How does the endocrine system work?

Your endocrine system is a set of glands that make hormones: chemical messages that travel in your blood. We are facing the person, so their right side is on your left.

In the head, the hypothalamus at the floor of the brain is the boss. Just under it, on a thin stalk, hangs the pea-sized pituitary, often called the master gland because its hormones switch other glands on. Deep in the middle of the brain sits the tiny pineal gland, which makes melatonin at night.

In the neck, the butterfly-shaped thyroid sets how fast your cells burn fuel. Four rice-sized parathyroids on its back keep your blood calcium steady. Behind the breastbone, the thymus helps train immune cells while you are young, then slowly shrinks.

Lower down, an adrenal gland sits on top of each kidney, making cortisol and adrenaline. Tiny islets scattered through the pancreas make insulin and glucagon (PancreasClear shows them up close). Finally, the ovaries or testes make the hormones that drive puberty.

Glands like these have no tubes: they pour hormones straight into the blood. That is what "endocrine" means, "secreting inside". Many organs join in too: the kidneys make EPO and renin (KidneyClear), the heart, stomach and fat make hormones, and your skin makes vitamin D (SkinClear).

A gland releases its hormone into the blood, and the heart pumps it all round the body in about a minute. So a hormone reaches every cell. Why doesn't every cell react?

Because each hormone is like a key, and only cells with the matching lock respond. The lock is a protein called a receptor. TSH from the pituitary fits receptors found on thyroid cells, so it only wakes up the thyroid. ACTH only fits the adrenal glands. Adrenaline fits receptors on the heart (beat faster) and the liver (release sugar).

Thyroxine is different: it slips inside cells and fits receptors in the nucleus of almost every cell, so it changes the pace of your whole body. Steroid hormones like cortisol also work from inside the cell.

The more hormone in the blood, the more receptors are filled and the stronger the response. When the level drops, the keys fall out and the cell calms down.

Compare that with nerves (see NervousClear): a nerve is a private wire to one target, and its message arrives in milliseconds. Hormones are more like a radio broadcast: slower (seconds to days), but they reach the whole body and can last for hours, weeks or years.

How does your body keep a hormone at the right level, not too much and not too little? The same way a thermostat keeps a room warm: it measures, compares with a set point, and corrects.

The hypothalamus is the thermostat. When thyroid hormone is low, it sends TRH down a tiny private set of blood vessels to the pituitary. The pituitary answers by releasing TSH into the blood. TSH reaches the thyroid, which makes thyroxine (T4). That is a chain of command: brain, master gland, worker gland.

Then the clever part: thyroxine travels back to the brain, and when there is enough, it tells the hypothalamus and pituitary to ease off. This is negative feedback. Too much hormone turns its own supply down; too little turns it up. So the level settles back to normal.

The stress axis works the same way: CRH → ACTH → cortisol from the adrenal glands, with cortisol switching the chain off. Cortisol also follows a daily rhythm: it is highest soon after you wake up and lowest around midnight.

Because the pituitary reacts so strongly, doctors check TSH to see how the thyroid is doing: a tired thyroid makes TSH climb long before thyroxine falls very far.

Your brain burns about 120 g of glucose a day and can't store any, so your blood sugar has to stay steady, usually between about 70 and 140 mg/dL. Two hormones from the islets of the pancreas steer it, like two hands on a see-saw (PancreasClear shows them up close).

After a meal, glucose pours in from the gut. The islets' beta cells release insulin, and insulin tells muscle, fat and the liver to take glucose in. The liver stores it as glycogen. Within 2 to 3 hours your blood sugar is back to normal.

When you fast, insulin falls and the alpha cells release glucagon. Glucagon tells the liver to break its glycogen back into glucose. The liver holds only enough for about a day; after that it makes new glucose from other building blocks. So even after 36 hours without food, blood sugar only dips a little.

A sudden scare is different. Nerves speed your heart within a second, and the adrenal glands squirt adrenaline into the blood within seconds: faster heart, more sugar released for your muscles. Minutes later cortisol arrives through the stress axis and keeps sugar up for an hour or more. This is fight or flight (NervousClear shows the nerve side).

Some hormones act in seconds. Others work slowly for years and shape who you become.

Growth hormone from the pituitary makes your bones and muscles grow. It works through the growth plates near the ends of long bones (SkeletonClear shows them). It is released in bursts, the biggest in deep sleep, one more reason sleep matters. Thyroid hormone is needed too: it sets how fast your cells burn fuel and helps the brain and bones develop.

Puberty starts in the brain. Usually somewhere between about 8 and 13 in girls and 9 and 14 in boys, the hypothalamus starts sending pulses of GnRH. The pituitary answers with LH and FSH, which wake up the ovaries (making oestrogen and progesterone) or the testes (making testosterone). These bring the growth spurt, body hair, a deeper voice in boys and periods in girls. Everyone's timing is different, and that is normal.

At night the pineal gland makes melatonin, the "it's dark, time to sleep" signal. Bright screens late at night can push it later and lower it, which makes it harder to fall asleep. Teenagers need about 8 to 10 hours of sleep.

The thyroid needs iodine to make thyroxine: each molecule carries four iodine atoms. Without enough, the thyroid swells into a goitre, and children's growth and learning can suffer. That is why India adds iodine to salt: the iodised salt programme began in 1962, and since 1992 all salt for eating is meant to be iodised.

Because hormones reach the whole body, a gland that makes too much or too little can cause changes almost anywhere. The good news: most hormone problems can be found with a blood test and looked after well. This chapter explains common ones; it cannot tell you what is happening in your own body. For that, see a doctor.

Diabetes means blood sugar stays too high. In type 1, the immune system destroys the insulin-making cells, often in childhood, so insulin is needed for life. In type 2, the most common kind, insulin works poorly and too little is made. India has about 101 million people with diabetes (ICMR-INDIAB, 2023) and many more at risk; being active, eating well and keeping a healthy weight lower the risk. PancreasClear tells this story in depth.

The thyroid can be underactive (hypothyroidism: tired, cold, slow) or overactive (hyperthyroidism: racing heart, weight loss, shaky). About 1 in 10 Indian adults has an underactive thyroid. Many hospitals check newborns with a heel-prick blood test, because an underactive thyroid at birth harms growth and learning if it is missed. A goitre is a swollen thyroid, most often from too little iodine.

Growth disorders are rare: too little growth hormone slows growth, and too much makes a child grow very tall. Most short children are simply healthy and short, like their parents. PCOS (polycystic ovary syndrome) is a common hormone condition that can cause irregular periods; it is another reason to talk to a doctor.

Never start, stop or share hormone medicines without a doctor. Hormones are powerful at tiny amounts.