Levothyroxine to Liothyronine Dose Converter
Levothyroxine to Liothyronine Dose Converter
Interpret an existing levothyroxine dose in liothyronine-equivalent terms, using the published but contested potency ratios.
Levothyroxine to Liothyronine Dose Converter
T4 dose ÷ potency ratioLevothyroxine 100 µg/day, ratio 1:4
Formula
- potency ratio
- published estimates range from 1:3 to 1:5 — there is no single agreed value
- purpose
- this converter interprets an existing prescription; it is not a recommendation to switch
Worked example
Levothyroxine 100 µg/day, ratio 1:4
100 ÷ 4 = 25.0 µg/day liothyronine-equivalent
Between 20 and 40 → moderate dose
T4 versus T3 pharmacokinetics
| Levothyroxine (T4) | Liothyronine (T3) | |
|---|---|---|
| Approximate half-life | 7 days | 1 day |
| Serum level pattern | Stable across the day | Peaks and troughs after each dose |
| TSH as a monitoring tool | Reliable | Unreliable — TSH can look normal despite fluctuating T3 |
| Guideline stance on routine use | First-line | Not recommended for routine combination therapy |
Why this needs care, not just arithmetic
Liothyronine is roughly three to five times more potent than levothyroxine by weight, but there is no single agreed equivalence — published ratios span 1:3 to 1:5, and different centres and different studies use different values. Any liothyronine-equivalent figure calculated here is therefore an estimate bounded by real uncertainty, not a precise conversion.
The pharmacokinetics compound the difficulty. T3 has a half-life of about one day against roughly seven for T4, so an oral T3 dose produces a genuine peak-and-trough pattern across the day rather than the flat, steady serum level that once-daily levothyroxine achieves. TSH — the standard tool for monitoring thyroid replacement — cannot be interpreted the same way when T3 is fluctuating, because a single TSH measurement may not reflect the T3 exposure the patient has actually had over the preceding hours.
Major guidelines — the American Thyroid Association, the European Thyroid Association and the British Thyroid Association — do not recommend routine combination therapy with levothyroxine and liothyronine. The randomised trials comparing combination therapy against levothyroxine alone have not shown a consistent benefit in symptoms or quality of life, despite the persistent belief among some patients and some practitioners that impaired T4-to-T3 conversion explains residual symptoms on levothyroxine alone.
This converter exists to help interpret an existing T3-containing prescription — for example, converting a patient’s total intake into levothyroxine-equivalent terms when planning a medication reconciliation — not to encourage starting or switching to combination therapy. Any change to a thyroid hormone regimen should be specialist-supervised, and the reasoning behind an existing prescription should be understood before it is altered.
Frequently asked questions
How much more potent is liothyronine than levothyroxine?
Roughly three to five times more potent by weight, but there is no single agreed ratio — published estimates range from 1:3 to 1:5, which is why this converter offers all three.
Can TSH be used to monitor liothyronine dosing the way it monitors levothyroxine?
Not reliably. T3’s half-life of about one day produces peaks and troughs that a single TSH measurement does not capture the way it does for the stable levels achieved with once-daily levothyroxine.
Do guidelines recommend combination T4/T3 therapy?
No. The ATA, ETA and BTA do not recommend routine combination therapy — randomised trials have not shown a consistent benefit over levothyroxine alone.
Is this converter a recommendation to switch to liothyronine?
No. It exists to help interpret an existing prescription. Any change to a thyroid hormone regimen should be made with specialist supervision, after understanding why the current regimen was chosen.
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References
- Jonklaas J et al. Guidelines for the treatment of hypothyroidism. American Thyroid Association Task Force. Thyroid. 2014;24(12):1670–751.
- Wiersinga WM et al. 2012 ETA guidelines: the use of L-T4 + L-T3 in the treatment of hypothyroidism. Eur Thyroid J. 2012;1(2):55–71.