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[Biochemistry](https://durgical.com/en/biochemistry/) · Topic

# Hormone classes and mechanisms

Its chemistry decides whether a hormone travels free or protein-bound and whether it acts on membrane receptors or inside the cell.

Updated 2026-11-01

## Key facts

- Peptides and proteins (insulin, glucagon, GH, ACTH): water-soluble, stored in granules, short half-life, membrane receptors.
- Steroids (cortisol, aldosterone, oestrogens, testosterone) and vitamin D: made from cholesterol, not stored, carried by proteins, act on nuclear receptors.
- Amines: catecholamines behave like peptides; thyroid hormones behave like steroids.
- Most hypothalamic–pituitary–gland axes are controlled by negative feedback.

## In detail

- Membrane receptors: G protein-coupled (cAMP or IP₃/Ca²⁺), tyrosine kinase (insulin, IGF-1), JAK-associated (GH, prolactin, leptin) and guanylyl cyclase (atrial natriuretic peptide → cGMP).
- Intracellular receptors: nuclear, for fat-soluble hormones that cross the membrane.
- Negative feedback: cortisol holds back CRH and ACTH; T3 and T4 hold back TRH and TSH. Positive: the oestradiol peak triggers the LH surge before ovulation.
- Only free hormone acts; that is why free T4, not only total T4, is measured when binding proteins change (pregnancy, oral contraceptives, cirrhosis).
- Nitric oxide activates a soluble guanylyl cyclase; sildenafil inhibits phosphodiesterase 5 and prolongs cGMP.

## Clinical notes

- High TSH with low free T4 means thyroid failure (primary hypothyroidism); low TSH with low free T4 points to the pituitary.
- In Cushing syndrome from a pituitary tumour, cortisol does not hold ACTH back properly.
- Pituitary adenomas can overproduce one hormone or compress the neighbouring cells.

## Related conditions

- [Hypothyroidism](https://durgical.com/en/conditions/hypothyroidism/)
- [Cushing syndrome](https://durgical.com/en/conditions/cushing-syndrome/)
- [Pituitary adenoma](https://durgical.com/en/conditions/pituitary-adenoma/)

## Related

- [G protein-coupled receptors and cAMP](https://durgical.com/en/biochemistry/topics/g-protein-coupled-receptors-and-camp/)
- [IP₃, DAG and calcium pathway](https://durgical.com/en/biochemistry/topics/ip-dag-and-calcium-pathway/)
- [Receptor tyrosine kinases](https://durgical.com/en/biochemistry/topics/receptor-tyrosine-kinases/)
- [JAK–STAT pathway](https://durgical.com/en/biochemistry/topics/jak-stat-pathway/)
- [Nuclear receptors](https://durgical.com/en/biochemistry/topics/nuclear-receptors/)
- [Steroid hormone synthesis (overview)](https://durgical.com/en/biochemistry/topics/steroid-hormone-synthesis/)
- [Molecules derived from amino acids](https://durgical.com/en/biochemistry/topics/molecules-derived-from-amino-acids/)
- [Thyroid hormone synthesis](https://durgical.com/en/biochemistry/pathways/thyroid-hormone-synthesis/)
- [Catecholamines: synthesis and breakdown](https://durgical.com/en/biochemistry/topics/catecholamines-synthesis-and-breakdown/)

## Test yourself

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

## Why do steroid hormones travel bound to proteins in blood?

Because they are fat-soluble and poorly soluble in water; binding also prolongs their half-life.

## Which second messenger does atrial natriuretic peptide use?

cGMP, made by its receptor's guanylyl cyclase activity.

## Why is free T4 measured in a pregnant woman?

Because oestrogens raise the binding globulin: total T4 rises even when the active (free) hormone is normal.

## Which class does thyroid hormone resemble in its mechanism?

Steroids: it enters the cell and acts on a nuclear receptor.

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Educational information, not medical advice. For any symptom, see a health professional.

Sources: [Hormones | Hormone Function | MedlinePlus](https://medlineplus.gov/hormones.html) (MedlinePlus, US National Library of Medicine, 2026); [Free T4 test: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/003517.htm) (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).

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