T4 to T3 Ratio Calculator
This calculator tells you the optimal amount of T3 to add to your dose of Synthroid, levothyroxine, or Tirosint.
It’s created for patients on T4-only thyroid medications who still have persistent symptoms [1][2].
Here’s how to use it:
Enter the requested information, including your dose of T4 thyroid medication, along with your personal situation. Pick a target T4 to T3 ratio (the calculator will guide you on this). Flat any safety modifiers that will automatically adjust your dose on the fly and then submit.
Generic T4 to T3 ratio calculators provide you with standard low-dose T3 recommendations, but this one is different. It’s based on the physiologic output of a healthy thyroid gland and the clinical experience of thyroid experts[3].
Note: This calculator is a clinical tool, not a prescription. Don’t start or adjust T3 therapy based on the results without your doctor. Skip this calculator entirely if you have atrial fibrillation, are pregnant, or have untreated adrenal insufficiency.
T4 to T3 Ratio Calculator
Find your target Cytomel or liothyronine dose based on your current T4 dose and the 80/20 principle of healthy thyroid output. For patients exploring combination T4 plus T3 therapy.
Optional: Add your labs for a more detailed reading
What this means for you
Next steps
Combination therapy works best when your T4 dose is already optimized and your labs are in a functional range. Before adding T3, check:
Are your thyroid labs in the functional range?
Is your T4 dose already right before adding T3?
Is your T3 conversion impaired? Check your Reverse T3 ratio.
Written and medically reviewed by Dr. Westin Childs, D.O. Last reviewed: April 24, 2026.
How to Use This Calculator
Step #1: Enter Your Sex, Age, and Current T4 Dose
Your current T4 dose is used as the anchor for your T3 calculation (this is similar to how your own thyroid gland operates). Your age, sex, and risk factors are then used as modifiers to adjust this dose.
Enter your T4 dose in micrograms, not milligrams. A 100 mcg Synthroid tablet is the same dose whether it’s labeled generic levothyroxine, Tirosint, or Unithroid.
Different brands of levothyroxine do impact absorption, but not your microgram dosage[4]. If you don’t know your T4 dose, check your prescription bottle before you run the calculator.
I designed the calculator to output more conservative measures based on age and other risk factors like heart disease. This ensures that you get a heart-safe T3 dose, regardless of your situation.
Step #2: Pick What Best Describes Your Situation
This dropdown tells the calculator why you’re considering combination T4/T3 therapy.
The seven options cover the most common situations:
- T4 treatment alone with continued low thyroid symptoms
- Low free T3 with otherwise “normal” labs (poor T4 to T3 conversion)
- Hashimoto’s patients (usually undertreated)
- Post-thyroidectomy patients (full or partial)
- Thyroid cancer patients on TSH suppressive doses of thyroid medication
- And those with standard hypothyroidism who want a more comprehensive thyroid regimen[1][5]
Not sure which option fits? Pick the one closest to your primary reason for considering T3.
If you’re still symptomatic on T4, pick “ongoing symptoms”.
If your labs show a low free T3, pick “poor converter”.
If you don’t see any other label that fits you, pick “hypothyroidism exploring combo therapy”.
Step #3: Pick Your Target T4 to T3 Ratio
The default ratio is my recommended 5:1 T4:T3. This is based on the physiologic output of thyroid hormone from the thyroid gland, which is estimated to be around 80% T4 and 20% T3.
At 5:1, a patient on 100 mcg of T4 targets 20 mcg of T3 per day.
The other options exist for specific situations:
- Pick 4:1 if you have severe conversion issues (very low Free T3, high Reverse T3) and want more T3 per mcg of T4.
- Pick 6:1 if you want to start more conservatively than the recommended default.
- Pick 7:1 if you have a history of heart disease or you’re over 65 and want a lower starting T3 load.
- Pick 10:1 if you want the smallest T3 dose possible while still using combination therapy[6].
The ratio preview below the dropdown updates live as you change your T4 dose.
Before you hit calculate, you’ll see exactly what T3 dose your selected ratio produces. If the number looks too high or too low, feel free to change the ratio.
Step #4: Pick Your T3 Medication Type
Most patients take Cytomel (brand) or liothyronine (generic), which are both immediate-release T3 tablets available in 5 mcg and 25 mcg strengths.
The calculator defaults to once-daily dosing because that’s what most patients actually do, even though the textbooks still recommend twice-daily.
Twice daily dosing provides steadier T3 levels throughout the day[5], but is more complicated to fit into a busy schedule.
