What is a hormone?

Hormones are defined by what they do rather than what they are made from. So hormones aren’t a type of molecule in the way that proteins or carbohydrates are. Hormones can be made from different types of molecules, including proteins and steroids.

To be called a hormone, a molecule has to act as a signal that affects the activity of cells, causing them to change what they’re doing. It also needs to be sent out away from the cell where it was made, to affect cells in other parts of the body.

In humans, hormones are released directly into the bloodstream. This allows them to travel quickly around the body. Hormone molecules are released in very large numbers. From the bloodstream, they spread out to reach all parts of the body.

The cells that are able to respond to the hormones (their target cells) recognise them through the use of special receptors that only bind that one particular type of hormone.  

Organs that produce and release hormones are called endocrine glands . The entire system of endocrine glands and hormones is called the endocrine system.

Why do we need hormones?

From the viewpoint of a single cell, the human body is enormous. There are tens of trillions of different cells, with different shapes and abilities and roles, organised into different tissues and organs. And they all need to work together. Somehow, this huge pile of cells needs to behave as a single organism.

Our cells need to work as a team to keep the internal environment stable, and to respond to changes in the external environment. But how can they coordinate? Each individual cell is very simple - how can any one cell ever know exactly what it should be doing? Or when to do it?

There are a few different ways that messages are sent around the body to organise cells’ activity.

Nerves are perfect for sending signals to particular places – to a single muscle, or a gland. But they need to physically reach out to them. Nerves are a bit like old land-line telephone lines: you have to be wired in to hear the message. Calls can be received from multiple places, and can be sent out to multiple places, but you can only communicate with people that are wired into the network. On the plus side, messages can be sent almost instantly.

Sending a flood of hormone molecules out into the bloodstream is more like dropping a million paper notes out of an aircraft as you pass over a town, each with the same message written on it. There are multiple copies of the message, which spread out and get everywhere, reaching everyone. It might take a little time for the messages to flutter to the ground and be found. But they hang around for a while so their effect is longer-lasting than a phone call.

So, nerves are good for quick, directed communication. And hormones are good for broader, longer-lasting messages. But these two systems are not separate. The brain has a connection to the pituitary gland, through which it controls the release of many hormones. And many hormones affect the activity of nerve cells. 

What do cells do in response to hormones?

One of the great powers of hormones is that they can cause different responses in different cells. This means that a single hormonal signal can create a wide range of effects. This is really useful.

Imagine a fire alarm going off in a supermarket – this one simple signal will be interpreted differently by different people. The shoppers react by leave the building. The fire officer reacts by going to locate the source of the alarm, while other staff react by helping to evacuate the building. Having one single alarm with one single message (“possible fire!”) is much better than having separate alarms for the shoppers (“leave the building!”), the fire officer (“look for a fire!”), and other staff (“evacuate shoppers!”)

Similarly, the hormone adrenaline (find out more here) gives a single signal that the body should get ready for action (“prepare for action!”). Different tissues and organs react to this signal in different ways. When the heart detects adrenaline, it speeds up. So do the lungs. But the same signal diverts blood supply away from the gut. When the liver detects adrenaline, it releases sugar ready to fuel the muscles. Each organ and tissue responds in its own way to this single message.

Hormones are able to have so many different effects because they work by giving a message, rather than by taking direct action themselves. Different target cells respond to the message in different ways.Hormones can also affect the release of other hormones. The pituitary gland in the brain makes a lot of hormones that do this – these hormones are used to send signals to the ovaries, testes and other glands, to control their production of a wide variety of other hormones. This is why some people call the pituitary gland “The Master Gland”.

If this all sounds a bit complicated … that’s because it is! There are many complex interactions involved. But this complexity is what gives our bodies the power to be so responsive to the challenges of life, and to respond as a single multi-cellular organism. 

Receptors: how the message is passed to the cell

Hormones are simple molecules. They don’t know where they’re going, or where their target cells are. They are carried along by the flow of blood like rubber ducks in a stream. They spread out in the body and bump into many different cells and molecules. Sometimes, one of the cells they bump into just happens to be a target cell, and the molecule they bump into just happens to be a receptor.

