Explainer: What are endocrine disruptors?

Hormones are like the managers of the body’s organs and other tissues. These chemicals order cells — from your head to your toes — to switch on or off some particular activity. The brain usually coordinates the release of hormones, sending these managers to a particular job site when it’s time for work to begin. But sometimes industrial chemicals and pollutants can mimic these managers. When such imposters enter the body, they can alter when or how an organism develops, what it looks like — even whether it gets some disease.

Toxicologists — the scientists who study the action of poisons — have begun referring to these hormone mimics as endocrine disruptors. That’s because the endocrine system releases hormones. And these chemicals fake out the normal players in this system.

Hormones work in very small amounts. Still, they can have very big impacts. Hormones usually tell cells to start some task now. Or they turn on signals that report what’s going on in and around the body.

Thanks to hormones, our bodies know when to eat and when to stop. Hormones tell us when to sleep and when to wake. They turn on signals telling tissues when to grow and by how much. They also trigger changes that open a new chapter — or close an old one — in the story of our growth and development. For instance, they tell the body when it’s time to begin puberty. In older women, they turn off the reproductive cycling that formerly made pregnancy possible. Hormones can even control the expression of our gender and how tissues should use energy (calories) in ways that prevent disease.

So it’s no exaggeration to say that hormones, even in super-tiny quantities, play a major role in our well-being.

Scientists often compare hormones to keys. As these chemicals move throughout the body, they look for a particular “lock” on the outside of a cell. Biologists refer to that lock as a receptor. If the chemical key has the right size and shape, it can unlock that hormone’s receptor.

But sometimes a fake hormone will fit the lock. Like a skeleton key, an endocrine disruptor may unlock a receptor and turn on some activity — but at the wrong time.

In other instances, a hormone mimic may work like a bent key. It may fit the lock, but fail to turn on any action. And that’s bad, because the whole time this key sits in the lock, it blocks access to “working keys” — the body’s real hormones.

So depending on how many mimics the body encounters and how closely they resemble the real thing, these chemicals may convince our tissues that there’s too much or too little of a real hormone, or even a normal amount that has shown up at a totally inappropriate time.

Active at very low levels

A range of different pollutants can mimic hormones. Among them is nonylphenol (NON-ull-FEE-nul). It’s made when certain surfactants (chemicals that allow liquids to mix that would not ordinarily do so) break down. Nonylphenol can masquerade as estrogen, the primary female sex hormone. Studies as far back as the late 1990s showed that male fish exposed to nonylphenol could make vitellogenin (Vi-TEL-oh-JEN-in). That’s an egg-yolk protein normally produced only by females. Research with a chemical cousin of nonylphenol showed it could do the same thing to male fish. Some of those males actually made eggs.

Many other chemicals can similarly feminize fish — giving them traits or promoting behaviors normally seen only in females. Among such chemicals are the pesticides DDT; certain non-stick chemicals known as perfluorinated compounds (such as PFOA), and certain chemicals used to make plastics, such as polystyrene.

Dioxins represent another worrisome class of chemicals. These pollutants are produced during the creation and the burning of many chemicals that contain chlorine. Biologists first used the term “environmental hormones” to describe dioxins. In fact, it makes more sense to call them endocrine disruptors, because they don’t actually mimic hormones. Instead, dioxins modify how the body uses its hormones.

Since around 2000, scientists have found that many common products contain chemicals that can mimic hormones or alter their activity. In many cases, these chemicals have been used widely for 50 years or more. Consider bisphenol A, better known as BPA. It’s a building block of clear, polycarbonate plastics. A resin made from BPA lines some food cans. Some cash-register receipts use it for printing. It also can be present in trace amounts in the material that dentists use to seal teeth from the effects of cavity-causing germs.

Endocrine-disrupting pollutants can get flushed down the drain and sent to water-treatment plants. Since these plants were never designed to remove hormone mimics, such pollutants flow along with cleaned water into lakes and rivers. There, experts worry, hormone mimics may alter the growth and behavior of fish and other aquatic life.

