Hello byronicbear,
Tyrosine-derived amine hormones are divided into two major classes: catecholamines and thyroid hormones. It's important to know this because one class behaves like peptides and the other like steroids.
Catecholamines (epinephrine and norepinephrine) are produced in the adrenal medulla and act like peptide hormones, i.e. act on cell membranes in order to activate second messenger G proteins through adrenergic receptors. They are minimally-modified tyrosine monoamines (with an extra hydroxyl on the tyrosine side-chain ring, increasing its polarity) so they do not cross the cell membrane.
Thyroid hormones (T4, T3) are more processed and include two iodinated benzyl rings, making the molecules sufficiently non-polar that they can easily diffuse through the cell membrane and act on nuclear receptors to cause metabolic changes (like steroid hormones). Think of them as being more steroid-like because of the extra ring.
I hope this helps!