Pick a compounded sustained-release or SR T3 if you want once-daily dosing with a reduced risk of heart palpitations. Compounded T3 formulas are not available at regular box office pharmacies and must be obtained at a compounding pharmacy.
The medication type changes the dosing schedule in the output but not the total daily dose.
20 mcg of T3 is 20 mcg of T3, whether you take it all at once or split it.
Step #5: Flag Any Modifiers That Apply
The modifier checkboxes are there for safety.
They adjust how aggressively the calculator doses T3 and, in two cases, block the calculation entirely[1].
Atrial fibrillation is one of those. T3 raises heart rate and cardiac output, both of which can trigger arrhythmias in afib patients.
The calculator will not generate a T3 dose with this flag checked. Pregnancy is also a hard stop. Maternal T3 does not cross the placenta the way T4 does, which means fetal brain development depends on the T4 your body converts locally[7]. Current guidance favors T4 monotherapy during pregnancy, even though many providers use and recommend T4/T3 combos during pregnancy.
If you flag heart disease, your age at 65 or older, a history of anxiety or heart palpitations, the calculator will lower your total recommended dose of T3.
A 70-year-old with heart disease on 100 mcg of T4 doesn’t get 20 mcg of T3 at a 5:1 ratio. The safety cap drops the target to 5 mcg.
The calculator shows you both numbers so you understand what’s happening.
Adrenal insufficiency or low cortisol triggers a warning banner but doesn’t block the dose.
Adding T3 on top of low cortisol can trigger an adrenal crisis, so make sure to optimize your cortisol first[8].
Understanding Your Results
Your Target T3 Dose
The big number at the top of your results is your daily T3 target in micrograms (mcg).
It’s your T4 dose divided by the ratio you picked. For a patient on 100 mcg of T4 at the recommended 5:1 ratio, the target is 20 mcg of T3 per day.
The three cells below the main number show you the full picture: your T4 dose (unchanged), your T3 dose, and the actual ratio the combination produces.
If any safety cap is applied, the actual ratio will look more conservative than the target ratio you selected. This is by design.
When hitting your target T3 dose, you shouldn’t lower your T3 dose. This is a common mistake.
The 5:1 T4:t3 pattern that I recommend adds T3 on top of your existing T4 dose to help overcome conversion issues, not to replace that T4.
In general, reducing T4 while adding T3 often makes thyroid patients feel worse, not better[9].
Why the 80/20 Principle Matters
The 80/20 principle is the physiological output of a healthy human thyroid gland. Your thyroid secretes approximately 80 percent T4 and 20 percent T3 by mass directly into the bloodstream[3]. That’s the ratio your body was made to run on.
T4-only thyroid medication treatment assumes your body is capable of converting the right amount of T4 into T3 on its own. For some patients, this works, but for many, it doesn’t.
Impaired conversion (from stress, inflammation, dieting, a DIO2 polymorphism, or just being post-thyroidectomy) means the T3 output stays low even when the T4 dose looks adequate on labs[5][10].
Adding T3 at the 80/20 ratio bypasses the broken conversion step and gives your body the hormone in the form it needs.
Conventional doctors recommend combination therapy at a ratio of 14:1 or 16:1[11], based on the biological activity of T3 compared to T4.
I use the 5:1 ratio because that’s what I’ve seen works best for thyroid patients.
A 100 mcg T4 plus 20 mcg T3 combination resolves symptoms in most patients far more reliably than 100 mcg T4 plus 5-7 mcg T3.
The Titration Plan
The calculator gives you a three-step titration plan:
Week 1 starts at 50% of your target dose.
Week 2 puts you at your full recommended T3 dose.
And labs are then recommended after 6-8 weeks of use.
This titration schedule accounts for your body to adapt to your new T3 dose to minimize side effects like palpitations, jitters, or anxiety, which may appear with rapid titrations.
Labs at week 6 to 8 give your body long enough to reach a steady state on the new combination.
My recommended TSH target is between 0.5 and 1.0, with a free T3 and total T3 in the upper 1/3 of the reference range provided.
If TSH drops below 0.5 and you feel overactive (racing heart, heat intolerance, insomnia), your T3 dose likely needs to be reduced.
When Your Dose Gets Capped
Three flags trigger a safety cap on your T3 target: a history of heart disease, age 65 or older, and anxiety or heart palpitations. Any one of them lowers the cap. Two of them lower it further.