And evidence has been emerging that this is already happening.

One change linked to endocrine disruptors: “feminized” animals. These include male fish that develop female features — or whose male features and reproductive organs may develop abnormally. Such feminized fish have been turning up in rivers across the United States and off of the coasts of northern Europe.

And fish aren’t the only species vulnerable to the gender-bending effects of hormone-mimicking chemicals. In one 2010 study, for instance, Tyrone Hayes’ team at the University of California, Berkeley, feminized male frogs. They did this by exposing them to supposedly safe concentrations of the weed killer atrazine. The males took on female traits. Some even mated with untreated males — and produced live young.

Dose really matters

There’s an old saying in toxicology: The dose makes the poison. For many decades, scientists interpreted that to mean that there was a safe level below which exposure to a chemical would do no harm. Only as exposure to a chemical grew above that level might the risk of toxicity also grow. In fact, that concept does not quite apply to hormone mimics.

Hormones are active at extremely low levels. In most cases, so are any impacts of their mimics. In fact, hormone-like action can occur at levels well below those where any visible poisoning would occur. So scientists have recently begun revising their tests to scout for endocrine disruptors. They must now probe for effects at exposures once considered harmless.

An added twist can make this particularly confusing: Endocrine disruptors may have impacts only at very low levels. In contrast to other poisons and substances that can cause cancer, higher levels may actually reduce the risk — at least of hormone-like action. It just goes to show that the effects of pollutants on the body can be complex and very hard to predict.

Power Words

bisphenol A (BPA)     A building block of polycarbonate plastics and many commercially important resins. This chemical gained widespread public attention when research showed it could mimic the activity of estrogen, a female sex hormone.

chemical      A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H₂O.

DDT      (short for dichlorodiphenyltrichloroethane) This toxic chemical was for a time widely used as an insect-killing agent. It proved so effective that Swiss chemist Paul Müller received the 1948 Nobel Prize (for physiology or medicine) just eight years after establishing the chemical’s incredible effectiveness in killing bugs. But many developed countries, including the United States, eventually banned its use for its poisoning of non-targeted wildlife, such as birds.

demasculinize    (in biology) A change in a male organism that causes it to lose some features that are typical of its gender. Some changes are outwardly visible. In some fish, for instance, demasculinization may change a male’s coloration or the shape of its head or fins. In mice, it may shorten distance between a male’s genitals and anus (making it resemble the distance typical of females). Other changes may be outwardly invisible. In certain fish and alligators, for instance, internal gender-related tissues or organs may not fully develop or may develop abnormally.

dioxins     A family of chlorine-based pollutants, all sharing a similar chemical structure. They form during the combustion of a number of products, including many plastics. They readily dissolve into fat and can be stored in the body of many vertebrates (including people), and later released into the blood where these pollutants can bathe tissues. Although not an estrogen mimic, these chemical can alter the way the body uses such hormones. Studies have linked dioxins, especially the most toxic one — known as TCDD — to reproductive changes in animals.

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.

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.

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 too. But that will be largely because it had too little exposure to male hormones, not an excess of female hormones.

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. 

nonylphenol       The name for a family of pollutants that can survive in the aquatic environment for a long time. These persistent chemicals are used primarily to make NPE surfactants and to strengthen certain plastics. Studies have shown these chemicals can mimic the action of estrogen, a female sex hormone. Animals can accumulate these pollutants from the environment. Nonylphenols can be extremely toxic to aquatic organisms.

nonylphenol exothylates(NPEs)      A family of chemicals that are widely used in industry as surfactants and wetting agents. When they break down, NPEs produce nonylphenols, a family of chemical compounds that can be toxic to plants and aquatic animals.

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.

surfactant      A chemical that decreases the attraction between water molecules. Manufacturers use such compounds to make it easier for water to spread on surfaces and to mix with other substances (such as oil).

toxicology  The branch of science that probes poisons and how they disrupt the health of people and other organisms. Scientists who work in this field are known as toxicologists.

vitellogenin     A protein found in the yolk of the eggs of many fish and birds.