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
Related
- G protein-coupled receptors and cAMP
- IP₃, DAG and calcium pathway
- Receptor tyrosine kinases
- JAK–STAT pathway
- Nuclear receptors
- Steroid hormone synthesis (overview)
- Molecules derived from amino acids
- Thyroid hormone synthesis
- 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.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Hormones | Hormone Function | MedlinePlus (MedlinePlus, US National Library of Medicine, 2026); Free T4 test: MedlinePlus Medical Encyclopedia (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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Representative model, for educational purposes only.