Your Active Thyroid Hormone
If T4 is the raw material, T3 is the finished product. T3 is 3-5x more potent and does the actual work in your cells. Most T3 is made by converting T4 in your liver, kidneys, and muscles. You can have normal T4 but low T3 if conversion is impaired—and you'll still feel hypothyroid.
What is Free T3 (Free Triiodothyronine)?
Free T3 is the unbound, active form of triiodothyronine. About 80% of T3 comes from conversion of T4 in peripheral tissues. It's the most metabolically active thyroid hormone with a short half-life of about 1 day.
↑ What High Free T3 (Free Triiodothyronine) Means
Too much active thyroid hormone. Everything speeds up: metabolism, heart rate, nervous system. Anxiety, weight loss, palpitations.
Common symptoms:
Anxiety · Rapid heartbeat · Weight loss · Tremor · Heat intolerance · Insomnia
↓ What Low Free T3 (Free Triiodothyronine) Means
Not enough active hormone reaching your cells. Even with normal T4, poor conversion to T3 means your cells aren't getting the metabolic signal.
Common symptoms:
Fatigue despite normal T4 levels · Brain fog · Cold intolerance · Hair loss · Depression · Weight gain
Why It Matters
When normal:
Directly controls cellular metabolism
Reveals conversion problems (normal T4, low T3)
Best correlates with how you actually feel
Guides combination T4/T3 therapy decisions
Risks if abnormal:
Low with normal T4: poor conversion, persistent symptoms
High: thyrotoxicosis, cardiac strain, bone loss
What Can Cause Abnormal Levels?
Poor T4-to-T3 Conversion (low)
50% likelyStress, inflammation, selenium deficiency, liver disease, and medications impair the deiodinase enzyme.
Graves' Disease (high)
45% likelyAutoimmune stimulation increases both T4 and T3.
Chronic Stress
Cortisol inhibits T4-to-T3 conversion and increases inactive reverse T3.
Selenium Deficiency
Selenium is required for the deiodinase enzyme that converts T4 to T3.
Caloric Restriction
Your body downregulates T3 to conserve energy during starvation—even intentional dieting.
What You Can Do
Selenium: 200mcg daily (Brazil nuts or supplement)
Impact: Essential cofactor for T4-to-T3 conversion enzyme \u00B7 Timeline: 4-8 weeks
Adequate caloric intake (avoid extreme diets)
Impact: Prevents starvation-triggered T3 suppression \u00B7 Timeline: 2-4 weeks
Zinc: 15-30mg daily
Impact: Supports thyroid hormone metabolism \u00B7 Timeline: 4-8 weeks
If lifestyle changes aren't enough:
Stress management (reduce cortisol)
Impact: Cortisol directly impairs T4-to-T3 conversion \u00B7 Timeline: 4-8 weeks
Support liver health
Impact: Liver is the main T3 production site \u00B7 Timeline: 4-8 weeks
Iron optimization (ferritin >50)
Impact: Iron needed for thyroid hormone production \u00B7 Timeline: 8-12 weeks
Recommended retest: 6-8 weeks after treatment change
Related Markers
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