A message in the blood
A hormone is a chemical messenger made by one set of cells, released into the bloodstream, and carried to distant cells that can read it. That last part — released into the blood and acting at a distance — is what makes it endocrine. This is endocrine signaling: the gland and the target may be in completely different parts of the body, connected only by circulation.
Here is the key move for this whole track: a hormone is defined by its job (carry a message), but it is built out of ordinary chemistry. There is no special "hormone material." Instead, the body reuses four kinds of molecules it already knows how to make — and which kind it picks determines how the hormone is stored, how it travels, and how a cell must listen for it.
The four families
Peptides and proteins. The biggest group. These are chains of amino acids, from short peptides to large protein hormones like insulin and growth hormone. They are made the way all proteins are made — by reading a gene and stringing amino acids together — and they are water-loving.
Steroids. All carved from cholesterol: cortisol, aldosterone, testosterone, estradiol, progesterone. Because cholesterol is a fat, every steroid hormone is fat-loving. They share a recognizable four-ring skeleton; the small chemical decorations on that skeleton are the entire difference between a stress hormone and a sex hormone.
Amines. Tiny molecules made by tweaking a single amino acid. From tyrosine come the catecholamines — epinephrine and friends — which behave like water-loving messengers. From tyrosine, after a long and unusual route, also come the thyroid hormones T4 and T3, which (oddly for such small molecules) behave like fat-loving ones. We treat thyroid hormones as their own special case throughout this track.
Eicosanoids. Short-lived signals built from fatty acids in the cell membrane — prostaglandins and their relatives. Each eicosanoid mostly acts locally, on neighboring cells (that is paracrine signaling) or on the very cell that made it (autocrine signaling), rather than traveling far through blood. We mention them so the picture is complete; they are the family least like a classic circulating hormone.
Sorting them by the only split that matters
If four families feel like a lot to track, collapse them onto one axis: does this molecule dissolve in water, or in fat? Peptides, proteins, and catecholamines are hydrophilic (water-loving). Steroids and thyroid hormones are lipophilic (fat-loving). Almost every other property in this track lines up behind that one question.
FAMILY EXAMPLE WATER/FAT ACTS AT A DISTANCE? --------------------------------------------------------------------- peptide/protein insulin, GH water-loving yes (classic endocrine) steroid cortisol, estradiol fat-loving yes (classic endocrine) amine: catechol epinephrine water-loving yes amine: thyroid T4, T3 fat-loving yes (the odd one out) eicosanoid prostaglandins fat-derived mostly local (para/autocrine)