T3 (Liothyronine): The Active Thyroid Hormone
Last updated: March 2026
The metabolically active form of thyroid hormone. 4-5x more potent than T4. Short half-life requires divided dosing. The T4+T3 combination therapy debate continues.
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Than T4
(vs 7 days T4)
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What Is T3?
T3 (liothyronine) is the body's active thyroid hormone. While T4 (levothyroxine) is the major secretion from the thyroid, T3 is the form that actually binds to thyroid receptors and drives metabolic rate.
T3 binds directly to thyroid hormone receptors (TRα and TRβ) with high affinity. This direct activation is why T3 is 4-5x more potent than T4 on a molar basis.
The body converts T4 to T3 via deiodinase enzymes (mainly type 1 and type 2). Some patients have genetic variations affecting this conversion, leading to the "T3 trial" concept.
T3 has a half-life of approximately 1 day (vs T4's 7 days). This requires divided dosing (usually 2x/day) to maintain stable levels. Also causes faster reversal if discontinued.
Some patients and physicians advocate adding T3 to standard T4 therapy. This is controversial — studies show mixed results. Patient preference plays a significant role in treatment decisions.
What the Research Shows
Context: Most hypothyroidism patients do well on T4 monotherapy. The subset who don't respond fully may have conversion issues or T3 receptor polymorphisms. Combination therapy remains controversial with mixed study results.
T3 vs T4
- Active hormone — binds receptors directly
- 4-5x more potent than T4
- Half-life: ~1 day
- Requires 2x daily dosing
- Not standard monotherapy
- Brand: Cytomel, Triostat
- Prohormone — converts to T3 in tissues
- Standard treatment for hypothyroidism
- Half-life: ~7 days
- Once-daily dosing
- FDA-approved first-line
- Brand: Synthroid, Levoxyl
Dosing
- Start: 5mcg once or twice daily
- Typical range: 5-25mcg/day total
- Divide into 2 doses (morning + early afternoon)
- Take on empty stomach
- Wait 30-60 min before food
- First dose: morning on waking
- Second dose: early afternoon (before 3pm)
- Don't take late in day (can disrupt sleep)
- Consistent timing important
Key Takeaways
- FDA-approved for hypothyroidism treatment
- 4-5x more metabolically potent than T4
- Useful for patients with conversion issues
- Some patients strongly prefer combination therapy
- Optimal T4:T3 ratio for combination therapy
- Long-term effects of T3 supplementation
- Which patients benefit most from T3 addition
- T3-only therapy not well-studied
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Thyroid medication requires a prescription and medical supervision. T3 (liothyronine) is FDA-approved but must be monitored with blood tests (TSH, Free T3, Free T4). Never adjust thyroid medication without consulting your healthcare provider. This is not medical advice. FDA Approved Rx Required