Heart disease caps the target at 7.5 mcg of T3 per day. Age 65 or older caps at 10 mcg. The combination of both caps at 5 mcg. Anxiety or palpitations cap at 10 mcg[1][12]. T3 raises heart rate and cardiac output within an hour of dosing. For a healthy patient, that’s not a problem. For someone with cardiac disease or reduced cardiovascular reserve, even a physiological dose of T3 can feel like too much or trigger symptoms.
When a cap applies, the results show both the uncapped and capped numbers so you can see what was adjusted and why. You can always titrate slowly upward from the capped dose if your doctor monitors your response and your labs. The cap isn’t a ceiling for life. It’s a safer starting point than the ratio math alone would give.
What Your Labs Are Telling You
The optional labs section unlocks tailored interpretation based on your TSH, Free T4, Free T3, and Reverse T3. Skipping it doesn’t change the T3 dose calculation, but adding your labs tells you whether combination therapy is the right next step for your specific situation.
A TSH above 2.5 means your T4 dose may not be optimized yet. Adding T3 before optimizing T4 is usually the wrong order of operations. Use the Levothyroxine Dose Adjustment Calculator to fix T4 first, then come back here to layer in T3. A TSH below 0.5 (outside of cancer suppression) means you may already be slightly over-replaced on T4. Adding T3 on top of that can push you into an overactive thyroid state.
The Free T3 to Reverse T3 ratio is the single best lab indicator of whether combination therapy will help you. A ratio below 20 means your conversion is impaired and T3 is well-indicated. A ratio above 20 means your conversion is adequate, and adding T3 may not change much[13]. To run this ratio on its own, use the Reverse T3 Ratio Calculator.
T4 to T3 Ratio Reference Table
The table below shows the T3 dose your current T4 dose maps to at each of the five ratio options. Use it as a quick lookup without running the calculator. The calculator still applies safety caps on top of these numbers when relevant.
| T4 Dose (mcg) | 4:1 (more T3) | 5:1 (Dr. Childs recommended) | 6:1 (slightly less T3) | 7:1 (cautious start) | 10:1 (very cautious) |
|---|---|---|---|---|---|
| 50 mcg | 12.5 mcg | 10 mcg | 7.5 mcg | 7.5 mcg | 5 mcg |
| 75 mcg | 17.5 mcg | 15 mcg | 12.5 mcg | 10 mcg | 7.5 mcg |
| 88 mcg | 22.5 mcg | 17.5 mcg | 15 mcg | 12.5 mcg | 7.5 mcg |
| 100 mcg | 25 mcg | 20 mcg | 17.5 mcg | 15 mcg | 10 mcg |
| 112 mcg | 27.5 mcg | 22.5 mcg | 17.5 mcg | 15 mcg | 10 mcg |
| 125 mcg | 30 mcg | 25 mcg | 20 mcg | 17.5 mcg | 12.5 mcg |
| 137 mcg | 35 mcg | 27.5 mcg | 22.5 mcg | 20 mcg | 15 mcg |
| 150 mcg | 37.5 mcg | 30 mcg | 25 mcg | 22.5 mcg | 15 mcg |
| 175 mcg | 45 mcg | 35 mcg | 30 mcg | 25 mcg | 17.5 mcg |
| 200 mcg | 50 mcg | 40 mcg | 32.5 mcg | 27.5 mcg | 20 mcg |
The T3 doses in the table are rounded to the nearest prescribable Cytomel or liothyronine increment. Compounded T3 can be made at any half-mcg increment if the exact ratio target matters for your situation.
Want to check whether your T4 dose is optimized before adding T3? Use the Optimal Thyroid Lab Test Calculator.
Switching from T4 to a different thyroid medication entirely? Use the Thyroid Medication Conversion Calculator.
Want to know if your T3 conversion is impaired before committing to combination therapy? Use the Reverse T3 Ratio Calculator.
Frequently Asked Questions
A good T4 to T3 ratio for combination therapy is 5:1 by mass, which matches the 80/20 output of a healthy human thyroid gland[3]. At 5:1, a patient on 100 mcg of T4 per day adds 20 mcg of T3 per day. Conventional guidelines sometimes use 14:1 to 16:1, which come from a T4-equivalent potency interpretation rather than a direct mass ratio[11]. In clinical practice, the mass ratio is more reliable for resolving symptoms.
The ratio isn’t a rigid rule. Patients with severe conversion issues sometimes need a more T3-heavy ratio (4:1). Patients with cardiac history or who are over 65 usually need a more conservative ratio (7:1 or 10:1). Use the calculator above to run your specific numbers.
