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How hormones actually work — and why they

How hormones actually work — and why they're the key to your health

9 min read

We’ve got a confession for you: We’re hormone obsessed. Okay, fine, maybe it’s not much of a confession — if you’ve spent any time with us, you’ve probably figured out that hormones are pretty essential to what we do here at Modern Fertility, and we think they’re pretty rad.

But what are hormones anyway? Where do they come from? How do they affect our bodies? Do we all have hormones? What different things can hormones affect?

In this post, we’ll demystify these seemingly magical substances. By the time you’re done reading this, we’re pretty sure you’ll be hormone obsessed, too.

What are hormones?

The cells in our bodies talk to each other using chemical “messengers,” and the type of messengers used depends on what system we’re talking about. The immune system uses things called cytokines and the brain and spinal cord use things called neurotransmitters. The endocrine system is powered by hormones.

Remember the game “telephone?” You whisper something silly to the person next to you, and they pass the message along to the person next to them, until the last person finally gets the message (which is often 100% different than what you originally said). Think of the messengers in the brain and spinal cord (neurotransmitters) as working like a never-ending game of telephone: They plug into one cell to create some sort of change, and this cell passes the message along to another cell, and on and on.

Hormones are a different story. Think of hormone-based communication like using a megaphone to get a message across — only it’s a megaphone that not everyone’s paying attention to. Rather than the message being passed along from person to person, one at a time, all the people who are paying attention hear the megaphone message simultaneously. That’s what hormones being released into the bloodstream are like. Once they get released, they meander throughout the body, simultaneously acting on the many cells that have receptors for those hormones (more on what receptors are below!).

Where do hormones come from?

Different hormones are released by different glands throughout the body. The thyroid gland releases hormones like thyroxine (T4) which regulate cell metabolism, the adrenal gland releases hormones like cortisol which regulates blood sugar and physiological stress responses, and the gonads (ovaries and testes) release gonadal hormones like estradiol and testosterone, which regulate things like ovulation and ovarian function. In all of these cases, though, there’s a chain of events that starts from the brain — no discussion of where hormones come from is complete without talking about the brain. (Quick caveat: Fat cells can release hormones too, but we’ll save that for another day.)

Let’s take the hormones estradiol and testosterone as an example — we’ll start from the very top of the chain to explain how these hormones come to be.

  • Step 1: The original initiator of it all is the hypothalamus, which is a small area located near the base of your brain. The hypothalamus produces what’s called gonadotropin-releasing hormone (GnRH).
  • Step 2: GnRH scoots down to the pituitary gland, which is located right at the base of the skull. Here, the GnRH tells the pituitary to produce and release luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
  • Step 3: LH and FSH are then released into the bloodstream, and eventually make their way to the ovaries and testes.
  • Step 4: Ovaries and testes then wrap up the process by producing and releasing hormones like estradiol and testosterone.

To make things even more complicated, this linear chain is bidirectional. At any point, when levels are detected to be high, feedback is sent to the previous step to “hold up!” and stop signaling. For example, if estrogen levels are high, your pituitary gland will release less LH and FSH.

Unfortunately, more steps in the process = more places in the process where things could go awry. If someone has really low estradiol, for example, it’s not always immediately clear why — it could be an issue with the ovaries, pituitary, hypothalamus, or any combination of the above. For a super thorough work-up, endocrinologists (doctors who specialize in glands and hormones) will likely run a whole bunch of tests to figure out exactly where the issue lies.

How do hormones actually do anything?

Cells have receptors that can be either external (on the cell’s surface) or internal (somewhere inside the cell). These receptors act as a lock, and hormones act as the key. When the lock and key meet (when the hormone plugs into the receptor), that’s when the magic (read: cool cellular biology) happens.

There are two main things that can take place depending on the type of cell involved:

  1. They take a little rendezvous together to the cell’s nucleus, where its genes are. There, they affect which genes get expressed, which affects which proteins are produced, which ultimately affects a whole bunch of biological processes.
  2. Rather than vacationing to nucleus land, the hormone that's plugging into the receptor changes the activity of the cell — how likely it is to fire or not fire — and it’s through this process that hormones can have really rapid effects on physiological functions, cognition, and behavior.

Locks are nothing without keys, and vice versa. For hormone-dependent processes to work, you need both the hormones and functioning receptors. (Here's one example of what happens when you’ve got the hormones, but not the receptors for them to plug into — genetically male [i.e., XY] people may look externally like genetic females [i.e., XX]).

Do we all have the same hormones?

Not only do all humans have the same hormones (more on the idea of “male hormones” and “female hormones” below), but a lot of really distantly related species have the same hormones too.

Many bacteria have the exact same insulin (the hormone that helps us use and store sugar for energy) that humans have — meaning you can take insulin from bacteria, put it in a human, and it’ll do its usual thing. This is the case for some other hormones too: Estrogen, for example, is basically identical in humans and horses. Fun fact: the post-menopausal estrogen-replacement medication Premarin is literally made from estrogens taken from pregnant mares’ urine (hence the name!).

While it’s true that all humans have the same hormones, the amount of each hormone you have depends heavily on your biological sex and age. Because reproductive hormones are Modern Fertility favorites (no offense, other types of hormones!), we’ll focus on these here.

It’s no secret that males and females across species differ in their reproductive hormones. For example, in adult men and women, men have testosterone levels over 10x as high as women, and women have much higher levels of estradiol. It’s because of these big sex differences in levels that testosterone has gotten the nickname of the “male hormone,” and estradiol the “female hormone.” What’s important to point out is that despite its nickname, the “male hormone” plays a role in body composition, energy levels, and fertility in women. Similarly, the “female hormone” plays a role in many functions, including fertility, in men.

