Free Research Tool
T3 Dosing &
Conversion Calculator
Convert between T4 (levothyroxine), immediate-release T3 (Cytomel), sustained-release T3 (SR-T3), and natural desiccated thyroid (NDT) based on published clinical conversion ratios.
Enter your current dose
Equivalent daily doses
Based on the widely-cited clinical ratio of 1 mcg T3 ≈ 4 mcg T4 by potency and an NDT label spec of ~38 mcg T4 + 9 mcg T3 per grain (Armour-style). These are starting estimates only - actual titration should always be guided by labs and symptom response.
T4 (Levothyroxine)(your input)
100.0 mcg/day
Immediate-Release T3 (Cytomel)
25.0 mcg/day
Sustained-Release T3 (SR-T3)
25.0 mcg/day
Natural Desiccated Thyroid (NDT, grains)
1.35 grains/day
≈ 51.4 mcg T4 + 12.2 mcg T3
SR-T3 dosing breakdown
SR-T3 is typically dosed twice daily (every 12 hours) to mimic endogenous T3 release. For your equivalent of 25.0 mcg SR-T3/day:
Per-dose target
12.5 mcg × 2
Nearest standard strength
15.0 mcg
Important
This calculator returns potency-equivalent doses based on published clinical conversion ratios. Individual conversion efficiency varies with deiodinase activity, inflammation, and other factors, so actual titration requires lab monitoring (TSH, free T3, free T4, reverse T3) and symptom response. Not medical advice. All compounds discussed are sold strictly as research reference standards and are not approved for human consumption.
Conversion methodology
How the math works
The calculator uses two widely-cited clinical conversion ratios:
- 1 mcg T3 ≈ 4 mcg T4 by potency. This is the most commonly cited approximation (Cooper, Wartofsky, and others) used in clinical practice when transitioning between T4 monotherapy and combination therapy. T3 is roughly 4x more potent than T4 mcg-for-mcg because T4 must first be deiodinated to T3 to be metabolically active.
- 1 grain NDT (~60-65 mg) ≈ 38 mcg T4 + 9 mcg T3. This is the standardized Armour-style label specification for natural desiccated thyroid. Other NDT brands (NP Thyroid, WP Thyroid, compounded NDT) cluster around the same ratio with small batch variations.
SR-T3 and immediate-release T3 (Cytomel) contain the same active molecule - liothyronine - so they convert mcg-for-mcg. They differ in pharmacokinetics, not potency: SR-T3 spreads release over 4-8 hours via an HPMC matrix, while Cytomel releases within ~30 minutes. For the same daily dose, SR-T3 produces a flatter serum curve with fewer side effects.
The calculator returns potency-equivalent doses as a starting estimate. Real titration requires lab monitoring (TSH, free T3, free T4, reverse T3) and symptom response. Conversion efficiency varies between individuals based on deiodinase activity (DIO1, DIO2, DIO3), inflammation, micronutrient status, and stress load.