Explainer: What is a hormone? | Science News for Students

Explainer: What is a hormone?

These unsung heroes of the body inform cells when it’s time to go to work
Nov 1, 2017 — 2:31 pm EST
insulin structure

This is an artist’s representation of a molecule of insulin. This hormone helps the body use its food as energy.


All of us started as a single cell. Along the way, that cell divided and morphed in very individual ways. Some of us may have ended up short or tall, dark skinned or light, clever or slow, night owls or early birds. Scientists like to attribute most of those traits to inherited genes. But much of the work in crafting the traits that make each of us unique is performed by a family of chemicals known as hormones.

Various tissues of the body secrete hormones into fluids, like blood. From there, the hormones travel far from the place they were made until they reach cells that read the chemical as an instruction.

Estrogen formula
This is the molecular structure of estrogen, a primary reproductive hormone. Estrogen plays a role in crafting the body of females and helps to support fertility during what are known as a woman’s reproductive years.

That hormone might tell the cell to grow — or to stop. It might direct a cell to change its shape or activity. These instructions might cause the heart to pump more rapidly or signal hunger to the brain. Another hormone might let you know that you’re full. One hormone latches onto sugar in the bloodstream and then helps ferry that sugar into cells to fuel their work. Yet another might tell your body to burn some nutrients as fuel — or instead store their energy as fat for use at a later date.

What’s more, a hormone can have more than one role. For instance, estrogen is a hormone made by a woman’s ovaries. It helps to shape her body during puberty to look — and function — differently than a man’s. Indeed, during her reproductive years, monthly pulses of estrogen will prepare her breasts for the potential production of milk, something that would be needed if she became pregnant. But estrogen also sends signals to bone to become stronger. Different types of estrogens can even promote or thwart the growth of would-be cancers.

Receiving those messages

cell surface
Various tissues of the body secrete hormones into fluids, like blood. From there, the hormones travel far from the place they were made until they reach cells that read the chemical as an instruction.

Hormones essentially whisper their instructions to affected cells. The “ears” through which cells listen for that instruction are known as receptors. These are special structures on the outside of a cell. If a hormone’s chemical recipe and shape are just right, it will dock into the receptor, like a key into a lock. These receptors are known as “gatekeepers.” If and only if the right hormonal key arrives will that receptor unlock. Now some important, newly specified action will turn on.

Or at least that’s how it’s supposed to work.

Sometimes imposters arrive. Like fake keys, these may inappropriately turn on some cellular action.

Clover, soybeans, fungi and marijuana, for instance, evolved compounds that resemble the estrogen in mammals. Those molecules resemble hormones well enough that consuming some of these can fool the body into thinking it got a legitimate estrogen signal. In fact, it didn’t. This could even happen in males. Since estrogen is hormone that promotes feminine traits, that faulty signal could work to effectively feminize some male traits.

Some estrogen mimics may sit in the lock but fail to turn it on — or perhaps only slightly turn it on. They act like a bad key, stuck in the lock. Now if a true key shows up, it can’t enter the blocked receptor. So it can’t instruct the cell that it’s time to do its job. Some pesticides as well as chemicals used in plastics can do this. If these chemicals mimic testosterone, a male sex hormone, they may block some of the activity that would be turned on when true testosterone showed up. The result could be a male animal that now looks like a female.

Over the past three decades, scientists have been uncovering a growing number of chemicals that the body may mistake for hormones. These include a large number of commercial chemicals, such as pesticides, plasticizers and combustion byproducts. Together, scientists have come to refer to such materials as “environmental hormones.” Other times, they’re called hormone mimics or “endocrine disruptors.” That last term reflects that chemicals are central players in the body’s endocrine — or hormone — system.

Not just for humans

Hormones act throughout the living world.

One reason scientists often use animals as stand-ins for people is because their bodies work similarly. Their bodies often rely on the same hormones to do the same things as in the human body. From mice and pigs to fish, insects, birds and reptiles, creatures throughout the animal kingdom rely on hormones to develop, grow and live out healthy lives.

A number of hormones instruct plants when to grow up — or grow old and die. Others inform a plant that it’s time to form flowers, fruit and seeds so that it can reproduce. Still others trigger the plant to heal some wound or to enter dormancy.

Fungi rely on chemicals to signal when their tissues need to take certain actions, such as communicate with microbes in its root zone or begin spore formation (reproduction). Many such chemicals work as hormones. Sometimes, these chemicals will be identical to hormones produced by plants.

There are even bacteria that make hormones. Those hormones may be help a bacterium sense if has entered a host’s gut and now should attach to the intestinal wall so that it can settle in for a long stay. However, some of the signaling chemicals bacteria make can work primarily in their host (which may even be a human). For instance, some bacteria in the gut can fashion androgens (male reproductive hormones, such as testosterone) from inflammation-fighting chemicals in their environment.

