Thyroid Medication Conversion Calculator
This calculator simplifies the process of converting from one thyroid medication to another, with just a few inputs and clicks.
Here’s how to use it:
Enter your current medication (or combination of medications), pick the medication you want to switch to, and instantly see your equivalent dose using the conversion method of your choice.
The calculator uses Dr. Westin Childs’ recommended conversion metrics, which differ from conventional pharmaceutical recommendations, but if you want to see those, you can toggle between them at the top.
Note: If you take more than one thyroid medication, use the “Add another medication” button to enter all of them before calculating. The calculator will add up your total thyroid hormone exposure and convert it into the single target medication you pick.
Written and medically reviewed by Dr. Westin Childs, D.O. Last reviewed: April 22, 2026.
How to Use This Calculator
Step #1: Pick Your Conversion Method
At the top of the calculator, you’ll see three options: my recommended conversion, the updated 2013 study conversion, and the conventional one.
If you don’t know which one to pick, go with mine. It’s the conversion that syncs up with both clinical experience and existing research, and it reflects how natural desiccated thyroid (NDT) and T3 actually behave in the body, not how textbooks assume they should.
You can switch between the three at any time. The results are recalculated on the spot, so you can see exactly how much your equivalent dose changes depending on which chart your doctor (or the pharmacy) is using.
Step #2: Enter Your Current Medication(s)
Pick your current medication from the dropdown and enter your dose.
T4 medications (levothyroxine sodium) like Levothyroxine, Synthroid, Tirosint, Tirosint-SOL, Levoxyl, and Unithroid are dosed in mcg.
T3 medications like Liothyronine and Cytomel are also dosed in mcg.
NDT medications (natural desiccated thyroid, also known as porcine thyroid extract) like Armour Thyroid and NP Thyroid can be entered in either mg or grains, whichever matches your prescription bottle.
Compounded thyroid medications can be entered using their T4 and T3 content.
If you’re on more than one thyroid medication, which is common with combination T4 and T3 therapy, click the “+ Add another medication” button. You can add as many as you need.
Double-check your unit before you hit calculate. Entering 75 mg of Synthroid when you meant 75 mcg will throw the calculation off by a factor of 1,000.
Step #3: Pick Your Target Medication and Calculate
Pick the medication you want to switch to from the “Medication you want to convert to” dropdown, then hit the “Calculate My Equivalent Dose” button.
You’ll get your calculated equivalent dose, the closest single-tablet strength you can actually get from a pharmacy, a 2-tablet combination if one lands closer to your ideal dose, a breakdown of your total T4 and T3 before and after the switch, and timing notes specific to the medication you’re moving to.
From there, play with it. Change the method, change the target, tweak your doses. Everything updates automatically.
Understanding Your Results
Your Current Thyroid Hormone Intake
The first box in your results breaks down your total thyroid hormone intake into T4 and T3.
This matters because T4 and T3 are not the same hormone. T4 (thyroxine) is the storage form, an inactive precursor that has to be converted to T3 before your body can use it. T3 (triiodothyronine) is the active form that actually does the work at the cellular level.
Most thyroid medications contain only T4 (Levothyroxine, Synthroid, Tirosint). A few contain only T3 (Liothyronine, Cytomel). NDT medications like Armour Thyroid and NP Thyroid contain both T4 and T3 in a fixed ratio.
When you switch thyroid hormone replacement medications, your total T4 and T3 exposure changes. This box shows you exactly how much of each you’re getting now and exactly how much you’ll be getting after the switch. That’s the real comparison, not the milligrams on the bottle.
Your Equivalent Dose
This is the calculated equivalent dose of the medication you’re moving to, based on the conversion method you picked.
For T4 and T3 medications the number shows up in mcg. For NDT medications it shows up in both mg and grains.
This exact number almost never matches an available tablet strength. That’s what the next box is for.
Nearest Commercially Available Strength
Thyroid medications come in specific tablet strengths, so your calculated dose is almost always going to land between two of them.
This box rounds to the closest tablet you can actually pick up at the pharmacy. It’s the simplest starting point for a conversion.
Closer Match With 2 Tablets
Sometimes combining two tablets gets you closer to your ideal dose than any single tablet can.
Here’s an example: if your calculated dose works out to 75 mg of Armour Thyroid, the closest single tablet is 60 mg. But a 60 mg plus a 15 mg tablet lands you exactly on 75 mg, which is a perfect match. The calculator will flag that combination for you.
This box only shows up when the 2-tablet combo is meaningfully closer to your ideal dose than the nearest single tablet. If the nearest tablet is already close enough, you won’t see it.
In my practice I actually prefer the 2-tablet approach most of the time. It lets you fine-tune more precisely, and it gives you flexibility to adjust up or down without chasing a new prescription every time you want to nudge the dose.
Conversion Method Explanation
Below the dose boxes you’ll see a short explanation of why the method you picked produced the result it did. This block changes depending on which of the three methods is active.
