Hormones: How the Body Sends Messages

Your body is made of trillions of cells, all doing different jobs. For everything to work together, cells need to communicate. Hormones are one of the body's main communication systems — they are chemical messengers that are made in one place, travel through the blood, and cause a change somewhere else. Think of a hormone like a radio signal: - The gland that makes the hormone is the transmitter - The bloodstream is the airwaves - The target cell with the matching receptor is the receiver - Only cells with the right receptor can "hear" the signal The study of hormones is called endocrinology. The glands that produce hormones make up the endocrine system: the hypothalamus, pituitary gland, thyroid, parathyroid, adrenal glands, pancreas, and gonads (testes/ovaries). Hormones are involved in virtually everything — growth, metabolism, mood, reproduction, stress response, blood sugar control, water balance, and much more.

All hormones can be grouped into two major types based on their chemistry. Understanding this distinction is one of the most important concepts in endocrinology because the type of hormone determines HOW it works. Peptide hormones (also called protein hormones) are made from amino acids — they are essentially small proteins. Examples: insulin, glucagon, growth hormone, ADH (antidiuretic hormone), oxytocin, TSH, FSH, LH, ACTH. Steroid hormones are made from cholesterol (a lipid). Examples: cortisol, testosterone, oestrogen, progesterone, aldosterone, vitamin D (technically a steroid). These two types work in completely different ways, which we'll explore in the next sections.

Peptide hormones are made of amino acids, so they are water-soluble (they dissolve in blood easily) but cannot cross the cell membrane (which is a lipid bilayer — fat-based, not water-friendly). Because they can't get inside the cell, peptide hormones bind to receptors on the surface of the cell — the receptor sits in the cell membrane. Think of it like knocking on the door but never entering the house. When a peptide hormone binds its surface receptor, it triggers a chain of reactions INSIDE the cell through second messengers — molecules that relay the signal from the membrane to the inside. The most common second messenger is cAMP (cyclic AMP). The signalling cascade in simple terms: 1. Hormone (first messenger) arrives and binds surface receptor 2. Receptor activates a protein called a G protein inside the membrane 3. G protein activates adenylyl cyclase enzyme 4. Adenylyl cyclase converts ATP → cAMP (second messenger) 5. cAMP activates Protein Kinase A (PKA) 6. PKA adds phosphate groups to target proteins, switching them on or off 7. This changes what the cell does (e.g. breaks down glycogen, contracts a muscle, secretes a substance) Key point: The hormone itself never enters the cell. The signal is amplified at each step — one hormone molecule can trigger hundreds of second messenger molecules, causing a large cellular response. Examples in action: - Glucagon binds liver cells → cAMP → PKA activation → glycogen breakdown → glucose released into blood - Adrenaline (epinephrine) binds heart and muscle cells → cAMP → increased heart rate, enhanced muscle performance ("fight or flight")

Steroid hormones are made from cholesterol, so they are lipid-soluble (fat-soluble). Because the cell membrane is made of lipids, steroid hormones can pass straight through it — like a key that can walk through walls. This means steroid hormones bind to receptors INSIDE the cell — either in the cytoplasm or in the nucleus itself. Once bound, the hormone-receptor complex travels to the nucleus and directly influences gene transcription (which genes get read to make proteins). The signalling cascade in simple terms: 1. Steroid hormone crosses the cell membrane freely (no receptor needed on the surface) 2. Binds an intracellular receptor in the cytoplasm or nucleus 3. Hormone-receptor complex travels to the nucleus 4. Complex binds specific DNA sequences called hormone response elements (HREs) 5. This switches certain genes ON or OFF 6. New mRNA is made → translated into specific proteins 7. These new proteins change what the cell does Key point: Steroid hormone effects are SLOW but LONG-LASTING. Because they change which genes are expressed, building new proteins takes hours. But the effects persist for hours to days. Examples in action: - Testosterone/Oestrogen → enter cells in reproductive organs → switch on genes for growth and differentiation → development of sex characteristics - Cortisol (stress hormone from adrenal glands) → enters immune cells → switches off genes for inflammation → suppresses immune response. This is why synthetic cortisol (like prednisolone, hydrocortisone) is used to treat inflammatory diseases. - Aldosterone → enters kidney cells → switches on genes for sodium channel proteins → more Na⁺ is reabsorbed → blood volume and pressure increase Steroid hormones in medicine: Corticosteroids (like prednisolone and dexamethasone) are among the most widely used drugs in medicine. They treat asthma, rheumatoid arthritis, allergic reactions, organ transplant rejection, and many autoimmune diseases — all by suppressing genes for inflammation.