Examples of some human hormones and roles they play

The human body makes around 50 different hormones, which direct the timing of actions by cells and tissues throughout the body. Here are some of them:

NamePrimary roleMain activities
AdrenalineStress hormoneKnown as the fight-or-flight hormone, it helps the body respond to stress by increasing heart and breathing rates and preparing muscles for exertion.
Estradiol (also known as estrogen)Sex hormoneIn females, this hormone promotes the growth of feminine traits (such as breasts and padded hips) and prepares the body — from puberty to menopause — to release eggs and nurture a developing fetus through birth. In males, this hormone helps in development of sperm and a healthy sex drive.
GhrelinHunger hormoneProduced mostly in the stomach, it alerts the brain that the body is running low on energy and it’s time to eat.
InsulinMetabolic hormoneIt helps the body move sugar in the bloodstream into cells where that sugar can be used as fuel.
LeptinSatiety hormoneSecreted mainly by fat cells, it tells the body when it’s had enough to eat. Leptin also signals when incoming food should be burned or stored as fat.
MelatoninSleep hormoneThis hormone is produced by the brain’s pineal gland and readies the body for sleep.
TestosteroneSex hormoneProduced by the testes in males, it tells the male body to develop masculine characteristics, such as facial and body hair, a deep voice and muscle strength. Produced in females by the ovaries and adrenal glands, it promotes such traits as the growth of underarm hair.
Thyroxine (also known as thyroid hormone or TH)Growth hormoneThis is the primary hormone secreted by the thyroid. It plays a role in promoting growth of the brain, bone and muscle. It also helps regulate activities of the heart and digestive tract.

Power Words

(more about Power Words)

adrenaline    A hormone produced by glands (adrenal) when someone is stressed by fear, anger or anxiety. It can make the heart beat faster and allow muscles to perform better than normal. Adrenaline is part of the body’s “fight or flight” response to stress. It can briefly help someone run faster or temporarily boost the performance of muscles (as for lifting weights).

androgens    A family of powerful male sex hormones, including testosterone.

bacteria     (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

cancer     Any of more than 100 different diseases, each characterized by the rapid, uncontrolled growth of abnormal cells. The development and growth of cancers, also known as malignancies, can lead to tumors, pain and death.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical also can be an adjective to describe properties of materials that are the result of various reactions between different compounds.

combustion     (adj. combustible ) The process of burning.

commercial     (in research and economics) An adjective for something that is ready for sale or already being sold. Commercial goods are those caught or produced for others, and not solely for personal consumption.

compound     (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.

develop     (in biology) To grow as an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape.

endocrine disruptor     A substance that mimics the action (sometimes well, sometimes poorly) of one of the body’s natural hormones. By doing this, the fake hormone can inappropriately turn on, speed up or shut down important cellular processes.

endocrine system     The hormones (chemicals secreted by the body) and the tissues in which they turn on (or off) cellular action. Medical doctors who study the role of hormones in health and disease are known as endocrinologists. So are the biologists who study hormone systems in non-human animals.

estradiol    The primary female sex hormone, a type of estrogen. Produced throughout life in both males and females. Among adult women, it’s produced mostly by the ovaries, to help the body prepare for the potential to reproduce (have a baby).

estrogen     The primary female sex hormone in most higher vertebrates, including mammals and birds. Early in development, it helps an organism develop the features typical of a female. Later, it helps a female’s body prepare to mate and reproduce.

fat     A natural oily or greasy substance occurring in plants and in animal bodies, especially when deposited as a layer under the skin or around certain organs. Fat’s primary role is as an energy reserve. Fat also is a vital nutrient, though it can be harmful if consumed in excessive amounts.

feminize     (in biology) For a male animal to take on physical, behavioral or physiological traits typical of females. It usually results from exposure to an abnormal amount of female sex hormones — or pollutants that mimic these hormones. Feminizing is sometimes used as a synonym for demasculinizing. In fact, they can be different. A demasculinized male may appear more feminine. But that will be largely because it had too little exposure to male hormones, not an excess of female hormones.

fruit     A seed-containing reproductive organ in a plant.

fuel     Any material that will release energy during a controlled chemical or nuclear reaction.

gene     (adj. genetic) A segment of DNA that codes, or holds instructions, for a cell’s production of a protein. Offspring inherit genes from their parents. Genes influence how an organism looks and behaves.

ghrelin    Known primarily as the “hunger” hormone, it is produced by the gut and pancreas to signal when the body needs to eat. It also plays a role in triggering the release of growth hormone. People who cut back on calories often produce excess quantifies of ghrelin — essentially the body’s automatic attempt to avoid what it senses may be the beginning of starvation.

gland    A cell, a group of cells or an organ that produces and discharges a substance (or “secretion”) for use elsewhere in the body or in a body cavity, or for elimination from the body.

hormone     (in zoology and medicine) A chemical produced in a gland and then carried in the bloodstream to another part of the body. Hormones control many important body activities, such as growth. Hormones act by triggering or regulating chemical reactions in the body. (in botany) A chemical that serves as a signaling compound that tells cells of a plant when and how to develop, or when to grow old and die.

inflammation    (adj. inflammatory) The body’s response to cellular injury and obesity; it often involves swelling, redness, heat and pain. It also is an underlying feature responsible for the development and aggravation of many diseases, especially heart disease and diabetes.

