How Much T3 to Add to Levothyroxine: The Complete Conversion Reference

Search any thyroid research forum for combination therapy and the same question appears in near-identical wording, month after month, usually unanswered or answered three different ways in the same thread: how do I dose a T4/T3 combination equivalent to 125 mcg of levothyroxine?

It is an arithmetic question with a genuinely unsatisfying answer, which is why it keeps being asked. There is no exact conversion factor, because T4 and T3 are not two strengths of the same thing. But there are conventions in wide use, the arithmetic those conventions produce is straightforward, and understanding what the numbers do and do not mean is more useful than any single figure.

This is the reference table set. The rationale for the ratios themselves is covered in why combination tablets use a 4:1 ratio, and the question of whether to add T3 at all, as opposed to how much, is covered in when levothyroxine is not working.

The One Distinction That Causes Most of the Confusion

Before any table is useful, two operations have to be separated, because they are routinely described with the same words and produce entirely different results.

Adding T3 means keeping the levothyroxine dose where it is and introducing liothyronine on top. Total thyroid hormone exposure rises.

Substituting T3 means removing a portion of the levothyroxine and replacing it with a potency-equivalent amount of liothyronine. Total exposure is intended to stay approximately constant while the proportion delivered as T3 rises.

Someone who reads a substitution table and applies it as an addition has increased their dose, possibly substantially, while believing they have held it constant. This is the single most common error in the whole topic and it is entirely a vocabulary problem.

Everything below is written as substitution, because that is what the conversion question is actually asking.

The Conversion Factor and Where It Comes From

The convention in circulation is that 1 mcg of liothyronine is treated as displacing roughly 3 to 4 mcg of levothyroxine. A frequently quoted shorthand is that 10 mcg of T3 stands in for about 30 mcg of T4.

That figure is a potency estimate, not a measured equivalence, and it is worth being precise about why no exact number exists.

Levothyroxine is a prohormone. Its effect depends on peripheral conversion to T3 by deiodinase enzymes, and the efficiency of that conversion varies between individuals, across tissues, and with nutritional and inflammatory state. Liothyronine is the active hormone and requires no conversion. Substituting one for the other is therefore not a unit conversion between two strengths of the same drug; it is a substitution of a finished product for a precursor, in a person whose conversion efficiency is the unknown variable.

Genetic contribution to that variance is documented. Polymorphism in DIO2, the gene encoding type 2 iodothyronine deiodinase, has been associated with reduced enzyme activity and lower serum T3 in thyroid-deficient individuals. Someone with reduced conversion capacity gets less from a given quantity of T4 than the population average that any conversion table is built on, which is precisely the population most interested in the question.

The mechanism is covered in the deiodinase dysfunction guide.

Conversion Tables

The tables below apply the 1:3 and 1:4 conventions across the common levothyroxine strengths. Read them as starting points for calculation, not as prescriptions.

At 1 mcg T3 : 3 mcg T4

At 1 mcg T3 : 4 mcg T4

Note what happens at the higher strengths. At 200 mcg levothyroxine under the 1:4 convention, the resulting 120 / 20 sits close to the strengths that manufactured combination products actually use, which is not a coincidence: those products were designed around the replacement range.

The T3 conversion calculator handles strengths and ratios not covered above.

Worked Example: The 125 mcg Question

Take the question as it is usually asked. A protocol runs 125 mcg levothyroxine daily. What does a combination equivalent look like?

Step one, decide the T3 quantity. A conventional starting substitution is 10 mcg of T3, which is a modest fraction of the total and the level most commonly described as an entry point.

Step two, remove the corresponding T4. At 1:3, 10 mcg of T3 displaces 30 mcg of T4, leaving 95 mcg. At 1:4, it displaces 40 mcg, leaving 85 mcg.

Step three, check what is actually obtainable. Levothyroxine is manufactured in fixed strengths. 95 mcg is not one of them, and 85 mcg is not one of them. The realistic options are the nearest available strength, a combination of tablets, or a fixed-ratio combination product whose strengths were chosen to land on manufacturable numbers.

Step four, note what the resulting ratio is. A 95 / 10 protocol is a 9.5:1 ratio by weight. An 85 / 10 protocol is 8.5:1. Both are considerably more T4-weighted than a 4:1 combination tablet, which is why moving from a self-assembled substitution to a manufactured combination product is usually a larger change in T3 fraction than people expect. That comparison is set out in why combination tablets use 4:1.