Do hormones change over time?

If you’re here, there’s a good chance you know that some hormones — like anti-Mullerian hormone (AMH) — change across your lifespan. But that’s just the tip of the iceberg on how reproductive hormones change over time. Some hormones like FSH, LH, and even others unrelated to reproductive health (like cortisol) can fluctuate in the course of one day or over a reproductive cycle. This forces clinicians to develop protocols for when to best measure each of these hormones.

Hormones can also change over longer periods of time. Early on in prenatal development, the biological axis that controls reproductive hormone development is pretty active — this is crucial because it guides the development of male-typical and female-typical reproductive structures. In the few months after birth, too, reproductive hormone production is so high that this period is referred to as ‘mini-puberty’ because babies are literally producing puberty-esque levels of reproductive hormones.

Things calm down for a while after that. Reproductive hormone production is super low (often undetectable with the current measurement methods we’ve got) after the first few postnatal months, until puberty hits. This means that if I measured reproductive hormones in a 5-year-old boy and a 5-year-old girl, they’d look the same. But once puberty hits, this allllll changes. Reproductive hormone production goes into overdrive, which is what we can thank for things like acne and oily skin (thanks a lot, hormones!), breast development, and periods.

Across the reproductive lifespan in women (i.e., from puberty to menopause), reproductive hormones like LH, FSH, estradiol, and testosterone stay pretty consistent. The one exception to this rule is AMH, which decreases.

After menopause, things change pretty dramatically — there are no more developing follicles producing estradiol and testosterone. Because it’s usually estradiol that tells the brain to chill on LH and FSH production (a process called negative feedback), after menopause, when estradiol production is much lower, LH and FSH have zero chill. Levels of these two hormones go way up, which is why doctors may measure LH and FSH when they’re assessing menopausal status.

Okay, but what do hormones actually do?

Great question, but an even greater question is: “What do hormones NOT do?” Hormones are involved in basically any process you can think of. How hungry or satiated you feel? Hormones. Your blood sugar level? Hormones. The “fight or flight” response you might get in stressful situations? Hormones. Uterine contractions during birth? Hormones. Your circadian rhythms? Also hormones (didn’t see that one coming!).

Let’s focus a bit more on what reproductive hormones do. We already know that they guide prenatal development of reproductive structures and are hyper-important for fertility later on in life. Probably one of THE coolest things about reproductive hormones, though (in our humble opinion), is that they simultaneously guide our bodies, our brains, and our behavior toward the same reproduction-related goals. Don’t believe us? Keep reading.

Hormones change across the menstrual cycle, and that’s how we can do things like figure out when a woman is ovulating. As we get closer and closer to ovulation (which is the only time it’s physically possible to get pregnant), estradiol levels increase. And, the most recent science tells us that it’s estradiol that’s responsible for sexual desire in women — as estradiol goes up, so does sexual desire.

Putting these two facts together, we see that the same hormone that signals to the body that it’s possible to conceive also makes us feel more in the mood, gearing our psychology toward thoughts and behaviors that would make conception more likely.

Another example: Prolactin is best known for its role in facilitating milk production during breastfeeding (hence its name). But prolactin serves a sneaky second function. Recent work suggests that it’s also necessary for securing that mother-infant bond, meaning the same hormone that helps you deliver nutrients to your baby also helps you become completely obsessed with and want to provide care for them. Prolactin while breastfeeding also suppresses ovulation — while you are intensively breastfeeding, your body may shut down your ability to get pregnant again, presumably so you can focus your efforts (in terms of both physiology and behavior) on the baby you’re currently caring for.

How to find out if you have a hormone imbalance or if your hormone levels are “normal”

There’s a lot of variability in what’s “normal” for hormones. But before we get into that, how is normal even defined? In the context of hormone values, the normal range is usually calculated by looking at what the values are for most people (usually, the middle 95% of a distribution) of a given age and sex (for more on this, check out our post on reference ranges). Hormones above or below where we’d expect them to be can be indicative of things not functioning as they should.

Many well-known conditions are associated with hormone imbalances. Take diabetes, for one, which is characterized by problems with the body’s production of insulin, or the body’s ability to respond to insulin. Then there’s polycystic ovarian syndrome (PCOS), which affects an estimated 10% of women, characterized by elevated androgens (those “male hormones,” like testosterone), and, in many cases, higher-than-normal AMH. There are also conditions associated with the over- and under-production of thyroid hormones (hyper- and hypothyroidism, respectively). All of these conditions can be treated, so if you suspect there may be something going on, it’s always a good idea to get your hormone levels checked out (and pssst, the Modern Fertility test is one way to do it!) and talk to a doctor.

Bottom line: Hormones = real-life magic

Hormones play a role in too many physiological functions to count across a crazy wide range of organisms, and measuring them can give us clues about whether things are working as they should.

They’re also able to coordinate reproductive physiology, psychology, and behavior toward the same goals.

Luckily, it’s never been easier to learn about these magical little messengers — so easy, in fact, that you can do it in your jammies. Check out what else we’ve got to say about reproductive hormones, and take our quiz if you’re ready to get a customized hormone panel sent your way!

This article was medically reviewed by Dr. Eva Marie Luo, an OB-GYN at Beth Israel Deaconess Medical Center and a Health Policy and Management Fellow at Harvard Medical Faculty Physicians, the physicians organization affiliated with the Beth Israel-Lahey Health System.

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Talia Shirazi

Talia is a biological anthropology PhD candidate at Penn State, passionate about women's reproductive health and behavioral neuroendocrinology.

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