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How hormones actually work — and why they

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

10 min read

This article was last updated April 8, 2021.

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.

Put simply, hormones are tiny chemical "messengers" the endocrine system uses to communicate with the cells in our bodies.

In this post, we’ll demystify these seemingly magical substances and answer the following questions:

By the time you’re done reading this, we’re pretty sure you’ll be hormone-obsessed too.

First things first: 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, which communicate with your organs and tissues to make your body function. (We’ll talk more about the endocrine system a little later.)

So... what do hormones do exactly?

Great question, but an even greater question is: “What do hormones NOT do?” A range of different hormones are involved in basically any process you can think of:

  • How hungry or satiated do you feel? Hormones.
  • Your blood sugar level? Your metabolism? Hormones.
  • The “fight or flight” response you might get in stressful situations? Your blood pressure level? Hormones.
  • Your mood and emotions? Hormones.
  • Uterine contractions during birth? Hormones.
  • Physical growth and development? Hormones.
  • Your circadian rhythms? Also hormones (didn’t see that one coming!).

At Modern Fertility, we’re focused on the types of hormones that regulate menstrual cycles to make conception possible, then guide prenatal development of reproductive structures. These specific chemicals are your reproductive hormones.

The coolest thing about reproductive hormones? They simultaneously guide our bodies, our brains, and our behavior toward the same reproduction-related goals:

Anti-Mullerian hormone (AMH): This reproductive hormone is a proxy for your ovarian reserve, signifying how many eggs you have.

Estrogen: There are three types of estrogen, but we’re talking about estradiol, also known as E2. Estrogen is a “female” sex hormone responsible for the development and maintenance of female sex characteristics (breast growth, menstruation, pregnancy, etc.). E2, which is produced by your ovaries, specifically helps your body prepare for egg fertilization. As we get closer to ovulation (which is the only time it’s physically possible to get pregnant), estradiol levels increase.

Recent research tells us that estradiol is responsible for the sex drive in women — meaning the same hormone signaling to our 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.

Follicle-stimulating hormone (FSH): Also produced by the pituitary gland, FSH helps your ovaries produce eggs by allowing your follicles to grow and mature.

Luteinizing hormone (LH): Produced by the pituitary gland, luteinizing hormone (LH) preps the body for possible pregnancy and surges 24-48 hours before ovulation to let your maturing ovary follicle know it can release an egg. (Measuring LH levels with ovulation tests, like ours, allows you to track ovulation.)

Progesterone: This is another “female” sex hormone secreted by your ovary, and it’s an important part of your menstrual cycle. Progesterone regulates your endometrium, or the inner lining of your uterus, and helps the uterus maintain early pregnancies.

Prolactin: This hormone facilitates milk production during breastfeeding/chestfeeding and suppresses ovulation while potentially securing the mother-infant bond. This means nursing helps you get completely obsessed with your child and shuts down your ability to get pregnant again so you can focus all your efforts on the baby in front of you.

Testosterone: This is typically considered a “male” sex hormone since it’s produced in the testes and is responsible for the development and maintenance of male sex characteristics, but it’s also produced by the ovaries in smaller amounts.

How do hormones actually work? Meet the endocrine system

You can’t talk hormones without discussing the endocrine system — hence why the medical field that studies hormones is called endocrinology. The endocrine system refers to the series of glands and organs that secrete hormone chemicals, releasing them into your bloodstream so they can reach other tissues and organs in different parts of the body.

The endocrine system can be thought of as one of your body’s communication systems. In any endocrine process, there’s a chain of events that starts from the brain — no discussion of hormones is complete without talking about the brain. (Quick caveat: Fat cells can release hormones too, but we’ll save that for another day.)  

As an example, let's start from the very top of the chain with the reproductive hormones estradiol, testosterone, FSH, and LH plus four very important endocrine glands:

  • Step 1 is the hypothalamus: 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 is the pituitary gland: 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 LH and FSH — which eventually make their way to the ovaries and testes.
  • Step 3 are the gonads (ovaries and testes): Ovaries and testes then wrap up the process by producing and releasing hormones like estradiol and testosterone.

Other glands in the endocrine system include the adrenal gland (which releases hormones like cortisol, the stress hormone, and regulates blood sugar and physiological stress responses), the thyroid gland (which releases hormones like thyroxine, or T4, to regulate cell metabolism), the parathyroid gland, thymus gland, and the pancreas.

How do hormones and glands communicate with one another?

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 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.

But hormones are another story. Imagine a person using a megaphone to spread a message — but not everyone is paying attention. 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 it’s like when hormones are released into the bloodstream.

Once hormones get released, these secretions meander throughout the body, simultaneously acting on the many cells that have receptors for those hormones.

