Free T3 to Reverse T3 Ratio Calculator
Free T3 to Reverse T3 Ratio Calculator
Divide free T3 by reverse T3 in matched molar units — and see why the same pair of results gives four different “ratios” depending on the units, and why no cut-off is published here.
Free T3 : reverse T3
Two results, matched units → molar ratioFree T3 4.6 pmol/L and reverse T3 18 ng/dL — both comfortably mid-interval on their own reference ranges
The arithmetic, and the units that decide the answer
- One molecular weight, two hormones
- reverse T3 is an isomer of T3 — the same formula, C₁₅H₁₂I₃NO₄, with the iodine atoms in different positions — so both convert on the same formula mass of 650.97 g/mol
- Free T3
- reported in pmol/L or pg/mL. 1 pg/mL = 1000 ÷ 650.97 = 1.53617 pmol/L
- Reverse T3
- reported in ng/dL or nmol/L. 1 ng/dL = 10 ÷ 650.97 = 0.0153617 nmol/L, so 1 ng/dL = 15.3617 pmol/L
- Why matched units matter
- pmol/L and ng/dL differ by a factor of about 15, and pg/mL and pg/dL by ten. Dividing one unit by another gives a number with no meaning and no comparator
- Reference interval
- none exists for the ratio. Reverse T3 itself has one — about 10–24 ng/dL by mass spectrometry — but a ratio of two analytes is not covered by either analyte’s interval
Worked example
Free T3 4.6 pmol/L and reverse T3 18 ng/dL — both comfortably mid-interval on their own reference ranges
Reverse T3 18 ng/dL × 15.3617 = 276.5 pmol/L (equivalently 18 × 0.0153617 = 0.2765 nmol/L)
Free T3 is already in pmol/L → 4.6 pmol/L
4.6 ÷ 276.5 = 0.0166 as a molar ratio
The same two results as pg/mL ÷ ng/dL: 2.99 ÷ 18 = 0.166
As pg/dL ÷ pg/mL: 299 ÷ 180 = 1.66
As pg/mL ÷ ng/dL × 100: 16.6
One patient, four numbers spanning three orders of magnitude — and none of them has a published cut-off to be compared against
One patient, four “ratios”
| How it is calculated | The number | Where that convention comes from |
|---|---|---|
| Free T3 pmol/L ÷ reverse T3 pmol/L | 0.0166 | The molar ratio — the only unit-independent form, and what this page prints |
| Free T3 pg/mL ÷ reverse T3 ng/dL | 0.166 | The most common naive form: divide the two numbers as printed |
| Free T3 pg/dL ÷ reverse T3 pg/mL | 1.66 | The form a commercial biomarker directory quotes an “optimal” range of 1.53–2.80 against |
| Free T3 pg/mL ÷ reverse T3 ng/dL × 100 | 16.6 | The form behind the rule of thumb in widest circulation |
What the professional bodies say about measuring reverse T3 at all
| Source | Position |
|---|---|
| ATA-commissioned review, Thyroid 2023 | “Measurement of rT3 is widely cited in the lay press as a potential marker to guide T4 or T3 therapy, however, there is currently no evidence to support this application.” And: “except for these three uncommon situations, there is no need to measure rT3 in routine clinical practice” |
| The three exceptions it allows | Infrequent genetic thyroid syndromes; consumptive hypothyroidism from overexpression of deiodinase 3; and nonthyroidal illness confounding a diagnosis of central hypothyroidism |
| ATA, Thyroid Function Tests (patient information) | “In healthy, non-hospitalized people, measurement of reverse T3 does not help determine whether hypothyroidism exists or not, and is not clinically useful” |
| AACE/ATA hypothyroidism guideline, via a laboratory utilisation review | “Reverse T3 (triiodothyronine) testing is not useful and should not be used to diagnose sick euthyroid syndrome” |
| A laboratory that sells the test (ZRT, 2019) | “There appears to be little clinical justification for routine testing of rT3 for thyroid function assessment”, and on the ratio: “there is no clinical basis for the use of this ratio in routine thyroid function assessment in non-critically ill people” |
| Thyroid 2018 practice-variation study | Most reverse T3 tests are ordered by functional medicine practitioners rather than by conventional specialists, and of the published evidence on the test “90% were published before 2000” |
If the question is peripheral conversion, this is not the only page
| Question | Where to look |
|---|---|
| Is T4 being converted to T3? | FT3:FT4 ratio calculator — which carries the same warning about treating a ratio, and does not need a reverse T3 at all |
| What is this free T3 in other units? | Free T3 unit converter |
| Is this actually hypothyroidism? | TSH and free T4 — TSH remains the screening test |
| The TSH is off target on treatment | Levothyroxine dose adjustment from TSH |
| Is binding-protein disturbance the explanation? | Free thyroxine index and TBG converter |
No cut-off is published here, and the units are the reason the circulating ones disagree
Reverse T3 is the inactive isomer of T3. The same fifteen carbons, twelve hydrogens, three iodines, one nitrogen and four oxygens, arranged with the iodine atoms in different positions — which is precisely what makes it inactive, as the American Thyroid Association’s own patient material puts it. Because it is an isomer, it has the same formula mass as T3, 650.97 g/mol, and one conversion factor serves both hormones. That is the first thing this page does: convert free T3 from pg/mL and reverse T3 from ng/dL onto a single molar scale, pmol/L, before dividing. A ratio of a picomole per litre over a nanogram per decilitre is not a small unit problem. It is not a number.