Step three is where most forum answers stop being useful, because the arithmetic produces a number that no product is made in. It is also the reason fixed-ratio combination tablets exist at all.

The Six Errors That Account for Most Bad Conversions

Conversion questions go wrong in a small number of repeatable ways. Recognising them is more useful than any single table.

Applying a substitution table as an addition. The most common and the most consequential. A table that says 125 mcg becomes 95/10 describes a protocol with less T4 in it. Someone who keeps 125 and adds 10 has raised total exposure rather than redistributed it.

Assessing at two weeks. The T3 side has settled and the T4 side has not, so the result reflects one variable at steady state and one mid-transit. It is not comparable against pre-conversion values.

Adjusting during the transition. Every change restarts the four-to-six-week T4 clock. Three adjustments inside six weeks produce a protocol nobody can interpret, including the person running it.

Treating a population conversion factor as a personal one. The 1:3 and 1:4 conventions are estimates across groups. The individuals most interested in combination therapy are, by hypothesis, those whose conversion differs from the group, so the factor is least reliable exactly where it is most used.

Rounding silently. When arithmetic produces 95 mcg and the available tablet is 100, the protocol is 100, not 95. Rounding is legitimate; recording the rounded figure as though it were the calculated one is how a protocol drifts away from its own documentation.

Changing two things at once. A conversion combined with a brand change, or with a formulation change, produces a result that cannot be attributed to either. Where several changes are wanted, sequencing them is the only way to learn anything from any of them.

Two Timescales, One Change

A conversion is a single decision that expresses itself at two completely different speeds, and this catches people out repeatedly.

Liothyronine has a biological half-life of roughly 2.5 days. Levothyroxine's is around a week. After a conversion, the T3 side of the protocol reaches its new steady state within a few days. The T4 side takes approximately four to six weeks.

The consequences are practical.

Anything noticed in the first week reflects the T3 change, not the T4 reduction, because the T4 level has barely begun to move. Attributing an early effect to the reduced levothyroxine is attributing it to the variable that has not yet expressed itself.

Bloodwork drawn at two weeks is uninterpretable for the same reason: it captures a settled T3 picture against a T4 level still in transit. It is not a steady-state result and cannot be compared cleanly against pre-conversion values.

And every mid-transition adjustment resets the T4 clock. A conversion adjusted three times in six weeks produces far less usable information than one left alone, which is the argument for choosing a starting point deliberately and then holding it.

Converting Back

The arithmetic runs in both directions, and reverse conversions come up more often than the forward-only framing of most discussion suggests: a protocol that did not suit, a supply interruption, or a return to a simpler regimen.

The same conventions apply inverted. Removing 10 mcg of T3 requires replacing it with roughly 30 to 40 mcg of levothyroxine to hold total exposure approximately constant.

The 120/30 row deserves a note. A full combination tablet at the replacement range carries enough T3 that unwinding it produces a large levothyroxine figure, above the range many protocols run. This is a useful sanity check on what a 4:1 tablet is actually delivering: the T3 fraction is substantial, not a garnish on a T4 dose.

Reverse conversions carry the same two-clock problem in reverse. The T3 leaves the system within days; the added T4 takes four to six weeks to arrive at its new level. There is therefore a window in the middle of a reverse conversion where T3 has gone and its T4 replacement has not yet accumulated, which is the single most uncomfortable part of the manoeuvre and the reason it is normally staged rather than done in one step.

Why Equal Micrograms Are Not Equal Effect

Even holding the arithmetic constant, a matched dose is not a matched result, and there are three separate reasons.

Conversion capacity varies. The whole premise of substituting T3 for T4 is that the recipient's conversion step is a bottleneck. If it is, the conversion factor understates what the T3 delivers relative to the T4 removed. If it is not, the substitution overshoots. The table cannot know which applies.

Formulation and brand differ. A cohort study of a forced dose-equivalent levothyroxine brand switch found measurable movement in plasma thyrotropin despite the substitute being nominally dose-equivalent. If that holds between two levothyroxine products, it certainly holds across a change of hormone.

Delivery profile differs. Immediate-release liothyronine produces an early serum peak; extended-release formulations distribute the same daily quantity across a longer window. Equal daily micrograms with different release profiles are not equivalent exposures, which is the subject of slow-release versus immediate-release combination therapy.

