The endocrine system is your body’s chemical messaging network. It consists of glands scattered throughout your body that produce hormones, release them into your bloodstream, and deliver instructions to organs and tissues far from where those hormones originated. This system controls some of the most fundamental processes in your body: metabolism, growth, reproduction, sleep, mood, and how you respond to stress.
How the System Works
The endocrine system operates on three components: glands that manufacture hormones, the bloodstream that carries them, and receptors on target cells that recognize and respond to them. Hormones travel through your entire circulatory system, but only cells with the right receptors can pick up the signal. Think of it like a radio broadcast that reaches everywhere, but only a tuned receiver plays the station.
This makes the endocrine system fundamentally different from your nervous system. Nerve signals travel along specific wires (neurons) and arrive in milliseconds. Hormonal signals spread through the blood and can take seconds to hours to produce their effects, but those effects often last much longer. The two systems work together constantly, with the brain’s hypothalamus serving as the primary bridge between them.
Two Types of Hormones
Hormones fall into two broad categories based on their chemical structure, and that structure determines how they interact with your cells.
Steroid hormones, like estrogen and cortisol, are made from lipids (fats). Because cell membranes are also made of lipids, steroid hormones can pass directly through the membrane and into the cell. Once inside, they bind to receptor proteins and travel to the nucleus, where they influence which genes get turned on or off. This is why steroid hormones tend to produce slower but more lasting changes in the body.
Non-steroid hormones, like insulin, are made from amino acids. They can’t cross the cell membrane, so instead they attach to receptors on the outside surface of the cell. That binding triggers a chain reaction inside the cell using what’s called a “second messenger,” a molecule that relays the hormone’s instructions internally. These signals tend to produce faster, shorter-lived effects.
The Major Glands and What They Do
Seven primary glands make up the core of the endocrine system, each with a distinct role.
The hypothalamus, located at the base of the brain, is the control center. It links the nervous system to the endocrine system by producing hormones that tell the pituitary gland what to do. The pituitary gland, a pea-sized structure just below the hypothalamus, is often called the “master gland” because its hormones regulate most of the other endocrine glands. It produces growth hormone, among others, which drives bone elongation and muscle development during childhood and adolescence. During puberty, growth hormone works alongside sex hormones to fuel the adolescent growth spurt.
The thyroid gland in your neck controls your metabolic speed. Its hormones increase your basal metabolic rate, the amount of energy your body burns at rest. They stimulate carbohydrate metabolism, promote protein building, and raise your body temperature by increasing oxygen consumption across tissues. When your thyroid is underactive, everything slows down: you feel cold, tired, and sluggish. When it’s overactive, your metabolism runs too hot, causing weight loss, rapid heartbeat, and excess heat production.
The parathyroid glands, four tiny glands behind the thyroid, regulate calcium levels in your blood, which is critical for bone health, muscle contraction, and nerve function. The adrenal glands, sitting on top of your kidneys, produce cortisol (your primary stress hormone) along with small amounts of sex hormones. The pancreas produces insulin and glucagon, the two hormones that keep your blood sugar in balance. And the gonads, the ovaries in women and testes in men, produce the sex hormones that drive reproduction, sexual development, and bone density.
Organs You Wouldn’t Expect
The endocrine system extends beyond the traditional glands. Your fat tissue produces leptin, a hormone that signals your brain to reduce appetite and increase energy expenditure. The more fat cells you have, the more leptin you produce, though the brain can become less responsive to it over time. Your stomach produces ghrelin, a hormone that does the opposite: it stimulates hunger and triggers growth hormone release. The gut also contains specialized cells that secrete hormones involved in blood sugar regulation after meals. Even the heart produces a hormone that helps regulate blood pressure.
The Stress Response
One of the endocrine system’s most dramatic functions is orchestrating your body’s reaction to stress. The process follows a specific chain of command. When you perceive a threat, the hypothalamus releases a signaling hormone that travels to the pituitary gland. The pituitary responds by sending another hormone (ACTH) into the bloodstream, which reaches the adrenal glands and triggers cortisol production.
Cortisol’s job is to mobilize energy. It tells the liver to release stored glucose, promotes the breakdown of fat reserves, and redirects resources away from non-essential functions toward muscles and the brain. This system evolved to help you survive physical danger, releasing stored fuel so you could fight or run. The response works for both immediate threats and situations your brain merely anticipates could become threatening. Once the stressor passes, cortisol levels feed back to the hypothalamus and pituitary to shut the cycle down. Chronic stress disrupts this feedback loop, keeping cortisol elevated and contributing to problems like weight gain, high blood sugar, and weakened immune function.
Common Endocrine Disorders
Because the endocrine system touches nearly every process in your body, problems with hormone production or signaling can have wide-ranging effects. The most prevalent endocrine disorder by far is diabetes, a condition where the pancreas either stops producing insulin (Type 1) or the body’s cells stop responding to it effectively (Type 2).
About 15.8% of U.S. adults now have diabetes, according to CDC data from 2021 to 2023. That includes 11.3% who have been diagnosed and another 4.5% who have it but don’t know it yet. The prevalence rises sharply with age: 3.6% of adults in their 20s and 30s have diabetes, compared to 27.3% of those 60 and older. Weight plays a significant role as well. Adults with obesity have a diabetes prevalence of 24.2%, roughly four times the rate of those at a normal weight. Overall, diabetes rates in the U.S. have climbed from about 9.7% in 1999-2000 to 14.3% in 2021-2023 after adjusting for age.
A common screening tool is the A1C test, which measures your average blood sugar over the past two to three months. A result below 5.7% is normal, 5.7% to 6.4% indicates prediabetes, and 6.5% or higher means diabetes.
Thyroid disorders are also common. Hypothyroidism (an underactive thyroid) causes fatigue, weight gain, cold sensitivity, and depression. Hyperthyroidism (an overactive thyroid) causes the opposite: anxiety, weight loss, heat intolerance, and a racing heart. Both conditions are diagnosed with blood tests that measure thyroid hormone levels and are typically managed with medication that either replaces or suppresses thyroid hormone production.
How the System Stays in Balance
The endocrine system relies heavily on feedback loops to maintain stable hormone levels. Most of these are negative feedback loops, meaning that when a hormone reaches a certain concentration, it signals the gland to stop producing more. Your thermostat works the same way: once the room hits the target temperature, the heater shuts off.
For example, when thyroid hormone levels rise too high, the pituitary gland reduces its signal to the thyroid, slowing production. When levels drop, the signal increases. This constant adjustment keeps hormone levels within a narrow range, and most endocrine disorders come down to a breakdown somewhere in this feedback cycle, whether from autoimmune damage to a gland, a tumor that overproduces a hormone, or cells that stop responding to a hormone’s signal.