The Endocrine System Is Every Triathlete’s Best Friend (or Worst Enemy)

As triathletes, we focus on a few of the body's systems—skeletal, muscular, cardiovascular, respiratory, and even digestive—but did you know the single biggest make-or-break system for triathletes could be the endocrine system?

Photo: Lemono/Canva

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Many years ago, when I first became an endurance athlete, I asked an older training partner why he only did one super-distance race per season. “Even if I had the time, I know that the body can’t take that kind of hit to the endocrine system more than once a year,” he replied. I nodded and probably muttered some kind of agreement, however, the truth is that I had no idea what he was talking about. This ignorance led to many terrible training decisions, missed opportunities to fix my mistakes, and ultimately, a face-first run-in with Overtraining Syndrome (OTS). I know I’m not alone. It’s likely that many of us are not aware of what the endocrine system is or what it does, and therefore do not know that it is the most important bodily system to an endurance athlete. Learning about how this complex, yet delicate, system has the power to make or break our performance, may have prevented my OTS–and it just may prevent yours.

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The Endocrine System, in a nutshell

The endocrine system is made up of eight glands, the hypothalamus, pituitary, thyroid, parathyroids, adrenals, pineal body, ovaries, and testes. These glands secrete hormones like prolactin, thyrotropin, growth hormone, antidiuretic hormone, and oxytocin, that act as chemical messengers throughout the body. The hormones control things like mood, growth and development, organ function, metabolism, and reproduction.

A diagram of the endocrine system for triathletes and endurance athletes
(Photo: Getty Images)

What do all of these systems do for a triathlete? The triathlete’s hypothalamus takes in information about habits, behaviors, and environment; how we eat, sleep, train, what we think, and even the temperature and amount of daylight we’re exposed to. It then sends a message to the pituitary gland, which will then make hormones to regulate the body based on what we’re putting it through. If we are managing stress, training appropriately, eating enough, and taking enough rest, our endocrine system functions normally. Hormones will carry out actions like regulating the amount of water in the body, repairing muscle tissue during sleep, making sure reproductive hormones are on track, and hitting us with just enough cortisol so that we wake up refreshed from a good night’s sleep. Basically all of the things we know, as triathletes, that should be going right if we want to train, recover, and race at our best.

All of this healthy function requires a balance between the hypothalamus, the pituitary, and the adrenal glands, known as the HPA axis. The job of the HPA axis is to decide how much stress the body is under and increase or decrease the amount of cortisol, or stress hormone, in response. It is critical to remember that the hypothalamus does not distinguish between physical and mental stress – it’s all the same. No matter if you just got fired, didn’t sleep enough last night, or went on a ten-mile run, stress hormones will increase and if left unchecked, can eventually lead to HPA axis dysfunction.

RELATED: Life Stress, Work Stress, Training Stress—Your Body Can’t Tell the Difference

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Are triathletes more susceptible to endocrine system dysregulation?

Dr. Nicky Keay, a sports endocrinologist, Lecturer in Medicine at University College London, and Research Fellow in the Department of Sport and Exercise Sciences at Durham University, says that triathletes are more likely to become dysregulated simply because they are asking their bodies to do more, for a longer period of time. If one or more of our habits and behaviors is out of balance, the endocrine system can become dysregulated as it attempts to fix the problem.

Hormones are the link between the outside, or what we do, and inside, or how our body reacts to it,” Keay said. “so our behaviors around exercise, sleep, and nutrition are communicated to the inside of your body to tell it what to do through hormones. Quite sensibly, the body recognizes that the behaviors are not great for an athlete and will take steps to correct it.”

What does this look like for a triathlete? If we are not eating enough for the amount of exercise we’re doing, training too much, and/or not getting enough recovery, the following examples are just some of the potential outcomes.

  • We can expect a downregulation of thyroid function and a decrease in metabolism.
  • We may see a decrease in the amount of sex hormone, which will affect erections and sperm production (men) or prevent you from getting a period (women).
  • We will likely suffer from insomnia, as well as a decrease in growth hormone released during sleep, which will lead to ongoing soreness or injury.
  • Overtraining Syndrome becomes a threat as our habits continue, which will only exacerbate symptoms.
  • Everyday life will be affected, since research shows that as the HPA axis becomes inundated with high physical and mental stress, we can experience depression, anxiety, lowered libido, disordered eating, and even disturbed cognitive function.

Let’s be clear about something: Every one of these dysregulations is the result of our behaviors. It’s not because our body is doing something wrong, it’s actually doing everything right and trying to protect us…from ourselves. This may be hard to hear, but it also means we can fix what we broke.

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How can triathletes avoid endocrine dysregulation?

