Cytomel and Liothyronine Dosage Calculator (T3 Dosing)
Your T3 numbers from my ratio system, in a form you can bring to your doctor:
Written and medically reviewed by Dr. Westin Childs, D.O. Last reviewed: August 28, 2026
How To Use This Calculator:
#1. Enter Your Current Medication
Your levothyroxine dose, and your T3 dose if you already take one. Your T4 dose is the anchor: the T3 target comes from it as a ratio, the same system my T4 to T3 ratio calculator uses.
#2. Add Your Safety Context
Age, heart history, and anxiety or palpitations each put a hard cap on your T3 number, because T3 works on your heart faster than T4 does. The calculator applies those caps for you.
#3. Get Your Numbers
Your target T3 dose at your chosen ratio (5:1 is my recommendation, because it mirrors the 80/20 output of a healthy thyroid gland), your week 1 starting dose at half the target, and how to split it through the day.
#4. Bring Them To Your Doctor
Every result ends in the questions for your appointment, because T3 is prescription medication and dose changes happen with your doctor, on labs, on a schedule.
Why T3 Dosing Confuses Everyone
Levothyroxine dosing has charts everywhere. T3 dosing has almost nothing, which is why I use a ratio system for it: your T4 dose is the anchor, and your T3 target comes from it.
My recommended ratio is 5:1, five parts T4 to one part T3 by dose, because it mirrors the roughly 80/20 split a healthy thyroid gland actually secretes. A patient on 100 mcg of levothyroxine comes out at a 20 mcg T3 target. Severe conversion problems can justify 4:1, and cautious situations call for 6:1, 7:1, or 10:1.[1]
Safety puts a ceiling on all of it. A heart history caps the target at 7.5 mcg, age 65 or older caps it at 10, anxiety or palpitations caps it at 10, and the combination of age and heart history caps it at 5, until you and your doctor have real-world data on how you respond.
And nobody starts at the target. Week 1 is half the target (never below 2.5 mcg), held for 6 to 8 weeks with labs before stepping up. Above 5 mcg, fast-release T3 splits into morning and early afternoon doses, because T3 peaks within hours.
Frequently Asked Questions
In my ratio system, your T4 dose divided by 5 is the T3 target: 100 mcg of levothyroxine points at 20 mcg of T3 per day, reached in steps, starting at half. Safety caps override the math when they apply: 7.5 mcg with a heart history, 10 mcg at 65 or older or with anxiety and palpitations, 5 mcg when age and heart history combine.
Not at the start, in this system. Your T4 dose is where the ratio starts, and it stays put while the T3 comes in. What decides any later T4 change is your labs and your symptoms at the 6 to 8 week re-checks, and that call belongs to your doctor with the numbers in hand.
Once you’re above about 5 mcg per day, usually yes. T3 has a short half-life, peaking within a few hours, so a single morning dose gives you an afternoon dip. Morning and early afternoon is the common split. Not at bedtime for most people, because T3 too late can disturb sleep.
For most people, no, and it’s much harder to do well. T3-only removes your stable T4 reserve, makes your TSH and T4 labs look alarming to any doctor who doesn’t know the plan, and demands careful divided dosing. It has a place, usually after combination therapy has genuinely failed, and it belongs with a doctor experienced in it.
Related Tools
- T4 to T3 Ratio Calculator: the ratio math on its own
- T4 to T3 Conversion Calculator: find out if poor conversion is why T4-only is not enough
- Thyroid Medication Conversion Calculator: switching between medications
- All Thyroid Calculators & Quizzes: the full set of free tools
Scientific References
- Wiersinga WM, Duntas L, Fadeyev V, Nygaard B, Vanderpump MP. 2012 ETA Guidelines: The Use of L-T4 + L-T3 in the Treatment of Hypothyroidism. Eur Thyroid J. 2012;1(2):55-71.
- Jonklaas J, Bianco AC, Bauer AJ, et al. Guidelines for the treatment of hypothyroidism: prepared by the American Thyroid Association Task Force on Thyroid Hormone Replacement. Thyroid. 2014;24(12):1670-1751.
- Hoang TD, Olsen CH, Mai VQ, Clyde PW, Shakir MK. Desiccated thyroid extract compared with levothyroxine in the treatment of hypothyroidism: a randomized, double-blind, crossover study. J Clin Endocrinol Metab. 2013;98(5):1982-1990.