Fast and brief | Slow and lasting | Depends on the subclass |

## Clinical notes

- Peptide hormones are digested in the gut and so are not given by mouth (insulin); steroid and thyroid hormones are absorbed.
- Changes in binding proteins (pregnancy, cirrhosis, nephrotic syndrome) alter the total level without changing the free fraction.

## Memory aid

"Water is fast, fat is slow": the water-soluble hormone knocks on the door; the fat-soluble one walks into the house and changes the blueprints.

## Test yourself

Think of the answer, then tap the question to see it.

## Why does cortisol secretion equal its synthesis, while insulin can be released in a burst?

Steroids are lipid-soluble and cross membranes, so they cannot be kept in vesicles; insulin, a peptide, is stored in granules ready to go.

## Why does continuous GnRH shut the gonadal axis down while pulsatile GnRH stimulates it?

Continuous exposure desensitizes and down-regulates GnRH receptors on gonadotrophs; pulses leave recovery time between stimuli.

## Why does thyroid hormone, an amine, behave like a steroid?

It is lipid-soluble, travels on proteins, enters the cell and acts on a nuclear receptor that changes transcription, with slow and lasting effects.

## What feature marks positive feedback in physiology?

It amplifies the change instead of correcting it and needs an event to end it: ovulation ends the LH surge, and delivery of the fetus ends the oxytocin reflex.

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## Related topics

- [The hypothalamic–pituitary system: overview](https://durgical.com/en/physiology/the-hypothalamic-pituitary-system-overview/)
- [Measuring hormones: stimulation and suppression tests](https://durgical.com/en/physiology/measuring-hormones-stimulation-and-suppression-tests/)
- [Actions of thyroid hormones](https://durgical.com/en/physiology/actions-of-thyroid-hormones/)
- [Homeostasis and feedback control](https://durgical.com/en/physiology/homeostasis-and-feedback-control/)
- [Actions of cortisol](https://durgical.com/en/physiology/actions-of-cortisol/)

## In Biochemistry

- [Hormone classes and mechanisms](https://durgical.com/en/biochemistry/topics/hormone-classes-and-mechanisms/)
- [G protein-coupled receptors and cAMP](https://durgical.com/en/biochemistry/topics/g-protein-coupled-receptors-and-camp/)
- [Nuclear receptors](https://durgical.com/en/biochemistry/topics/nuclear-receptors/)
- [Receptor tyrosine kinases](https://durgical.com/en/biochemistry/topics/receptor-tyrosine-kinases/)
- [JAK–STAT pathway](https://durgical.com/en/biochemistry/topics/jak-stat-pathway/)
- [IP₃, DAG and calcium pathway](https://durgical.com/en/biochemistry/topics/ip-dag-and-calcium-pathway/)

[← All of physiology](https://durgical.com/en/physiology/)

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: [17.2 Hormones](https://openstax.org/books/anatomy-and-physiology-2e/pages/17-2-hormones) (OpenStax (Rice University), 2022); [17.1 An Overview of the Endocrine System](https://openstax.org/books/anatomy-and-physiology-2e/pages/17-1-an-overview-of-the-endocrine-system) (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 75). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

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Representative model, for educational purposes only.

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