insulin    A hormone produced in the pancreas (an organ that is part of the digestive system) that helps the body use glucose as fuel.

leptin    A type of hormone made by fat cells that inhibit hunger.

masculinize     (in biology) For a female human or animal to take on physical, behavioral or physiological traits typical of males.

melatonin    A hormone secreted in the evening by a structure in the brain. Melatonin tells the body that it is nearing time to sleep. It plays a key role in regulating circadian rhythms.

menopause    The time in an older woman’s life that occurs following the permanent end of her menses (periods). It can occur at any point during a woman’s 40s or 50s, but usually starts around age 51. It happens as the production and cycling of reproductive hormones wanes, ending her ability to bear children. And there may be symptoms that show up for months or years, such as hot flashes (where the body’s thermostat goes haywire and can lead to sweating for no reason), and emotional symptoms that can disrupt sleep, lower a woman’s energy level or trigger anxiety or sadness.

metabolism    (adj. metabolic)  The set of life-sustaining chemical reactions that take place inside cells and bigger structures, such as organs. These reactions enable organisms to grow, reproduce, move and otherwise respond to their environments.

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

nutrient     A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.

pesticide     A chemical or mix of compounds used to kill insects, rodents or other organisms harmful to cultivated plants, pets or livestock; or unwanted organisms that infest homes, offices, farm buildings and other protected structures.

plasticizer     Any of several chemicals added to certain synthetic materials to make them soft and/or pliable.

primary    An adjective meaning major, first or most important.

puberty     A developmental period in humans and other primates when the body undergoes hormonal changes that will result in the maturation of reproductive organs.

receptor     (in biology) A molecule in cells that serves as a docking station for another molecule. That second molecule can turn on some special activity by the cell.

reptile     Cold-blooded vertebrate animals, whose skin is covered with scales or horny plates. Snakes, turtles, lizards and alligators are all reptiles.

satiety    A feeling of fullness. It’s the opposite of being hungry. The body tends to register satiety through the release of certain brain hormones. Someone who is full, after a meal, is said to be sated.

secrete     (noun: secretion) The natural release of some liquid substance — such as hormones, an oil or saliva — often by an organ of the body.

sex     An animal’s biological status, typically male or female. There are a number of indicators of biological sex, including sex chromosomes, gonads, internal reproductive organs, and external genitals.

stress    (in biology) A factor — such as unusual temperatures, movements, moisture or pollution — that affects the health of a species or ecosystem. (in psychology) A mental, physical, emotional or behavioral reaction to an event or circumstance (stressor) that disturbs a person or animal’s usual state of being or places increased demands on a person or animal; psychological stress can be either positive or negative.

testosterone    Although known as a male sex hormone, females make this reproductive hormone as well (generally in smaller quantities). It gets its name from a combination of testis (the primary organ that makes it in males) and sterol, a term for some hormones. High concentrations of this hormone contribute to the greater size, musculature and aggressiveness typical of the males in many species (including humans).

thyroxine    Also known as T4, it's a hormone made by the thyroid gland. This hormone plays a pivotal role in the growth and development of many organisms, including humans.

tissue     Made of cells, any of the distinct types of materials that make up animals, plants or fungi. Cells within a tissue work as a unit to perform a particular function in living organisms. Different organs of the human body, for instance, often are made from many different types of tissues.

trait     A characteristic feature of something. (in genetics) A quality or characteristic that can be inherited.

unique     Something that is unlike anything else; the only one of its kind.


Journal: G. Tanou, I.S. Minas and A. Molassiotis. Exploring priming responses involved in peach fruit acclimation to cold stress. Scientific Reports. Vol. 7, Sept. 12, 2017, article 11358. doi: 10.1038/s41598-017-11933-3.

Journal: M. Schulster, A. M. Bernie and R. Ramasamy. The role of estradiol in male reproductive function. Asian Journal of Andrology. Vol. 18, May-June 2016, p. 435. doi:  10.4103/1008-682X.173932.

Journal: J.M. Ridlon et al. Clostridium scindens: A human gut microbe with a high potential to convert glucocorticoids into androgens. Journal of Lipid Research. Vol. 54, September 2013, p. 2437.

Journal: G.W. Gooday and D.J. Adams. Sex hormones and fungi. Advances in Microbial Physiology. Vol. 34, Available online April 15, 2008. p. 69. doi: 10.1016/S0065-2911(08)60028-4.

Journal: E. Takano et al. A bacterial hormone (the SCB1) directly controls the expression of a pathway-specific regulatory gene in the cryptic type I polyketide biosynthetic gene cluster of Streptomyces coelicolor. Molecular microbiology. Vol. 56, April 2005, p. 465.

Book: L.I. Gilbert. Insect hormones. A chapter in Endocrinology, Springer Pub., 2005. doi. 10.1007/978-1-59259-829-8_9

Book: J. Raloff. “Environmental Hormones: Threats to health and reproduction.” Radio and Television News Directors Foundation and the Environmental Journal Center. Washington, D.C. 1999, 74 pp.