Scientists have identified more than 50 hormones in the human body. These chemical messengers travel through your bloodstream, carrying instructions from one part of the body to another. They regulate nearly everything: your energy levels, mood, growth, blood sugar, sleep, reproduction, and how you respond to stress. Most people think of hormones as coming from a handful of glands, but your heart, gut, fat tissue, skin, and kidneys produce them too.
How Hormones Work
Hormones are produced by specialized cells, released into the bloodstream, and picked up by target cells elsewhere in the body. Each hormone fits into a specific receptor on a cell’s surface or interior, like a key in a lock. Once that connection is made, the cell changes its behavior: it might start burning glucose, building protein, releasing another chemical, or dividing to grow new tissue. A tiny amount of hormone can trigger a large response, which is why even small imbalances can cause noticeable symptoms.
Your body keeps hormone levels in check through feedback loops. The thyroid system is a good example. The brain releases a signaling hormone that tells the thyroid to produce thyroid hormone. When thyroid hormone levels in the blood rise above a certain threshold, the brain detects this and stops sending the signal. Thyroid hormone production slows down. When levels drop again, the cycle restarts. This self-correcting mechanism keeps levels within a narrow range and applies to most hormones in the body.
The Major Endocrine Glands
The primary hormone-producing glands are the pituitary, pineal, thymus, thyroid, adrenal glands, and pancreas. The ovaries and testes round out the list. Each gland has a distinct job, though many of them influence one another in cascading chains of signals.
The pituitary gland, a pea-sized structure at the base of the brain, is sometimes called the “master gland” because it sends hormones that direct other glands to act. It controls growth, milk production after childbirth, and the activity of the thyroid and adrenal glands. The pineal gland, also in the brain, produces melatonin, which governs your sleep-wake cycle. The thymus, located behind the breastbone, plays its biggest role in childhood by releasing hormones that help immune cells mature.
Thyroid Hormones and Metabolism
Your thyroid gland, a butterfly-shaped organ in the front of your neck, is mainly responsible for controlling the speed of your metabolism. It does this by releasing two hormones, commonly known as T3 and T4, that affect every cell and organ in your body. These hormones regulate how quickly you burn calories, how fast your heart beats, how warm you feel, and even how efficiently your digestive system moves food through.
Too much thyroid hormone speeds up your metabolism, potentially causing weight loss, anxiety, and a racing heart. Too little slows it down, leading to fatigue, weight gain, and sensitivity to cold. A blood test measuring TSH (the brain’s signal to the thyroid) is one of the most common hormone tests. Normal TSH levels generally fall between 0.5 and 4.0 micro-units per milliliter.
Insulin, Glucagon, and Blood Sugar
The pancreas produces two hormones that work as a team to keep blood sugar stable. Insulin lowers blood sugar by helping glucose move out of the bloodstream and into cells, where it’s used for energy. Glucagon does the opposite: when blood sugar drops too low, the pancreas releases glucagon, which signals the liver to convert stored glucose into a usable form and release it back into the blood. Glucagon also prompts the body to make new glucose from other sources, such as amino acids from protein.
These two hormones constantly counterbalance each other throughout the day. After a meal, insulin rises to handle the incoming sugar. Between meals or overnight, glucagon takes over to keep levels from falling too far. When this system breaks down, as it does in diabetes, blood sugar swings become harder for the body to manage on its own. Fasting insulin levels in a healthy person typically stay below 20 micro-units per milliliter.
Stress Hormones: Adrenaline and Cortisol
Your two adrenal glands, which sit on top of your kidneys, handle the body’s stress response in two phases. The immediate reaction comes from adrenaline (also called epinephrine) and noradrenaline, produced in the inner part of the adrenal gland. These “fight or flight” hormones increase your heart rate, push more blood toward your muscles and brain, and help your body tap into glucose for quick energy. They also tighten blood vessels to maintain blood pressure. This response kicks in within seconds of a perceived threat.
Cortisol operates on a slower, longer timeline. Produced in the outer layer of the adrenal gland, cortisol helps your body sustain its response during ongoing stress by controlling how you use fats, proteins, and carbohydrates. It raises blood sugar, suppresses inflammation, and regulates blood pressure. Cortisol also follows a daily rhythm: it peaks in the early morning to help you wake up and gradually declines through the evening. Chronic stress can disrupt this pattern, keeping cortisol elevated when it should be dropping, which over time affects sleep, immune function, and weight.
Sex and Reproductive Hormones
The ovaries produce estrogen, progesterone, and a small amount of testosterone. Estrogen drives the development of female sex characteristics during puberty, regulates the menstrual cycle, and supports bone density. Progesterone prepares the uterine lining for pregnancy each month and, during pregnancy, the placenta takes over producing large amounts of both estrogen and progesterone to maintain the pregnancy and support the developing fetus.
The testes produce testosterone, which drives male puberty, sperm production, muscle mass, and bone strength. Normal testosterone levels in adult males range from about 291 to 1,100 nanograms per deciliter, while in females the range is much lower, roughly 18 to 54 nanograms per deciliter. Testosterone levels in all people decline gradually with age, which can affect energy, mood, and body composition.
Melatonin and Your Sleep Cycle
The pineal gland releases melatonin in response to darkness. As evening light fades, melatonin production rises, signaling to your body that it’s time to sleep. When light hits your eyes in the morning, your brain suppresses melatonin and starts ramping up cortisol instead, helping you feel alert. This interplay between melatonin and cortisol is central to your circadian rhythm. Blue light from screens can delay melatonin release, which is one reason late-night phone use makes it harder to fall asleep.
Hormones From Unexpected Places
Some of the body’s most important hormones come from organs you wouldn’t normally think of as hormone producers. Your heart releases natriuretic peptides when blood pressure rises. These hormones relax small blood vessels and tell the kidneys to release more sodium, both of which help bring blood pressure back down.
Your gut is especially active. The stomach and small intestine produce ghrelin, which stimulates appetite and triggers growth hormone release. After you eat, the small intestine releases GLP-1 (a hormone that enhances insulin production and helps regulate appetite), cholecystokinin (which aids digestion), and peptide YY (which signals fullness). GLP-1 has become widely recognized in recent years because medications that mimic its action are now used to treat type 2 diabetes and obesity.
Fat tissue is another major hormone source. Fat cells release leptin, which sends signals to the brain about how much energy the body has stored. They also produce adiponectin, which improves insulin sensitivity and helps protect against type 2 diabetes, and aromatase, an enzyme involved in sex hormone metabolism. The skin, liver, and kidneys work together to produce the active form of vitamin D, technically a hormone, which controls blood calcium levels and supports the immune system.
What Disrupts Hormonal Balance
Because hormones depend on tightly regulated feedback loops, a problem in one part of the system often ripples outward. An underactive thyroid doesn’t just slow metabolism; it can raise cholesterol and affect mood. Chronically high insulin from a diet heavy in refined carbohydrates can eventually make cells less responsive to insulin’s signal, a condition known as insulin resistance. Disrupted sleep throws off melatonin and cortisol rhythms, which in turn can affect appetite hormones like ghrelin and leptin.
Age, stress, sleep quality, nutrition, and body composition all influence hormone levels. Many hormonal shifts are a normal part of life, like the gradual decline in estrogen during menopause or the slow drop in testosterone that begins in a man’s 30s. Others reflect a treatable underlying condition. Blood tests can measure most major hormones, and patterns in those results, rather than any single number, typically tell the fuller story.