At my recommended 5:1 ratio, the starting Cytomel dose for a patient on 100 mcg of Synthroid is 20 mcg of Cytomel per day. At 75 mcg of Synthroid it’s 15 mcg of Cytomel. At 150 mcg of Synthroid it’s 30 mcg of Cytomel. The table earlier on this page shows the full range[1].
Cytomel (or its generic, liothyronine) comes in 5 mcg and 25 mcg tablets, so most doses involve splitting or combining tablets. A 20 mcg daily dose is four 5 mcg tablets. A 15 mcg dose is three 5 mcg tablets. The 25 mcg tablet is only used once your total daily dose exceeds about 15 mcg and needs to be split (a half-tablet is 12.5 mcg).
Start at 50 percent of your target dose for the first week to avoid palpitations or jitters, then step up to the full target in week 2. Recheck labs at week 6 to 8.
Yes, you can add T3 to levothyroxine. It’s a well-established treatment approach called combination therapy, and it’s endorsed by the American Thyroid Association and the European Thyroid Association for patients who remain symptomatic on T4 monotherapy[1][11]. The T3 comes from either Cytomel, generic liothyronine, or compounded T3.
You need a prescription from your doctor to add T3. Most conventional endocrinologists are reluctant to prescribe it because T4 monotherapy is still the first-line standard. Functional medicine doctors, integrative physicians, and some thyroid-focused endocrinologists are more comfortable with combination therapy. You may need to seek out a doctor who works with T3 if your current prescriber won’t consider it.
You need T3 with levothyroxine if you have persistent hypothyroid symptoms despite a “normal” TSH on T4 alone, a low Free T3 on adequate T4 dosing, a high Reverse T3 or a low Free T3 to Reverse T3 ratio, or a DIO2 polymorphism that impairs T4-to-T3 conversion[5][10]. Post-thyroidectomy patients also benefit from combination therapy because removing the thyroid permanently reduces conversion capacity.
You probably don’t need T3 if your labs are in the functional range (TSH 0.5 to 2.0, Free T3 in the upper third of the reference range) and you feel well on T4 alone. Adding T3 to a patient who’s already optimized doesn’t improve outcomes and sometimes makes them worse. The decision to add T3 should be driven by symptoms plus labs, not by symptoms alone.
Conventional doctors don’t like to prescribe T3 for three reasons. First, T4 monotherapy is still the first-line standard of care in the ATA and AACE guidelines, and most endocrinologists stick with first-line treatment by default[1]. Second, T3 has a shorter half-life and a narrower therapeutic window than T4, which means dose adjustments feel more sensitive. Third, T3 is often misused for weight loss, which has created a professional reluctance to prescribe it even for legitimate clinical reasons.
None of these reasons justify refusing T3 to a patient who genuinely needs it. Patients with impaired T4-to-T3 conversion, post-thyroidectomy status, or persistent symptoms on T4 alone often feel significantly better on combination therapy[5]. If your current doctor won’t consider T3 and your symptoms persist, look for a functional medicine doctor, integrative physician, or thyroid-focused endocrinologist who’s more open to combination therapy.
5 mcg of Cytomel is not enough for most combination therapy patients. At a 5:1 mass ratio, 5 mcg of Cytomel only makes sense if your T4 dose is 25 mcg per day, which is a very low replacement dose. Most patients on 75 to 150 mcg of T4 need 15 to 30 mcg of Cytomel per day to hit a physiological ratio[5].
5 mcg is a reasonable starting dose for week 1 of a titration plan, or as a starting cap for patients with cardiac history, age 65 or older, or severe anxiety. But 5 mcg as a final target dose usually leaves patients under-dosed on T3 and not feeling better than they did on T4 alone. Run your specific T4 dose through the calculator above to see the right target for you.
Most patients do fine on once-daily T3 dosing, which is what I recommend as the default even though textbooks still suggest twice-daily[5]. Once-daily is simpler, easier to remember, and works well for the majority of patients. Take your T3 with your T4 in the morning on an empty stomach and wait 30 to 60 minutes before eating.
Split the dose twice daily if you feel a distinct afternoon crash on once-daily dosing. T3 has a half-life of about 24 hours, but some patients feel the peak wear off by mid-afternoon. Splitting your dose into morning and early afternoon keeps hormone levels steadier through the day. Compounded sustained-release or slow-release T3 is a once-daily option that mimics the steadier curve of split dosing without the extra tablet.
