The reasoning: chemistry predicts physiology
| Property | Peptide/protein | Steroid (and calcitriol) | Thyroid hormone | Catecholamine |
|---|---|---|---|---|
| Synthesis | pre‑pro‑hormone → granules (stored) | from cholesterol on demand (not stored) | on thyroglobulin, stored in colloid | tyrosine, stored in granules |
| Transport | mostly free | bound (CBG, SHBG, albumin, DBP) | bound (TBG, TTR, albumin) | free/loosely bound |
| Half‑life | minutes | hours | days (T4 ~7 d) | seconds–minutes |
| Receptor | plasma membrane | intracellular (nuclear) | nuclear | plasma membrane |
| Response | rapid (phosphorylation, channels) | slower (transcription) | slow | rapid |