Expansion
Steroids act on intracellular receptors and alter gene transcription
Expansion
| Feature | Steroid and thyroid hormones | Peptide and amine hormones |
|---|---|---|
| Solubility | Lipid soluble | Water soluble |
| Transport | Bound to carrier proteins | Free in plasma |
| Storage | Not stored; made on demand | Stored in granules |
| Receptor | Intracellular (cytoplasmic or nuclear) | Cell surface |
| Mechanism | Alters gene transcription | Second messengers |
| Onset | Hours | Seconds to minutes |
| Half-life | Long | Short |
Steroids include cortisol, aldosterone, the sex steroids and vitamin D. Thyroid hormone behaves like a steroid despite being an amine, because it is lipid soluble and acts on nuclear receptors.
Practical consequences:
- Protein binding means only the free fraction is active, so binding protein changes (pregnancy, oestrogen, liver disease) alter total but not free levels
- Steroids cannot be given as a rapid rescue in the way adrenaline can; hydrocortisone in anaphylaxis or asthma takes hours
- Steroid receptors are ubiquitous, which is why systemic side effects are unavoidable and why targeted delivery, as with inhaled steroids, is valuable