Filter Out Noise

Step one of avoiding endocrine dysregulation is to filter out the health messages that do not apply to endurance athletes. Triathletes often get into bad habits when they follow general “health” messages or pieces of diet advice that they have absorbed from the media and culture around them. Dr. Keay, who works with both elite athletes and people struggling with type two diabetes, says that endurance athletes need dramatically different messaging than the average person.

“In the media, the idea is ‘eat less, move more’ or that we need to have a perfect physique. This message can be totally inappropriate for an athlete,” Keay said. “Just as a message of needing to eat more carbohydrate would be inappropriate for the sedentary person struggling with type two diabetes.”

This means that we need to shift our mindset from “How can I cut calories?” to “How can I avoid becoming energy deficient?” and from “How many steps can I take in a day?” to “How can I avoid additional wear and tear on my body?”

RELATED: The Myth of Calories In = Calories Out

Recover and Fuel

Another way to avoid dysregulation is to be sure that we increase opportunities for rest and good nutrition as we increase training volume and move to train for longer distance triathlons. It is true that we become more fit and therefore, adapted to increasing training demands over time. However, this does not mean that we need less rest. According to Dr. Keay, we need to be more diligent than ever to see our hard work pay off. 

You can train all hours, but you will not improve because the improvement takes place when you are sleeping through growth hormone,” said Keay. “Athletes need rest, sleep, days of full recovery, no active rest – there is no such thing.”

She also says that athletes should become “dedicated and systematic” about nutrition, suggesting that they never do fasted training, always have some kind of recovery nutrition within twenty minutes of a workout, and are sure to fuel up during longer training sessions. It may be useful to check in with a sport-informed registered dietitian since copious research will attest to the connection between low energy availability and the disruption of endocrine functionality known as RED-S (Relative Energy Deficiency in Sport) in men and women.

RELATED: Ask Stacy: How Can I Nail My Recovery Nutrition—and How is This Different for Men and Women?

Honesty is The Best Policy

Dr. Keay says that athletes need to be honest with themselves. Symptoms like fatigue, impotence, gastrointestinal issues, or mood disturbances will show up long before dysregulation occurs, but most athletes will choose to ignore them until it’s too late. It’s also important to know that we risk dysregulation whether our behaviors are intentional or unintentional. Some athletes may be actively trying to restrict calories, but some may just be diligently following a training plan that is not appropriate for them. Dr. Keay says that in either case, we need to take a hard look inside.

Sometimes people have underlying confidence or control issues. Some people have to understand that this is their tendency, in stressful situations they will default to this – overtraining and underfueling,” Keay said. “Unless they address all of this, I will be seeing them in the office again.“

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A day in the life of a dysregulated triathlete

Though every triathlete is different, Dr. Keay says there are some common themes and signs of endocrine dysregulation in triathletes. If any of these occurrences sound familiar, talk with a medical professional who is well-versed in endocrine dysregulation in athletes.

Waking up

When you wake up, you’re not going to feel refreshed. You’ll have lingering fatigue and a lack of motivation. If you’re tracking your heart rate variability (HRV), you might find a reduction against your baseline levels. You are having to force yourself to get moving. If you’re a man, you may not find a morning erection.

First training session

You might become pissed off during your training. It’s not a good session, but you’ll hope it’s a one-off. You might say “I didn’t even train very hard and therefore I don’t need my recovery drink, food, or fuel.”

RELATED: Losing Focus? Feeling Grumpy? You Might be Overtraining


You go off to work and things feel heavy. Everything feels like a struggle. Your pants are a bit tight and you wonder if you’re gaining weight, despite having cut calories and increased training recently. If you’re a woman, your period was supposed to come today but it didn’t–or it started, but it was very light. Since you didn’t do enough training today, you decide that maybe you don’t need any snacks or starchy carbohydrates tonight. You hope tomorrow will be better.


You have trouble falling asleep or you wake up in the middle of the night. You wake up because you’re hungry and actually too fatigued. You may also wake up very early, 3 or 4 in the morning because of high cortisol. Everything starts all over again, but the body is not happy.

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The bottom line

In many cases, we can dig ourselves out of the hole that we have put ourselves in, but not without having a real personal reckoning. Do we want to be the thinnest triathlete with the shortest career? Do we want to risk fertility for a too-packed race schedule? Do we want to struggle with fatigue because we’ve been told that carbohydrates are the enemy? Or do we want to be a healthy, adequately-fueled, well-rested member of this community for as long as possible? I know which option I would choose if I could do it over. Here’s hoping you make the right choice, too.

Jill Colangelo is a writer and researcher of mental health and ultra endurance sport. She has a BA and ALM in psychology and is a former triathlete and ultramarathoner. 

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