T3 is biologically active within 1 to 2 hours of taking it, but symptom improvement takes longer. Most patients notice better energy, clearer thinking, and warmer extremities within the first 1 to 2 weeks of starting T3. Weight loss, hair regrowth, and mood improvement usually take 6 to 12 weeks[5].
If you don’t feel any change by week 4, your dose is likely too low. Recheck your labs, compare your Free T3 to the upper third of the reference range, and discuss increasing the dose with your doctor. If you feel worse instead of better (palpitations, jitters, anxiety, insomnia), your dose is too high. Drop back to the week 1 starting dose and re-titrate more slowly, or reconsider whether T3 is the right fit for your situation.
Yes, taking T3 suppresses TSH. T3 is the active thyroid hormone, and your pituitary gland senses rising T3 levels and lowers TSH in response. This is normal and expected on combination therapy. A TSH between 0.5 and 2.0 with adequate Free T3 is the functional target I recommend for most combination therapy patients[1].
A TSH below 0.5 is not automatically dangerous. Thyroid cancer patients are intentionally suppressed below 0.1. Combination therapy patients sometimes run below 0.5 without symptoms of over-replacement. The risk of a suppressed TSH is usually overstated by conventional guidelines that still treat TSH as the only lab that matters. The labs that matter most on combination therapy are Free T4, Free T3, and Reverse T3, not TSH alone[13]. A low TSH with a Free T3 in the upper third of the range and no symptoms of over-replacement is usually fine.
The most common side effects of Cytomel are heart palpitations, jitters, anxiety, insomnia, heat intolerance, and a racing heart rate. These usually mean the dose is too high for your body or you titrated up too fast[1][12]. The fix is dropping back to the week 1 starting dose and increasing more slowly, or lowering the final target if the full dose was too aggressive for you.
Cytomel is not appropriate for patients with atrial fibrillation, active heart disease without cardiology clearance, or untreated adrenal insufficiency. Patients over 65 and patients with a history of heart disease need a lower starting cap, which the calculator applies automatically when you check those boxes. Bone density loss is a long-term concern if Cytomel is dosed high enough to suppress TSH below 0.1 for years, so monitor bone density every 2 to 3 years if you’re on long-term combination therapy.
References
- 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. View on PubMed
- Peterson SJ, Cappola AR, Castro MR, et al. An online survey of hypothyroid patients demonstrates prominent dissatisfaction. Thyroid. 2018;28(6):707-721. View on PubMed
- Shahid MA, Ashraf MA, Sharma S. Physiology, Thyroid Hormone. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023. View on NCBI
- Liwanpo L, Hershman JM. Conditions and drugs interfering with thyroxine absorption. Best Practice and Research Clinical Endocrinology and Metabolism. 2009;23(6):781-792. View on PubMed
- Escobar-Morreale HF, Botella-Carretero JI, Escobar del Rey F, Morreale de Escobar G. Treatment of hypothyroidism with combinations of levothyroxine plus liothyronine. Journal of Clinical Endocrinology and Metabolism. 2005;90(8):4946-4954. View on PubMed
- 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. Journal of Clinical Endocrinology and Metabolism. 2013;98(5):1982-1990. View on PubMed
- Alexander EK, Pearce EN, Brent GA, et al. 2017 Guidelines of the American Thyroid Association for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27(3):315-389. View on PubMed
- Bornstein SR, Allolio B, Arlt W, et al. Diagnosis and treatment of primary adrenal insufficiency: an Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology and Metabolism. 2016;101(2):364-389. View on PubMed
- Celi FS, Zemskova M, Linderman JD, et al. Metabolic effects of liothyronine therapy in hypothyroidism: a randomized, double-blind, crossover trial of liothyronine versus levothyroxine. Journal of Clinical Endocrinology and Metabolism. 2011;96(11):3466-3474. View on PubMed
- Panicker V, Saravanan P, Vaidya B, et al. Common variation in the DIO2 gene predicts baseline psychological well-being and response to combination thyroxine plus triiodothyronine therapy in hypothyroid patients. Journal of Clinical Endocrinology and Metabolism. 2009;94(5):1623-1629. View on PubMed
- 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. European Thyroid Journal. 2012;1(2):55-71. View on PubMed
- Biondi B, Cooper DS. The clinical significance of subclinical thyroid dysfunction. Endocrine Reviews. 2008;29(1):76-131. View on PubMed
- Wiersinga WM. Therapy of endocrine disease: T4 + T3 combination therapy: is there a true effect? European Journal of Endocrinology. 2017;177(6):R287-R296. View on PubMed