Why Does Low-Carb Kill Your Thyroid and Energy?
Why Does Low-Carb Kill Your Thyroid and Energy?
TL;DR
Low-carb diets suppress T3 (active thyroid hormone) conversion. Your body sees carb restriction as starvation and slows metabolism to conserve energy. Initial weight loss is great. Then you hit a wall: cold, tired, can't lose more weight, hair falling out, period gone. Add carbs back gradually while eliminating PUFAs. Metabolism restores. Energy returns.
You started keto six months ago.
The first month was amazing. Lost 15 pounds. Energy through the roof. Mental clarity.
Now you're freezing. Exhausted by 2 PM. Hair falling out. Can't lose another pound despite eating 1200 calories.
Women: your period stopped. Men: libido crashed.
Keto worked. Then it broke you.
Low-carb is like running your car in eco-mode. Great for efficiency. Terrible for performance. Eventually the engine fails.
What Happens on Low-Carb
Your body has two fuel sources: glucose (from carbs) and ketones (from fat).
Normal eating:
- Eat carbs → glucose → cells use glucose → thyroid function normal
Low-carb/keto:
- Restrict carbs → low glucose → liver makes ketones from fat → cells use ketones
- Initially: Works well, lose weight, feel good
- Over time: Thyroid downregulates to conserve energy
Why thyroid slows: Your body thinks you're starving. No glucose = scarce food environment. Metabolism slows to preserve energy for survival.
T4 (storage thyroid hormone) stops converting to T3 (active thyroid hormone). Instead, it converts to Reverse T3 (inactive form that blocks T3).
Result:
- High T4
- Low T3
- High Reverse T3
- All the symptoms of hypothyroidism
Initial Success of Low-Carb
Why it works at first:
- You stop eating carbs
- Insulin stays low
- Your body releases stored water (glycogen holds water)
- You lose 5-10 pounds fast (mostly water)
You feel energized.
- Ketones provide fuel
- No blood sugar crashes
- Mental clarity improves
- Appetite decreases
This is real. Low-carb works short-term.
But: It's not sustainable for most people. Especially if metabolism is already compromised from PUFAs.
When Low-Carb Breaks
Month 3-6:
- Energy starts dropping
- Cold hands and feet
- Sleep worsens
- Mood declines
- Weight loss stalls completely
- Hair starts thinning
Month 6-12:
- Severe fatigue
- Temperature drops below 97°F
- Brain fog
- Women: periods become irregular or stop
- Men: testosterone drops, libido crashes
- Muscle loss despite protein intake
- Can't tolerate exercise anymore
Your labs:
- TSH: Normal or slightly elevated
- Free T4: Normal or high
- Free T3: Low (bottom third of range)
- Reverse T3: High
Classic pattern of thyroid suppression from chronic stress (carb restriction = metabolic stress).
The Cortisol Problem
When you don't eat carbs, your body makes glucose from protein (gluconeogenesis). This requires cortisol.
Chronic low-carb = chronic cortisol elevation.
Effects of high cortisol:
- Breaks down muscle
- Suppresses thyroid
- Disrupts sleep
- Increases anxiety
- Raises blood sugar (despite no carb intake)
- Prevents fat loss
You're running on stress hormones instead of thyroid hormone.
This feels like:
- Wired but tired
- Can't relax
- Mind racing at night
- Elevated resting pulse (above 85-90)
- Low body temperature
Who Can Handle Low-Carb
Some people do fine:
- Metabolically healthy (rare)
- Adequate thyroid function going in
- Not chronically stressed
- Eating enough calories
- Strength training appropriately
Most people can't:
- Already have PUFA-suppressed thyroid
- Under chronic stress
- Under-eating
- Overtraining
- Women (more sensitive to carb restriction)
If you're in the second group, low-carb will eventually crash you.
How to Add Carbs Back Without Gaining Weight
Don't go from keto to SAD (Standard American Diet). That's a recipe for rapid weight gain.
Gradual reintroduction:
Week 1:
- Add 50g carbs per day
- From fruit and potatoes
- Eat them with meals, not alone
- Track temperature and energy
Week 2-3:
- Add another 50g carbs per day (total 100g)
- Continue tracking
Week 4-6:
- Increase to 150-200g per day
- Adjust based on how you feel
Goal:
- Temperature climbs toward 98°F
- Energy improves
- Sleep better
- Mood stabilizes
- Pulse normalizes to 75-85
You might gain 3-5 pounds initially. This is glycogen and water. Not fat. It stabilizes after 2-3 weeks.
Critical: Eliminate PUFAs while reintroducing carbs. Otherwise you just create insulin resistance and inflammation. Carbs work when metabolism is healthy.
Best Carbs for Thyroid Recovery
Easiest to digest:
- Fruit (especially ripe bananas, oranges, berries)
- White rice
- Potatoes (white or sweet)
- Honey
- Fruit juice (fresh-squeezed or not-from-concentrate)
Avoid while healing:
- Whole grains (harder to digest)
- Beans and legumes (cause bloating for many)
- Bread (usually contains seed oils)
Start simple. Add complexity later once thyroid recovers.
What About "Metabolic Adaptation"
Some argue your metabolism "adapts" to low-carb and you need to stay keto forever.
This is partly true:
- Your body becomes efficient at using ketones
- Glucose tolerance decreases (can't handle carbs as well)
- Insulin sensitivity can worsen paradoxically
But this isn't optimal. It's adaptation to chronic stress. Not health.
Better: Restore metabolic flexibility. Ability to use both glucose and fat efficiently. This requires adequate carbs to maintain thyroid function.
The Ray Peat Approach
Ray Peat (biologist) argued for high-carb, anti-stress metabolism.
Key principles:
- Adequate carbs to support thyroid
- Saturated fats (not PUFAs)
- Simple, easily digestible foods
- Temperature and pulse as metabolic markers
This works for many people who crashed on low-carb.
Not a cult. Just: eliminate PUFAs, eat enough carbs, support thyroid. Simple.
FAQ
Q: I feel great on keto. Should I add carbs anyway? A: Check your temperature and pulse. If temperature is 98°F+ and pulse is 75-85, you might be fine. If temperature is low (below 97.8°F) and pulse is either very low or very high, your thyroid is suppressed even if you "feel good" (likely running on stress hormones).
Q: Will I gain all the weight back? A: Not if you eliminate PUFAs and don't overeat. Initial water weight gain is normal (3-5 pounds). Beyond that, weight stabilizes or continues dropping as metabolism improves.
Q: What about carnivore diet? A: Zero-carb is even more extreme than keto. Some people thrive. Most crash harder and faster. Thyroid needs some glucose. If carnivore works for you long-term with good biomarkers, continue. But most people need carbs.
Q: Can I do cyclical keto (keto weekdays, carbs weekends)? A: This can work. Provides metabolic flexibility. But if you're already crashed from long-term keto, you need consistent carbs daily to recover thyroid function. Cyclical keto is for already-healthy people, not recovery.
This isn't medical advice. I'm not your doctor. Make informed decisions about your diet based on your metabolic state.
Want the complete metabolic recovery protocol?
The SugarSaint course includes detailed strategies for recovering from low-carb thyroid suppression, reintroducing carbs safely, and optimizing metabolism through proper nutrition.
Crashed on low-carb?