How do peptides get transported compared to steroid/thyroid hormones
Peptides (and catecholamines)are water soluble so they are dissolved in the plasma and so they travel in the circulation. They’ll diffuse out of the capillaries once they get to target organ, go to interstitial tissue. And to target cells.
Steroid & thyroid hormones are bound to plasma proteins. 10% are unbound (active) free portion. Just like drugs, bound fraction of the hormone acts as a reservoir. Protein binding keeps hormone away from elimination but also slows the clearance from the plasma