A hormone is a chemical messenger your body produces in one place, releases into the bloodstream, and uses to control something happening somewhere else entirely. A hormone made in your brain can change what's happening in your testes, your bones, your fat tissue, and your mood, all at the same time. That's what makes the endocrine system so powerful, and why disruptions to it have such wide-ranging effects. Hormones split into two main types based on what they're built from:

• Steroid hormones (testosterone, oestrogen, cortisol, progesterone, DHT) are built from cholesterol. They're fat-soluble, which means they pass straight through cell membranes and bind to receptors inside the cell, usually in the nucleus, where they directly influence which genes get read. This is why steroid hormones have such deep, structural effects, testosterone telling a muscle cell to make more protein, cortisol telling your liver to release glucose, oestrogen telling a fibroblast to produce more collagen, all the result of a steroid hormone reaching the nucleus and flipping switches on the DNA.
• Peptide hormones (LH, FSH, GnRH, insulin, growth hormone) are protein-based and water-soluble. They can't pass through the cell membrane, so they bind to receptors on the cell's surface and trigger signalling cascades inside. This is how LH hits a Leydig cell and triggers the entire testosterone production chain without ever entering the cell.

The endocrine system is a network of glands communicating through these messengers, most of them regulated by feedback loops. Hormone rises, the gland slows down. Hormone falls, the gland ramps back up.

The endocrine system

Each gland has a specific role, but they don't operate independently, they influence each other constantly through feedback loops and shared signalling pathways.

Hypothalamus sits at the base of the brain and is the master regulator. It links the nervous system to the endocrine system, translating signals from the brain (stress, sleep, temperature, energy status) into hormonal commands. It produces releasing hormones like Gonadotropin-Releasing Hormone GnRH (which controls sex hormones), CRH (which controls cortisol), and TRH (which controls thyroid hormones). Almost every hormonal cascade in the body starts here.

Pituitary gland. Sits just below the hypothalamus and relays its commands to the rest of the body. The anterior pituitary produces LH, FSH, ACTH, TSH, growth hormone, and prolactin. The posterior pituitary releases oxytocin and vasopressin. When the hypothalamus sends a releasing hormone, the pituitary responds by secreting the matching stimulating hormone into the bloodstream, which travels to the target gland.

Thyroid. Sits in the front of the neck and controls metabolic rate, energy, and body temperature through T3 and T4. Every cell in the body has thyroid receptors, which is why thyroid dysfunction affects everything, weight, energy, mood, hair, skin, heart rate, cognition. Controlled by TSH from the pituitary, which is controlled by TRH from the hypothalamus.

Pancreas. Sits behind the stomach and produces insulin and glucagon, the two hormones that regulate blood sugar. Insulin lowers it by telling cells to absorb glucose. Glucagon raises it by telling the liver to release stored glucose. Insulin resistance, where cells stop responding properly to insulin, is the underlying dysfunction in type 2 diabetes and is tightly linked to testosterone, body composition, and chronic inflammation.

Adrenals. Sit on top of each kidney. The outer cortex produces steroid hormones: cortisol, DHEA and DHEA-S (both controlled by ACTH from the pituitary), and aldosterone (controlled separately by the kidneys via the RAAS system). The inner medulla produces adrenaline and noradrenaline, the fight-or-flight hormones, and is wired directly to the sympathetic nervous system, which is why the response is almost instant.

Testes (in men) are both reproductive organs and endocrine glands. They contain Leydig cells, which produce testosterone in response to LH from the pituitary, and Sertoli cells, which support sperm production in response to FSH. The testes are the endpoint of the HPG axis and the primary source of testosterone in men.

Ovaries (in women) produce oestrogen, progesterone, and testosterone. They contain follicles that mature under FSH stimulation and release eggs during ovulation triggered by an LH surge. The ovaries are the endpoint of the HPG axis in women and cycle through different hormonal phases monthly, making female endocrinology more time-dependent than male.

Pineal gland sits deep in the centre of the brain and produces melatonin.

These glands don't operate in silos. The hypothalamus and pituitary sit on top of multiple parallel axes (HPG for sex hormones, HPA for stress, HPT for thyroid) that cross-talk constantly. Cortisol suppresses GnRH. Thyroid hormones influence SHBG. Insulin resistance affects testosterone. Inflammation suppresses the HPG axis at multiple levels.

The HPG axis