The three methods don’t agree with each other because they treat T3 and NDT as having different potencies compared to T4. This is the single biggest reason patients end up undertreated or overtreated when they switch medications[1]. The explainer helps you see exactly where the disagreement is and why it matters for your dose.
Dosing Notes
The dosing notes at the bottom give you timing and absorption guidance specific to the medication you’re switching to. Topics covered include:
- Whether to take your medication in the morning on an empty stomach or at bedtime, and which is more practical for your routine.
- How long to wait between your medication and food, coffee, calcium, iron, or magnesium.
- Whether to split the daily dose into two, which matters more with T3 and NDT because of the T3 component.
- Absorption considerations specific to the medication. Tirosint and Tirosint-SOL, for example, have higher bioavailability than standard levothyroxine tablets[4], so some patients need a slightly lower dose.
- When to recheck your thyroid labs (TSH, Free T3, Free T4, Reverse T3) after the switch.
How the Three Conversion Methods Compare
The three methods disagree on how potent T3 and NDT are relative to T4. Here’s how they stack up:
| Conversion Ratio | Dr. Childs’ Recommended | Updated (2013 Study) | Conventional |
|---|---|---|---|
| 1 grain (60 mg) of NDT equals | 75 mcg of T4 | 88 mcg of T4 | 100 mcg of T4 |
| 1 mcg of T3 equals | 3 mcg of T4 | 3.5 mcg of T4 | 4 mcg of T4 |
| Raw hormone content of 1 grain NDT[3] | 38 mcg T4 + 9 mcg T3 (all three methods agree on the raw content) | ||
What this means in practice: the same combination of medications produces different equivalent doses depending on which method you use.
If you’re going from T4 to NDT, my method gives you more NDT than the conventional chart does. If you’re going from NDT to T4, my method gives you less T4 than the conventional chart does.
That’s because my chart treats T3 and NDT as more potent than conventional charts do, which matches what thyroid patients consistently experience in the real world. A 2013 randomized double-blind crossover study comparing desiccated thyroid extract to levothyroxine landed at a 3.5:1 ratio[1], which sits between my clinical ratio and the textbook one.
The conventional chart was built on pharmaceutical assumptions that don’t hold up in clinical practice. If you want a deeper comparison of the two most common medications, read my Armour Thyroid vs Synthroid breakdown.
Common Thyroid Medication Dose Conversions at a Glance
Here are the most-searched conversion pairs using my clinical ratio (3:1 T3:T4, 1 grain NDT = 75 mcg T4). Plug your specific dose into the calculator above for an exact number and the closest available tablet strength.
| Current Medication and Dose | Equivalent Dose (Clinical) |
|---|---|
| 50 mcg Levothyroxine / Synthroid | 0.67 grains (40 mg) Armour Thyroid or NP Thyroid |
| 75 mcg Levothyroxine / Synthroid | 1 grain (60 mg) Armour Thyroid or NP Thyroid |
| 100 mcg Levothyroxine / Synthroid | 1.3 grains (80 mg) Armour Thyroid or NP Thyroid |
| 125 mcg Levothyroxine / Synthroid | 1.67 grains (100 mg) Armour Thyroid or NP Thyroid |
| 150 mcg Levothyroxine / Synthroid | 2 grains (120 mg) Armour Thyroid or NP Thyroid |
| 1 grain (60 mg) Armour Thyroid | 75 mcg Levothyroxine / Synthroid |
| 25 mcg Cytomel / Liothyronine (T3) | 75 mcg Levothyroxine / Synthroid (T4 equivalent) |
| 10 mcg Cytomel / Liothyronine (T3) | 30 mcg Levothyroxine / Synthroid (T4 equivalent) |
Combination therapy (T4 plus a small dose of T3) often produces better results than T4-only therapy for patients with poor conversion[5]. If you’re on a combination and want to consolidate to a single medication, enter both doses in the calculator above. It’ll add up your total T4 and T3 and convert everything into a single target medication in one calculation.
Frequently Asked Questions
No. In clinical practice, 1 grain of Armour Thyroid is closer to 75 mcg of Synthroid (Levothyroxine), not 100 mcg. One grain (60 mg) contains 38 mcg of T4 and 9 mcg of T3[3]. The conventional 100 mcg equivalence assumes T3 is 4 times more potent than T4, but clinical experience shows closer to 3 times.
That 25 mcg gap per grain is the reason so many patients switching from 100 mcg of Synthroid to 1 grain of Armour end up feeling undertreated. Most do better on about 1.3 grains (80 mg) of Armour.
To convert Levothyroxine (or Synthroid) to Armour Thyroid, divide your Levothyroxine dose in mcg by 75 to get the equivalent Armour Thyroid dose in grains. So 75 mcg of Levothyroxine equals 1 grain (60 mg) of Armour, 100 mcg equals about 1.3 grains (80 mg), and 150 mcg equals 2 grains (120 mg).
This uses my clinical conversion, which treats NDT as more potent than the conventional pharmaceutical chart does. The standard “1 grain equals 100 mcg” chart consistently underdoses patients making this switch. Run your specific dose through the calculator above for the exact number and the closest available tablet. For a detailed comparison of the two medications, read my Armour Thyroid vs Synthroid guide.
