Does Endurance Training Suppress Your Thyroid?
Does Endurance Training Suppress Your Thyroid?
TL;DR
Chronic endurance training (60+ minutes daily, 5-7x/week) suppresses thyroid in most people. High cortisol from long cardio blocks T3 conversion. Metabolism slows. Fat loss stalls despite high mileage. Strength declines. If training for event, mitigate with adequate calories, carbs, and periodic rest weeks. If training "for health," reduce volume significantly.
You run every day.
Six miles. Eight miles. Ten. Or cycle for hours. Swim laps.
High mileage. Consistent training. Dedicated athlete.
But you're not getting leaner. You're getting slower. Weaker. More tired.
Can't sleep. Always hungry. Mood is terrible.
You thought more cardio was better. It's not.
You're overtrained. Your thyroid is suppressed.
Endurance training is like revving your engine at redline constantly. Works short-term. Long-term, engine breaks down.
How Endurance Training Affects Thyroid
Chronic cardio = chronic stress.
Every long training session:
- Depletes glycogen
- Raises cortisol
- Triggers stress response
Cortisol blocks T4-to-T3 conversion.
T3 (active thyroid hormone) drops. Metabolism slows.
Your body adapts: "We're in energy scarcity. Slow metabolism to conserve fuel."
This is protective short-term. Destructive long-term.
The Endurance Athlete Pattern
Classic presentation:
Training:
- Running 30-50+ miles per week
- Or cycling 8-12+ hours per week
- Or other endurance sport (swimming, triathlon, ultrarunning)
Symptoms:
- Chronic fatigue despite training
- Can't lose body fat (especially women)
- Cold hands and feet
- Poor sleep quality
- Frequent illness
- Low libido
- Women: irregular or missing periods
- Mood issues (irritability, depression)
Labs:
This is RED-S (Relative Energy Deficiency in Sport).
Metabolism is broken.
Acute vs. Chronic Cardio
Acute (single session):
- T3 drops temporarily
- Cortisol spikes
- Metabolism returns to baseline within 24-48 hours
- No long-term damage
Chronic (daily, 60+ minutes):
- T3 stays suppressed
- Cortisol stays elevated
- Metabolism adapts by slowing
- Long-term thyroid suppression
Frequency and duration matter.
Running 3x per week (30-40 minutes): probably fine. Running 6x per week (60-90 minutes): likely suppresses thyroid.
Who This Affects Most
Women more sensitive than men.
Female hormones more vulnerable to energy deficit. Periods disappear with high training volume + insufficient calories.
Undereaters: Combining endurance training + calorie restriction = severe thyroid suppression.
Low-carb athletes: Endurance training without adequate carbs worsens thyroid function.
Already hypothyroid: If thyroid is already low, endurance training makes it worse.
How to Mitigate Thyroid Suppression
If training for specific event (marathon, triathlon):
1. Eat enough calories:
- Match expenditure
- Don't restrict to lose weight while training
- 2,500-4,000+ calories depending on volume
- 5-7g per kg body weight
- Replenish glycogen daily
- Don't go low-carb
- 1.6-2.0g per kg body weight
- Prevents muscle breakdown
4. Periodize training:
- Build weeks (high volume)
- Recovery weeks (50% volume)
- Don't hammer year-round
- Waking temperature (should stay above 97.8°F)
- Resting pulse (should stay 75-85)
- Sleep quality
- Women: period regularity
If markers decline, reduce volume immediately.
If Training "For Health"
Endurance training for general fitness:
You don't need high volume.
30-40 minutes, 3-4x per week is sufficient for cardiovascular health.
Better approach:
- Strength training 2-3x/week
- Moderate cardio 2-3x/week (30-40 minutes)
- Walking daily
- Sprints 1x/week (optional)
This supports metabolism. High-volume endurance training suppresses it.
When to Reduce Training Volume
Signs of overtraining:
- Temperature below 97.8°F
- Resting pulse above 90 or below 70
- Chronic fatigue
- Performance declining despite training
- Frequent illness
- Mood disturbances
- Women: period irregularity or loss
Take 1-2 weeks completely off.
Temperature should rise. Energy should improve. Sleep should normalize.
Then return at 50% volume. Build back slowly.
Supplements for Endurance Athletes
Won't fix overtraining. But may help:
Creatine:
- 5g daily
- Supports ATP production
- Helps performance
- 300-400mg before bed
- Supports recovery
- Helps sleep
Carbs during long sessions:
- 30-60g per hour for sessions over 90 minutes
- Prevents glycogen depletion
Adequate food is more important than supplements.
FAQ
Q: I love running. Do I have to quit? A: No. But reduce frequency or duration if metabolism is suppressed. 3-4 runs per week (30-50 minutes) is sustainable. Daily long runs often aren't.
Q: Can I train for a marathon without suppressing thyroid? A: Difficult. Marathon training volume often suppresses thyroid temporarily. Mitigate with adequate food, carbs, rest weeks. Plan recovery phase after race.
Q: Will my performance suffer if I reduce volume? A: Initially, maybe. But with optimized metabolism, quality improves. Many athletes PR after reducing volume and fixing thyroid.
Q: What about ultrarunners and Ironman athletes? A: Extreme endurance almost always suppresses thyroid. Accept this as trade-off, or reduce volume. Can't have both optimal metabolism and extreme endurance training.
This isn't medical advice. Work with qualified coach for training guidance.