Hormone receptors are proteins that can bind hormones. They are how the hormone’s message is passed on to the rest of the cell.

Different receptors bind different types of hormone. The receptor proteins are shaped so they can grab hold of the right hormone. The shapes of the receptor and hormone fit together perfectly when they bind. A nice way to imagine this is to think of how a glove moves around your fingers as you slide your hand inside it. Both parts wriggle around a bit but you end up with a perfect fit. This type of binding is also called lock-and-key binding, because the shape of the ‘key’ (the hormone) has to fit the ‘lock’ (the receptor). Differently shaped receptor proteins fit differently shaped hormones.

Some types of hormone receptor stick out from the cell, while other types are found inside the cell. The location of any particular type of hormone receptor will depend a lot on what its matching hormone is made from. 

Protein (and peptide) hormones

Some hormones, like insulin, are made from proteins (or mini-proteins called peptides).

Cells make proteins by joining amino acids together into long chains. These chains then fold up into 3D shapes. The pattern of amino acids in a protein controls how it folds up, so different proteins can have very different shapes (and sizes). This means that proteins can be used for a huge variety of diverse roles in the body. Including as hormones, and as hormone receptors.

Compared to most other proteins, protein hormones are very small. But even so, they can’t pass through cell membranes by themselves. They can reach their target cells, but they have no way of getting inside. At first this might sound like it should be a problem, but it isn’t, because they don’t need to get inside. They only need to pass on their message. And they can do this by binding to a hormone receptor on the outside of their target cell.

Hormone receptors are large proteins which sit within the cell membrane of the target cell. One end of the receptor sticks out of the cell (this is where the hormone will bind), and the other end sticks into the cell cytoplasm. When the hormone binds to the outside part of the receptor, it makes the inside part of the receptor change its shape. This shape change affects how it binds to other molecules, leading to a chain reaction inside the cell, which ends up with a change in the cell’s activity.

Examples of protein hormones include insulin, glucagon, follicle stimulating hormone (FSH) and oxytocin.

Steroid hormones

Some hormones, like progesterone, are made from cholesterol molecules. These hormones are small molecules with chemical properties that allow them to pass through cell membranes. This means that when they go around the body, they can actually go inside cells.

Remember these hormones are just tiny molecules. They have no brain, they don’t know which are their target cells or where their receptors might be. They just get washed around the body. Sometimes they’ll pass through cells that aren’t interested in them (which don’t have receptors). But there are billions of hormone molecules so this isn’t a big problem.

Steroid hormone receptors are found inside the target cells. Depending on the exact hormone, the receptor might be attached to the cell membrane, or floating around in the cytoplasm. Some steroid hormones bind to free-floating receptors that then go inside the nucleus. These then directly affect the way that the cell’s DNA is used.

Examples of steroid hormones include progesterone, oestrogen, testosterone and cortisol.

What happens to the hormones afterwards?

Most hormones get broken down in their target tissues, or by the liver. This is an important step as otherwise the hormone signal would go on for too long.

However, it takes a little while for this to happen. This means that even if hormone production is completely stopped, hormones that are already in circulation will continue to affect cell function for a little while. This is one of the big differences between the endocrine system (hormones) and the nervous system (nerves); nerves send quicker, shorter messages.

The liver is an important organ for hormone function. As well as helping to break down hormones, it also produces its own hormones, and activates others. It even produces special carrier proteins to help some hormones travel more easily in the blood.

How many different hormones are there?

Over 50 hormones have been identified in the human body, but there are likely more to be discovered. Many of the hormones we know about have complex functions. And many are still not well understood.

There are many medical conditions which are caused by hormone levels being too high or too low, including diabetes mellitus (find out more here). By better understanding hormones, the glands that produce them, and their target cells, endocrinologists can improve diagnosis and treatment for people with conditions related to the endocrine system.