The T4 to T3 conversion ratio is approximately 3 to 1, meaning 3 mcg of T4 equals 1 mcg of T3. So 25 mcg of T3 (Cytomel or Liothyronine) is roughly equivalent to 75 mcg of T4 (Levothyroxine or Synthroid), and 10 mcg of T3 equals about 30 mcg of T4.
The pharmaceutical textbook ratio is 4 to 1, but that number comes from limited data and undersells how active T3 actually is in the body. A 2013 clinical study landed closer to 3.5 to 1[1]. In my practice, 3 to 1 lines up best with how patients feel and how their labs respond.
One grain of NDT (natural desiccated thyroid, sometimes called desiccated thyroid extract or porcine thyroid) contains 38 mcg of T4 and 9 mcg of T3[3]. One grain equals 60 mg, so a 30 mg tablet delivers 19 mcg of T4 and 4.5 mcg of T3, and a 90 mg tablet delivers 57 mcg of T4 and 13.5 mcg of T3.
This ratio is fixed. It’s the same across Armour Thyroid, NP Thyroid, WP Thyroid, and Nature-Throid. Where the major conversion charts disagree is not the raw hormone content but how potent that T3 is compared to T4. That’s why the three methods in this calculator produce different equivalent doses.
Dr. Childs’ conversion differs from the conventional one because it treats T3 as 3 times more potent than T4, not 4 times. That means my method credits 1 grain of NDT with 75 mcg of T4 equivalence, while the conventional chart credits it with 100 mcg.
The conventional 4 to 1 ratio came from pharmaceutical textbooks built on limited data, and it doesn’t hold up in real clinical practice. It consistently underdoses patients moving from T4 to NDT and overdoses patients moving the other direction. A 2013 randomized clinical study landed at 3.5 to 1[1], which sits between my ratio and the textbook one. All three methods are in the calculator so you can compare your dose across them.
The most common reason you feel worse after switching thyroid medications is that the conversion was based on the conventional pharmaceutical chart, which underdoses patients moving from T4 to NDT and overdoses patients moving from NDT to T4.
Switching from 100 mcg of Levothyroxine to 1 grain of Armour using the conventional chart drops your total thyroid hormone. Hypothyroid symptoms creep back in: fatigue, weight gain, brain fog, hair loss, cold intolerance. Going the other direction pushes some patients into hyperthyroid symptoms like anxiety, heart palpitations, insomnia, and tremor.
Other common reasons include absorption changes (new timing, coffee, food, calcium, iron) and individual T3 sensitivity. Recheck your labs (TSH, free T3, free T4, reverse T3) and work with your doctor on an adjustment.
Most patients need 6 to 8 weeks to reach steady state after switching thyroid medications or changing a dose[2]. T4 has a half-life of about 7 days, so blood levels take 5 to 6 weeks to stabilize. T3 has a shorter 24-hour half-life and stabilizes faster, but the full symptom picture still takes 4 to 6 weeks.
Don’t panic if the first two weeks feel off. Symptoms can lag behind labs by weeks. Plan on a full thyroid panel at the 6 to 8-week mark with TSH, free T3, free T4, and reverse T3.
Yes. Combination T4 and T3 therapy is widely used and often produces better results than T4-only therapy, especially for patients with conversion issues[5]. The most common combination is Levothyroxine or Synthroid plus a low dose of Cytomel or Liothyronine.
Another common approach is combining a T4 medication with NDT, or taking two NDT medications at different times of day. If you want to switch from a combination to a single medication, use the “+ Add another medication” button in the calculator. It’ll add up your total T4 and T3 and convert everything into the single target medication in one calculation.
In theory, no. Brand-name and generic thyroid medications contain the same labeled dose of T4. In practice, some patients notice changes in symptoms or labs because the inactive ingredients (binders, fillers, dyes) vary between manufacturers and can affect absorption.
Tirosint and Tirosint-SOL are the exception. They have higher bioavailability than standard tablet forms[4], so patients switching to them sometimes need about a 10% dose reduction. If you switch between brand and generic, or between two different generic manufacturers, recheck your labs at the 6 to 8-week mark, even if the number on the bottle didn’t change.
No. Thyroid medications are prescription drugs, so any change in medication or dose requires a prescription from your doctor.
What this calculator gives you is a strong starting point for the conversation. Walking into your appointment with a calculated equivalent dose in hand makes it easier to push back if your doctor defaults to the conventional chart and the number feels off. If your doctor isn’t familiar with T3 or NDT conversions, a functional medicine or integrative doctor usually will be.
Scientific References
- 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
- 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
- U.S. Food and Drug Administration. Armour Thyroid (thyroid tablets, USP) prescribing information. DailyMed. View on DailyMed
- Santini F, Giannetti M, Ricco I, et al. Steady-state serum T4 concentrations following oral administration of levothyroxine from an oral thin film. International Journal of Endocrinology. 2014;2014:193819. 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