Hormones fall into three chemical categories based on what they’re built from: steroids made from cholesterol, peptides and proteins made from chains of amino acids, and amines made from a single amino acid. Each type behaves differently in your body because of its unique molecular makeup, affecting how it travels through your blood, enters your cells, and delivers its message.
Steroid Hormones Start With Cholesterol
Steroid hormones are built from cholesterol, the same fatty molecule you associate with heart health. Your body takes cholesterol and, through a series of enzyme reactions, reshapes it into hormones like cortisol (your stress hormone), testosterone, estrogen, progesterone, and aldosterone (which regulates blood pressure). The process begins inside the mitochondria of hormone-producing cells, where an enzyme converts cholesterol into a molecule called pregnenolone. From there, pregnenolone branches into different pathways depending on which gland is doing the work, ultimately producing whichever steroid hormone that tissue specializes in.
Because steroids are built from a fat-based molecule, they dissolve easily in fat but not in water. This matters for how they get around. Blood is over 90% water, so steroid hormones can’t just float freely through it. Instead, they hitch a ride on transport proteins that carry them to their target tissues. Once they arrive, their fat-soluble nature lets them slip right through cell membranes, which are themselves made of fat. Inside the cell, they bind to receptors in the nucleus or cytoplasm and directly influence which genes get turned on or off. This is why steroid hormones tend to produce slower but longer-lasting effects compared to other hormone types.
Your body doesn’t stockpile steroid hormones. Because they can pass through cell membranes so easily, storing them in a compartment inside the cell would be pointless. Instead, your glands make them on demand. When a signal arrives telling the adrenal glands to produce cortisol, for example, the machinery kicks in right then, converting cholesterol into the finished hormone and releasing it almost immediately.
Peptide and Protein Hormones Are Amino Acid Chains
The largest class of hormones is built from amino acids, the same building blocks that make up every protein in your body. Peptide hormones are short chains (fewer than 100 amino acids), while protein hormones are longer. They’re produced the same way your body makes any protein: DNA provides the instructions, which get copied into a template, which gets translated into a chain of amino acids. But these hormones go through extra processing steps before they’re ready to work.
A peptide hormone starts as a larger, inactive precursor molecule. As it moves through the cell’s internal packaging system, enzymes clip it down to its final, active form. Insulin is a good example. It begins as a single long chain that gets cut into two smaller chains (called the A chain and B chain), which are then locked together by chemical bridges called disulfide bonds. Those bonds are essential for insulin to hold its shape and function properly. Other well-known peptide hormones include glucagon (which raises blood sugar), oxytocin (involved in bonding and labor contractions), and growth hormone.
Unlike steroid hormones, peptide and protein hormones dissolve in water. That means they travel freely through the bloodstream without needing a transport protein. The trade-off is that they can’t slip through cell membranes the way steroids can. Instead, they dock onto receptors on the outside surface of target cells, triggering a chain of signals inside the cell. This relay system produces fast but typically short-lived responses.
Cells that produce peptide hormones have another trick: they can stockpile them. The finished hormones get packed into tiny storage compartments called secretory granules, which sit inside the cell waiting for a signal. When that signal comes, the cell dumps its stored supply all at once through a process called exocytosis. This is why your body can flood the bloodstream with insulin within minutes of eating, rather than having to build each molecule from scratch.
Amine Hormones Come From a Single Amino Acid
Amine hormones are the simplest structurally. Rather than chaining together many amino acids, your body takes just one amino acid and chemically modifies it. The two starting amino acids are tyrosine and tryptophan, and the hormones they produce have very different jobs.
Tyrosine is the more versatile of the two. It gives rise to the catecholamines: epinephrine (adrenaline), norepinephrine, and dopamine. These are secreted by the adrenal glands and parts of the nervous system and drive the fight-or-flight response, raising your heart rate, sharpening focus, and redirecting blood flow to muscles. Tyrosine is also the starting material for thyroid hormones, but with an unusual twist.
Tryptophan, the other starting amino acid, is converted into melatonin by the pineal gland in your brain. Melatonin regulates your sleep-wake cycle, rising in the evening as light fades and dropping in the morning.
Thyroid Hormones Are a Special Case
Thyroid hormones deserve their own mention because they blur the categories. Chemically, they start as amine hormones derived from tyrosine, but their final structure is unusual: two tyrosine molecules linked together with iodine atoms attached. The two main thyroid hormones are T4 (thyroxine), which carries four iodine atoms, and T3 (triiodothyronine), which carries three. The number and position of those iodine atoms determine how active the hormone is. T3 is the more potent form.
This iodine requirement is why iodine in your diet matters so much. Without enough of it, your thyroid can’t assemble functional hormones, leading to conditions like goiter or hypothyroidism. It’s also why iodine was added to table salt in many countries starting in the early 20th century.
Thyroid hormones also behave more like steroids than like other amine hormones when it comes to getting inside cells. Their molecular structure is large and nonpolar enough that they can cross cell membranes and bind to receptors inside the nucleus, directly affecting gene activity. Most other amine hormones (like adrenaline) are water-soluble and stick to receptors on the cell surface instead.
Why Chemical Makeup Matters
The raw ingredients of a hormone determine almost everything about how it works. A hormone’s composition dictates whether it dissolves in water or fat, which controls whether it floats freely in your blood or needs a carrier protein. It determines whether the hormone can pass through a cell membrane to work from the inside or has to knock on the door by binding to surface receptors. And it shapes whether your body can store the hormone in advance or needs to build it fresh each time.
Peptide hormones, being water-soluble and stored in granules, tend to act fast but briefly. Steroid hormones, traveling on carrier proteins and altering gene expression directly, tend to act more slowly but with effects that last hours or even days. Amine hormones span the range: adrenaline hits in seconds and fades quickly, while thyroid hormones influence your metabolism over weeks. All of these differences trace back to one thing: what the hormone is made of.