Cycling Science Digest ·
It’s Not Overtraining. It’s Your Thyroid.
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If you want real progress heading into 2026, now’s the time to set your path.
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What Overtraining and Underfueling Do to Your Thyroid
A new theory may explain why some riders quietly fall apart during heavy training blocks.
A recent paper from Hackney and colleagues proposes that the early driver of REDs in endurance sport isn’t a mystery mix of “fatigue” or “stress.” It’s a predictable hormonal sequence. Push training load too high, underfuel carbohydrates, and cortisol spikes. When cortisol stays high, your T3 thyroid hormone drops. Once T3 falls, everything downstream starts to slide: metabolic rate, recovery signals, sex hormones, muscle protein turnover, immune function, mood, and your ability to actually respond to training.
For cyclists, this matters because REDs is usually framed around low calories, but the paper argues carbs are the key control knob. Low carbohydrate availability seems to amplify cortisol far more than an overall calorie deficit. The IOC’s updated REDs guidance points to the same thing. Even brief windows of carb restriction, like doing a big morning ride and “catching up later,” can push cortisol up for hours.
The message is simple: a well-timed bowl of oats or rice might be protecting your thyroid more than you think. If you’re stacking long endurance rides, intensity blocks, or back-to-back days, carbs are not optional. They’re a hormonal safety mechanism.
If you see prolonged fatigue, cold hands/feet, lower morning body temperature, slower heart rate recovery, or unusually flat power late in rides, low T3 is one possible contributor. The fix is rarely medication. It’s load management, sleep, and consistent carbohydrate intake through the day.
This theory still needs testing, but it aligns with what we see in the real world: riders who fuel well handle load better. Riders who don’t, crack early.
Reference
Hackney AC, et al. (2025). A Proposed Thyroid Hormone–Based Model for Understanding Relative Energy Deficiency in Sport (REDs) in Endurance Athletes. International Journal of Sports Physiology and Performance. https://journals.humankinetics...
On my mindSteady Through Chaos
Lately I’ve been reminded that progress never comes from perfect conditions. Training, work, life — none of it lines up neatly. The real shift happens when you can stay steady while things move around you. That’s the skill we’re all building, in and out of training. If your plan only works when everything goes right, it’s not a plan. It’s a wish. Learning to train, create, and live through the messy middle is where the real growth sits.
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