T3 (Triiodothyronine)
The metabolic accelerator. T3 is the active thyroid hormone—3-4x more potent than T4. It enters cells and regulates metabolism, body temperature, heart rate, and energy production. Most T3 is made from T4 conversion in peripheral tissues, not directly from the thyroid.
T4 to T3 Conversion
Deiodinase Enzymes
D1, D2, D3 enzymes remove iodine from T4. D1/D2 create active T3. D3 creates reverse T3.
Selenium Required
Deiodinases are selenoproteins. Low selenium = poor T4 to T3 conversion. Common deficiency.
Conversion Sites
Liver, kidney, muscle, gut. Local conversion in brain, thyroid. Tissue-specific regulation.
What Blocks T4 to T3 Conversion
Common Blockers
- Stress/Cortisol: Shifts to reverse T3
- Calorie restriction: Body slows metabolism
- Inflammation: Cytokines impair D1/D2
- Selenium deficiency: Enzyme doesn't work
More Factors
- Iron deficiency: Impairs conversion
- Gut issues: 20% converted in gut
- Liver disease: Major conversion site
- Heavy metals: Mercury blocks deiodinases
Reverse T3 (rT3)
What It Is
Inactive form. Same structure as T3 but iodine in different position. Blocks T3 receptors.
When It Rises
Stress, illness, fasting, inflammation. Body's way of slowing metabolism during crisis.
Testing
Free T3/rT3 ratio matters. High rT3 with normal T4 = conversion problem. Often missed.
What T3 Does
Metabolism
Controls basal metabolic rate. How fast you burn calories. Low = weight gain, fatigue.
Body Temperature
Thermogenesis regulation. Low T3 = cold intolerance. Can't generate heat properly.
Heart Function
Heart rate and contractility. Low T3 = slow heart rate. High T3 = fast/palpitations.
Brain Function
Cognitive function, mood. Low T3 = brain fog, depression. Critical for development.
Gut Motility
Controls digestive speed. Low T3 = constipation. High T3 = diarrhea.
Mitochondria
Increases mitochondrial activity. More ATP production. Cellular energy.
Testing & Treatment
Free T3 Test
Measures unbound, active T3. Total T3 includes protein-bound. Free T3 more useful.
Optimal Range
Many feel best in upper third of range. "Normal" isn't always optimal. Symptoms matter.
Support Conversion
Selenium, zinc, iron. Reduce stress. Address inflammation. Heal gut.
T3 Medication
Liothyronine (synthetic T3). Or desiccated thyroid (has both T4 and T3). For non-converters.
Dosing Considerations
T3 is short-acting. Often split into 2-3 doses. Sustained-release options exist.
Monitor Carefully
Too much T3 = heart issues, bone loss. Start low, go slow. Regular testing.