Hormones as Chemical Messengers

Your body has two main ways to send messages. Nerves use electrical signals, fast and precise, along fixed wires. Hormones use chemistry: molecules released into the blood that travel everywhere, yet affect only the cells equipped to “hear” them. A hormone released from a gland the size of a pea can change the behaviour of billions of cells. How that works, and why different hormones act so differently, comes down to their chemical structures.

What a hormone is

A hormone is a chemical messenger that is:

• made by specialised cells, usually in an endocrine gland (such as the pancreas, thyroid, adrenal glands or pituitary);
• released into the blood;
• carried to target cells, often far away;
• recognised by receptors, proteins that bind the hormone specifically;
• effective at very low concentrations, often nanomolar or picomolar.

Only cells that have the right receptor respond. That’s how one hormone circulating through the whole body can affect just the liver, or just the kidneys.

Three chemical classes

Hormones fall into three main structural families, and their chemistry determines how they travel and act.

1. Peptide and protein hormones

Chains of amino acids, from a few to hundreds long (see proteins: chains of amino acids).

• Examples: insulin (51 amino acids), glucagon (29), growth hormone (191), oxytocin (9), antidiuretic hormone (vasopressin, 9).
• Chemistry: polar and often charged, so water-soluble. They dissolve easily in blood plasma but cannot cross the non-polar core of cell membranes (see phospholipids and cell membranes).
• Receptors: on the cell surface.
• Made: by ribosomes like any protein, then stored in vesicles and released on demand.
• Because they’re proteins, they’re digested in the gut, which is why insulin must usually be injected rather than swallowed.

2. Steroid hormones

Lipids built on the four-ring steroid skeleton, all made from cholesterol (see steroids).

• Examples: cortisol, aldosterone, testosterone, oestradiol, progesterone; the active form of vitamin D works the same way.
• Chemistry: largely non-polar, so fat-soluble. They travel in blood bound to carrier proteins.
• Receptors: inside the cell. Steroids diffuse straight through the membrane and bind receptors in the cytoplasm or nucleus.
• Made: on demand from cholesterol; they can’t be stored in vesicles because they’d leak through the vesicle membranes.
• Being fat-soluble, many steroid hormones and their synthetic versions can be taken orally.

3. Amino acid derivatives

Small molecules made from a single amino acid.

• From tyrosine:
  • adrenaline and noradrenaline (catecholamines): water-soluble, act on surface receptors (see adrenaline);
  • thyroid hormones (thyroxine, T4, and triiodothyronine, T3): contain iodine atoms, are fat-soluble, and act on receptors inside the nucleus, behaving more like steroids.
• From tryptophan: melatonin, which helps regulate the sleep cycle (see the chemistry of sleep).

Why solubility decides everything

	Water-soluble hormones (peptides, catecholamines)	Fat-soluble hormones (steroids, thyroid hormones)
Transport in blood	Dissolved freely	Bound to carrier proteins
Crosses cell membrane?	No	Yes
Receptor location	Cell surface	Inside the cell (cytoplasm or nucleus)
Mechanism	Second messengers change existing proteins	Hormone–receptor complex changes gene expression
Speed of response	Seconds to minutes	Hours to days
Duration	Usually short	Usually long
Half-life in blood	Often minutes	Often hours or longer

How surface receptors pass the message inside

A water-soluble hormone never enters the cell. Instead, binding changes the shape of its receptor, which passes the signal through the membrane.

A common pathway, used by adrenaline and glucagon:

1. The hormone (the first messenger) binds a receptor on the outside of the cell.
2. The receptor changes shape and activates a G protein on the inside of the membrane.
3. The G protein activates the enzyme adenylyl cyclase, which converts ATP into cyclic AMP (cAMP), the second messenger.
4. cAMP activates protein kinases, enzymes that attach phosphate groups from ATP to other proteins, switching them on or off (see ATP).
5. For example, in the liver, this cascade switches on the enzyme that breaks down glycogen, releasing glucose into the blood.

Amplification

Each step multiplies the signal. One hormone molecule activates one receptor, which can activate many G proteins; each adenylyl cyclase makes many cAMP molecules; each kinase phosphorylates many target proteins. A single adrenaline molecule can lead to the release of millions of glucose molecules. That’s why such tiny concentrations are effective.

Insulin uses a different surface receptor, one that is itself an enzyme (a tyrosine kinase), but the principle of amplification through phosphorylation is similar (see insulin).

How intracellular receptors work

A steroid hormone:

1. diffuses across the cell membrane;
2. binds its receptor protein in the cytoplasm or nucleus;
3. the hormone–receptor complex binds to specific DNA sequences (hormone response elements);
4. it switches particular genes on or off, so the cell makes more or less of certain proteins.

Because new proteins must be made, these effects are slower but longer lasting. Cortisol’s anti-inflammatory action, for example, takes hours.

Feedback control

Most hormone systems regulate themselves by negative feedback: the effect of the hormone reduces its own release.

• Blood glucose: rising glucose triggers insulin release; insulin lowers glucose; as glucose falls, insulin release falls.
• Thyroid hormones: the hypothalamus releases TRH, which stimulates the pituitary to release TSH, which stimulates the thyroid to release T4 and T3. High T4 and T3 levels then suppress TRH and TSH release.
• Calcium: low blood calcium triggers parathyroid hormone, which raises calcium (see calcium and bones).

This keeps levels stable, a principle called homeostasis. It’s much like how a thermostat controls a heating system, and it has parallels with chemical equilibrium (see Le Chatelier’s principle).

Some systems use positive feedback, which amplifies change, such as oxytocin during childbirth: contractions trigger more oxytocin, which strengthens contractions, until the baby is born.

Switching hormones off

Signals must also end. Hormones are removed by:

• breakdown by enzymes in the liver, kidneys and target tissues;
• excretion in urine (after modification);
• reuptake, for some small molecules.

Inside cells, second messengers such as cAMP are broken down by enzymes called phosphodiesterases. Caffeine partly inhibits these enzymes, which contributes to its stimulating effect.

Hormones and medicine

• Replacement therapy: insulin for type 1 diabetes, thyroxine for an underactive thyroid, cortisol-like drugs for adrenal failure.
• Synthetic analogues: designed with modified structures to last longer or act more strongly, such as long-acting insulins and potent anti-inflammatory steroids.
• Blocking hormones: drugs that block receptors (such as beta-blockers against adrenaline) or hormone synthesis (such as aromatase inhibitors in some breast cancers).
• Diagnosis: measuring hormones in blood or urine, including pregnancy tests, which detect the hormone hCG using antibodies.

Key takeaways

• Hormones are chemical messengers released into the blood that act on cells with matching receptors.
• Three classes: peptides/proteins (water-soluble), steroids (fat-soluble) and amino acid derivatives (either).
• Water-soluble hormones bind surface receptors and use second messengers such as cAMP; fat-soluble hormones enter cells and change gene expression.
• Cascades amplify signals enormously; negative feedback keeps hormone levels stable.
• Next, see a hormone in detail: insulin.