Here is a clear comparison to help you remember the key differences: Water solubility: - Peptide hormones: water-soluble (dissolve in blood) ✓ — NOT fat-soluble ✗ - Steroid hormones: fat-soluble (dissolve in membranes) ✓ — NOT water-soluble ✗ (they need carrier proteins in blood) Where receptor is located: - Peptide hormones: receptor on cell SURFACE (the hormone stays outside) - Steroid hormones: receptor INSIDE the cell (hormone crosses the membrane) How the signal travels inside the cell: - Peptide hormones: via second messengers (cAMP, calcium, etc.) → fast, amplified - Steroid hormones: directly into the nucleus → changes gene expression Speed of action: - Peptide hormones: FAST — seconds to minutes (second messengers act quickly) - Steroid hormones: SLOW — hours (new proteins must be made) Duration of effect: - Peptide hormones: SHORT — effect stops when hormone is removed - Steroid hormones: LONG-LASTING — gene expression changes persist Can be given orally? - Peptide hormones: generally NO — the gut digests them like any protein (insulin must be injected) - Steroid hormones: YES — they can be absorbed from the gut (e.g. the contraceptive pill, prednisolone tablets)

The body doesn't just produce hormones and leave them running. Hormone levels are tightly regulated by feedback loops — the same principle as a thermostat controlling room temperature. Negative feedback is the most common system. Here's how it works using thyroid hormone as an example: 1. The hypothalamus (brain) senses low thyroid hormone → releases TRH (thyrotropin-releasing hormone) 2. TRH travels to the pituitary gland → pituitary releases TSH (thyroid stimulating hormone) 3. TSH travels to the thyroid gland → thyroid produces T3 and T4 (thyroid hormones) 4. T3/T4 rise in the blood → detected by hypothalamus and pituitary → they REDUCE TRH and TSH production 5. Thyroid hormone levels come back down → the system self-corrects This hypothalamus-pituitary-target gland system is called the hypothalamic-pituitary axis and it controls: thyroid function, cortisol production, sex hormones, and growth hormone. Clinical relevance — why this matters: - Hypothyroidism (underactive thyroid): low T3/T4 → pituitary senses this → produces more TSH → HIGH TSH is the blood test finding. Patient feels cold, tired, gains weight. - Hyperthyroidism (overactive thyroid — e.g. Graves' disease): too much T3/T4 → pituitary suppressed → LOW TSH on blood test. Patient feels hot, anxious, loses weight, fast heart rate. - Testing TSH is usually the FIRST test done when a thyroid problem is suspected — because the pituitary is so sensitive, TSH changes before T3/T4 go obviously abnormal.

Diabetes mellitus — insulin and glucagon Blood glucose is controlled by two opposing hormones from the pancreas: - Insulin (peptide hormone from beta cells): lowers blood glucose — tells cells to take up glucose and tells the liver to store it as glycogen - Glucagon (peptide hormone from alpha cells): raises blood glucose — tells the liver to break down glycogen and release glucose Type 1 diabetes: The immune system destroys the beta cells — no insulin is made. Without insulin, cells can't take up glucose. Blood glucose skyrockets. Fat is broken down for fuel instead, producing ketones — in large amounts this causes dangerous diabetic ketoacidosis (DKA). Treatment: insulin injections for life. Type 2 diabetes: Cells become resistant to insulin — they don't respond properly even when insulin is present. Initially the pancreas compensates by making more insulin, but over time it can't keep up. Strongly linked to obesity and inactivity. Treatment: lifestyle changes, metformin, and other drugs. Cushing's syndrome — too much cortisol Cortisol is a steroid hormone from the adrenal glands. It's the main stress hormone. Excess cortisol causes: central obesity (fat around the belly and face — "moon face" and "buffalo hump"), stretch marks (purple striae), thin skin, high blood pressure, high blood glucose, and osteoporosis. Most often caused by long-term corticosteroid medication. Addison's disease — too little cortisol The adrenal glands fail to produce enough cortisol and aldosterone. Causes weakness, weight loss, low blood pressure, and darkened skin. An Addisonian crisis (acute adrenal insufficiency) is life-threatening — treated urgently with intravenous hydrocortisone.