  • Cell receptors can either be 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:

  • The hormone and receptor take a little rendezvous together to the cell’s nucleus, where its genes are. There, they affect which genes get expressed, which affects the types of proteins produced, which ultimately affects a whole bunch of biological processes.
  • Rather than vacationing to nucleus land, the hormone 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]).

What else can impact hormone levels?

The linear chain we mentioned above 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.

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 gland, hypothalamus, or any combination of the above. For a super thorough workup, endocrinologists will likely run a whole bunch of tests to figure out exactly where the issue lies. (More on these issues a bit later.)

Below, we'll cover both the expected changes in hormone levels and what changes might warrant a call to your healthcare provider.

Hormone levels change by day and across your lifespan

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.

Changes per day or per cycle: Reproductive hormones like FSH and LH can fluctuate over the course of one day or reproductive cycle. This forces clinicians to develop protocols for when to best measure each of these hormones (spoiler: it's day 3 for many).

Changes across the lifespan:

  • Prenatal: 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.
  • Postnatal: Those few months after birth are referred to as “mini-puberty” because babies are literally producing puberty-esque levels of reproductive hormones. This calms down after the first few postnatal months, and levels remain super low (often undetectable with the current measurement methods we’ve got) until puberty hits.
  • Before puberty: Levels continue to remain low in early childhood. If a doctor measured reproductive hormones in a 5-year-old boy and a 5-year-old girl, their levels would look the same. But once puberty hits, this allllll changes.
  • During puberty: 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.
  • During the "reproductive years": LH, FSH, estradiol, and testosterone stay pretty consistent (aside from day-to-day changes). The one exception to this rule is AMH, which decreases.
  • After menopause: There are no more developing follicles producing estradiol and testosterone. Since estradiol usually tells the brain to chill on LH and FSH production (a process called negative feedback), when estradiol production is much lower after menopause, those two hormones have zero chill. LH and FSH levels go way up, which is why doctors may measure both when assessing menopausal status.

Some types of birth control affect hormone levels too

Different hormonal birth control methods contain synthetic female sex hormones — estrogens and progestins, or only progestin — to block ovulation, thicken cervical mucus, and/or prevent the thickening of uterine lining:

When steps in the hormone-production process don't go as planned (aka "hormonal imbalance")

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 of a given age and sex. (Usually, the middle 95% of a distribution. 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.

So, what should you do if your hormone levels are out of "normal" range? And what can you do about that? A lot of conditions require medical intervention (we’ll discuss those below), but you can take some steps on your own to improve your endocrine system’s function:

  • Eat well-balanced meals and get in regular exercise: Nutritious foods and exercise can prevent type 2 diabetes (which is related to problems with production of the blood glucose-regulating hormone, insulin).
  • Include specific foods or supplements in your diet: For example, iodine (like iodized salt) can prevent thyroid issues.

Many well-known reproductive health-related conditions associated with out-of-range hormone levels can be managed specifically through medications and various treatments:

  • Polycystic ovary syndrome (PCOS): This condition affects an estimated 10% of women, and is characterized by elevated androgens (those “male hormones,” like testosterone), and, in many cases, higher-than-normal AMH.
  • Thyroid conditions: These conditions are characterized by the over- and under-production of thyroid hormones (hyper- and hypothyroidism, respectively). When these levels are too high or too low, the menstrual cycle may be disrupted.
  • Pituitary gland conditions: FSH and LH are produced by your pituitary gland, and high FSH levels can result in primary ovarian insufficiency (your ovaries stop functioning normally before the age of 40). High LH levels and low FSH levels can be associated with PCOS.

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 Hormone Test is one way to do it!) and talk to a doctor about what your results mean for your reproductive health.

Do we all have the same hormones?

We’ve used the phrases “female” and “male” sex hormones, but all humans (and a lot of distantly related species) actually have the same hormones. Many bacteria have the exact same insulin as humans — meaning you can take insulin from bacteria, put it in the human body, and it’ll do its usual thing. And 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!)

Despite these similarities, it’s no secret that males and females across species differ in their reproductive hormones. That’s because the amount of each hormone you have depends heavily on your biological sex and age:

  • 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.

Bottom line: Hormones = real-life magic

Hormones play a role in too many physiological functions to count across an incredibly wide range of organisms. They also coordinate reproductive physiology, psychology, and behavior towards the same goals. Measuring hormones can give us clues about whether or not things in our body are working as they should.

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

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, PhD

Talia is a clinical product scientist at Modern Fertility. She's passionate about reproductive health + behavioral neuroendocrinology. Talia received her PhD in biological anthropology at Penn State.

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