The Human Endocrine System and Key Glands
The endocrine system is a set of glands that release hormones, chemical messengers, directly into the blood, which carries them to target cells elsewhere in the body where they change the cells’ activity. In brief: the main endocrine glands are the pituitary, thyroid, adrenal glands, pancreas, and the ovaries and testes. The pituitary is often called the “master gland” because it controls several others, although it is itself controlled by the hypothalamus in the brain.
The endocrine system works more slowly than the nervous system, but its effects last longer and control long-term processes such as growth, metabolism and reproduction. This guide explains what hormones are and how they act, describes the key endocrine glands and their hormones, summarizes them in a table, works through a hormone-matching example and corrects frequent mistakes.
What are hormones?
A hormone is a chemical substance produced by an endocrine gland and released into the bloodstream. It travels all over the body, but affects only target cells, which have specific receptors for that hormone. Endocrine glands are ductless, which means they have no tubes to carry secretions out, in contrast to exocrine glands such as salivary glands.
Hormones are of two chemical types. Peptide and protein hormones (such as insulin) bind to receptors on the cell surface and trigger a series of reactions inside. Steroid hormones (such as estrogen and testosterone) are lipid-soluble, pass through the cell membrane and act on the genes in the nucleus. Hormones act at very low concentrations and are broken down by the liver or removed by the kidneys, so their effects can be switched off.
What does the pituitary gland do?
The pituitary gland is a pea-sized gland at the base of the brain, attached to the hypothalamus. It has two parts. The anterior pituitary produces hormones that control other glands, including growth hormone (GH), thyroid-stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), adrenocorticotropic hormone (ACTH) and prolactin. The posterior pituitary stores and releases two hormones made in the hypothalamus: antidiuretic hormone (ADH), which regulates water balance in the kidneys, and oxytocin, which causes contractions of the uterus in birth and the release of milk.
Too little growth hormone in childhood causes dwarfism, while too much causes gigantism. In adults, too much leads to acromegaly, in which the hands, feet and face grow larger.
What does the thyroid gland do?
The thyroid gland lies in the front of the neck, below the larynx. It produces thyroxine (T4) and triiodothyronine (T3), which contain iodine. These hormones regulate the metabolic rate, which is how fast cells use energy, and they are needed for normal growth and brain development. The pituitary controls the thyroid through TSH, in a negative feedback loop.
- Hypothyroidism (too little thyroxine) causes tiredness, weight gain and sensitivity to cold.
- Hyperthyroidism (too much thyroxine) causes weight loss, a fast heartbeat and sweating.
- Goiter, a swelling of the thyroid, can be caused by lack of iodine in the diet. Adding iodine to table salt has greatly reduced it.
What do the adrenal glands do?
The two adrenal glands sit on top of the kidneys. The inner part (medulla) releases adrenaline (epinephrine) and noradrenaline in response to stress or danger, in the “fight or flight” response. Adrenaline increases heart rate and breathing rate, raises blood glucose by causing the liver to release glucose, widens the pupils and redirects blood to the muscles. The outer part (cortex) produces steroid hormones, including cortisol, which helps the body respond to long-term stress and keeps blood glucose up, and aldosterone, which helps to control sodium levels and blood pressure.
What is the endocrine role of the pancreas?
The pancreas has endocrine cells in clusters called the islets of Langerhans. Beta cells release insulin, which lowers blood glucose, and alpha cells release glucagon, which raises it. Together, they keep the glucose concentration within narrow limits. Diabetes mellitus occurs when this control fails. Control of blood glucose is explained in more detail in the next article.
What do the ovaries and testes do?
The testes produce testosterone, which controls the development of male features at puberty and sperm production. The ovaries produce estrogen and progesterone, which control female development at puberty and the menstrual cycle, and they maintain pregnancy. Their activity is controlled by FSH and LH from the pituitary.
Key endocrine glands and hormones
| Gland | Hormone | Main effect |
|---|---|---|
| Pituitary (anterior) | GH, TSH, FSH, LH, ACTH | Growth; controls other glands |
| Pituitary (posterior) | ADH, oxytocin | Water balance; contractions, milk release |
| Thyroid | Thyroxine | Raises metabolic rate |
| Adrenal medulla | Adrenaline | Fight or flight |
| Adrenal cortex | Cortisol, aldosterone | Stress response; sodium balance |
| Pancreas | Insulin, glucagon | Lower and raise blood glucose |
| Ovaries | Estrogen, progesterone | Female development, cycle |
| Testes | Testosterone | Male development, sperm |
Worked example: matching symptoms to glands
Match each case to the likely gland and hormone problem.
- A child is much shorter than other children of the same age and grows slowly. Growth is controlled by growth hormone from the anterior pituitary. A lack of this hormone is likely.
- An adult is tired, gains weight and feels cold. These are signs of a low metabolic rate, which suggests too little thyroxine (hypothyroidism).
- A person has a fast heartbeat, sweating, wide pupils and a feeling of alertness after a sudden fright. This is the effect of adrenaline from the adrenal medulla.
- A person with constant thirst, frequent urination and high blood glucose. This suggests a failure of insulin action from the pancreas (diabetes mellitus).
- A person produces large volumes of dilute urine and is always thirsty, but blood glucose is normal. This points to too little ADH (diabetes insipidus), which is a pituitary problem.
The method is to ask what process the symptom involves (growth, metabolism, stress, sugar or water) and then recall which hormone controls it. Doctors confirm the diagnosis with blood tests, since several conditions can have similar symptoms.
How was the endocrine system discovered?
Arnold Berthold showed in 1849 that chicken testes release something into the blood that affects the comb and behavior. In 1902 William Bayliss and Ernest Starling discovered secretin, a substance made by the small intestine that stimulates the pancreas, and in 1905 Starling coined the word “hormone”, from a Greek word meaning “to set in motion”. Insulin was isolated in 1921 by Frederick Banting and Charles Best.
Which mistakes are common?
Mistake 1: Saying that hormones travel along nerves
They travel in the blood. Nerve signals travel in neurons.
Mistake 2: Thinking hormones affect all cells
Only target cells with matching receptors respond.
Mistake 3: Mixing up exocrine and endocrine glands
Exocrine glands have ducts (sweat, salivary). Endocrine glands release hormones into the blood.
Mistake 4: Calling the pituitary the top of the control system
The hypothalamus controls the pituitary. The pituitary controls many other glands.