What the Evidence Establishes, and What It Does Not

Established. That liothyronine requires no peripheral conversion while levothyroxine does. That DIO2 polymorphism is associated with reduced deiodinase-2 activity and lower serum T3. That the two hormones reach steady state on very different timescales. That a dose-equivalent brand switch can measurably move thyrotropin.

Contested. Whether combination therapy outperforms levothyroxine monotherapy at all. A consensus document, a 2026 network meta-analysis, and a 2004 randomised trial using a bioavailable molar ratio of 14:1 all bear on this and none settles it. A 2025 systematic review examined liothyronine safety specifically, and a 2025 study of treatment preferences examined what patients themselves report wanting. The literature is genuinely unresolved rather than merely disputed, and anyone acting on the tables above should read the negative results as carefully as the positive ones.

Not established. Any exact conversion factor. That 1:3 or 1:4 is correct for a given individual rather than a population approximation. That matched total exposure produces matched physiological effect.

Frequently Asked Questions

How much T3 replaces 125 mcg of levothyroxine?

There is no whole-dose replacement figure in general use, because full substitution of T4 by T3 is a different protocol rather than a conversion. For partial substitution at the common conventions, 10 mcg of T3 displaces about 30 to 40 mcg of levothyroxine, leaving roughly 85 to 95 mcg of T4 alongside it.

Is 10 mcg of T3 really equal to 30 mcg of T4?

It is a widely used approximation rather than a measured equivalence. The two hormones are not interchangeable units, because levothyroxine requires peripheral conversion and liothyronine does not, and conversion efficiency varies between individuals.

Do I reduce levothyroxine when I add T3?

That depends entirely on whether the intention is to add or to substitute. Adding T3 without reducing T4 raises total thyroid hormone exposure. Substituting holds total exposure roughly constant. Conversion tables describe substitution.

Why do the numbers never land on an available tablet strength?

Because levothyroxine and liothyronine are manufactured in fixed strengths that were not chosen to divide evenly into one another. This is the practical reason fixed-ratio combination products exist, and why self-assembled conversions usually require rounding to the nearest obtainable strength.

When can I assess whether a conversion worked?

Around six weeks. The T3 component settles within days but levothyroxine takes four to six weeks to reach a new steady state, so earlier measurements capture one settled variable against one still in transit.

Does the conversion factor change at higher doses?

The conventions are applied linearly, but that is a simplifying assumption rather than an established property. Deiodinase activity is regulated rather than fixed, so a proportional substitution at a high dose is not guaranteed to behave like the same proportion at a low one.

Is a fixed-ratio combination tablet better than dosing separately?

Neither is superior in principle. A fixed ratio is simpler to take and impossible to adjust independently. Separate preparations allow any ratio at the cost of managing two products. The choice is practical rather than pharmacological.

What ratio does a manufactured combination product use?

Commonly 4:1 or 5:1 T4 to T3 by weight, which is substantially more T3-weighted than most self-assembled substitutions produce. The reasoning is covered in why combination tablets use a 4:1 ratio.

Closing Note

The reason this question never resolves in forum threads is that it is being asked as a units problem when it is actually a physiology problem wearing a units problem's clothing.

The arithmetic is easy. Pick a substitution ratio, multiply, subtract. What the arithmetic cannot supply is the one number that actually determines the answer, which is how efficiently a given individual converts T4 to T3, and that number is not knowable from a dose chart.

What the tables above are genuinely good for is establishing a defensible starting point and, more importantly, making explicit what is being changed. A researcher who can state that they moved from 125 / 0 to 95 / 10, that this represents a 9.5:1 ratio, that the T3 side settled within a week and the T4 side took six, is in a position to interpret what follows. A researcher who added 10 mcg of T3 to an unchanged 125 mcg and is not sure whether their total exposure went up is not.

Where the resulting numbers do not correspond to an available strength, the SRT4+T3-120 slow release T4+T3 is manufactured at 120 mcg T4 with 30 mcg T3, and T4 levothyroxine at 100 mcg with the slow-release T3 range allows any ratio to be assembled from two components.

All compounds discussed here are research chemicals supplied for laboratory research use only. Nothing in this article is medical advice, a treatment protocol, a dosing recommendation, or a recommendation for human or animal consumption. Thyroid hormone dosing carries genuine cardiovascular risk, the clinical evidence on combination therapy remains actively contested, and the conversion conventions described are population approximations that do not account for individual variation. Researchers should consult the primary literature cited below and appropriate qualified professionals.