The Thyroid Brake Pedal
Reverse T3 is the mirror image of T3—it fits into T3 receptors but does nothing. Your body makes it instead of active T3 when it wants to slow metabolism down. It's a conservation mechanism: stress, starvation, and illness trigger the switch from T3 to reverse T3.
What is Reverse T3?
Reverse T3 (rT3) is an inactive metabolite of T4. Under stress, deiodinase type 3 converts T4 to inactive rT3 instead of active T3. The Free T3/Reverse T3 ratio is more clinically useful than rT3 alone.
↑ What High Reverse T3 Means
Your body is deliberately braking your metabolism. Common with chronic stress, crash dieting, severe illness, or high cortisol. You may feel hypothyroid even with normal TSH and T4.
Common symptoms:
Fatigue despite normal thyroid labs · Weight loss resistance · Brain fog · Cold intolerance · Depression · Hair loss
↓ What Low Reverse T3 Means
Good—your body is efficiently converting T4 to active T3.
Common symptoms:
Not typically symptomatic
Why It Matters
When normal:
Explains persistent hypothyroid symptoms with normal labs
Identifies stress-related metabolic suppression
Guides treatment when standard labs look normal
Risks if abnormal:
High rT3: functional hypothyroidism at cellular level
Associated with chronic fatigue, weight resistance, brain fog
What Can Cause Abnormal Levels?
Chronic Stress and High Cortisol
60% likelyCortisol directly shifts T4 conversion from active T3 toward inactive reverse T3.
Severe Illness
50% likelyYour body deliberately slows metabolism to conserve energy. rT3 is the mechanism.
Crash Dieting
Starvation triggers rT3 to slow metabolism. This is why extreme diets cause metabolic adaptation.
Chronic Inflammation
Inflammatory cytokines impair T4-to-T3 conversion.
Iron Deficiency
Iron is needed for proper deiodinase function.
What You Can Do
Stress management: sleep, meditation, boundaries
Impact: Reduces cortisol-driven rT3 \u00B7 Timeline: 4-8 weeks
Adequate caloric intake (no crash diets)
Impact: Prevents starvation signal \u00B7 Timeline: 2-4 weeks
Anti-inflammatory diet
Impact: Reduces inflammatory rT3 production \u00B7 Timeline: 4-8 weeks
If lifestyle changes aren't enough:
Selenium: 200mcg daily
Impact: Supports proper T4-to-T3 conversion \u00B7 Timeline: 4-8 weeks
Optimize iron stores (ferritin >50)
Impact: Iron needed for deiodinase function \u00B7 Timeline: 8-12 weeks
Zinc: 15-30mg daily
Impact: Supports thyroid hormone metabolism \u00B7 Timeline: 4-8 weeks
Recommended retest: 6-8 weeks after addressing causes
Related Markers
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