And that matters more here than on almost any other ratio, because the “optimal ranges” in circulation for this one are unit artefacts of each other. Take a perfectly ordinary pair of results: free T3 4.6 pmol/L, reverse T3 18 ng/dL, both mid-interval. As a molar ratio that is 0.0166. Divide the printed numbers as they stand — pg/mL over ng/dL — and it is 0.166. Express free T3 in pg/dL and reverse T3 in pg/mL, which is what one widely used biomarker directory does when it quotes an “optimal” range of 1.53 to 2.80, and it is 1.66. Multiply the naive form by a hundred, which is what the rule of thumb in widest circulation implicitly does, and it is 16.6. Four numbers, three orders of magnitude, one patient. A cut-off quoted without its units cannot be applied to anything, and this page prints all four forms side by side so that a reader who has been given one of them can see which one they have.
Then the harder point, which is that none of those cut-offs has a source. Searching for a derivation returns laboratory marketing pages, biomarker directories and calculator sites — not a cohort, not a reference-interval study, not a trial. So this page publishes no interval and no verdict, and the result above carries no band, no colour and no interpretation. That is a deliberate refusal rather than an omission: the alternative would be to print a threshold this site cannot attribute, which is the one thing a page like this must not do.
The position of the professional bodies goes further, and it is about the denominator rather than the ratio. The ATA-commissioned review of thyroid hormone testing, published in Thyroid in 2023, states that measurement of reverse T3 “is widely cited in the lay press as a potential marker to guide T4 or T3 therapy, however, there is currently no evidence to support this application”, and that outside three uncommon situations — rare genetic thyroid syndromes, consumptive hypothyroidism from deiodinase 3 overexpression, and nonthyroidal illness confusing a diagnosis of central hypothyroidism — “there is no need to measure rT3 in routine clinical practice”. The ATA’s patient material says that in healthy, non-hospitalised people reverse T3 “is not clinically useful”. A 2018 study of ordering patterns found that most reverse T3 requests come from functional medicine practitioners and that ninety per cent of the published literature on the test predates 2000. Even ZRT, a laboratory that sells the assay, writes that there is “no clinical basis for the use of this ratio in routine thyroid function assessment in non-critically ill people”.
None of which makes the number meaningless as physiology. Reverse T3 does rise as T3 falls in serious illness, starvation and after trauma, and with amiodarone, propranolol and glucocorticoids, and the ratio does track that shift — the ATA review’s own observation is that “serum rT3 typically rises as T3 falls during nonthyroidal illness”, while adding that measuring T3 is “more practical, cheaper, and as effective” if that is the question. What the ratio will not do is establish that a patient with a normal TSH and a normal free T4 has “poor conversion” that needs treating, and it should not be used to start or escalate liothyronine. The FT3:FT4 ratio page sets out that argument in full from two tests every laboratory reports, and this page defers to it rather than repeating it. This supports a clinician’s judgement rather than replacing it. It is arithmetic on the figures entered, and it knows nothing about the patient in front of you.
Frequently asked questions
What is a normal free T3 to reverse T3 ratio?
There isn’t one. No validated reference interval or cut-off for this ratio has been published, which is why this page shows the number and no interpretation. The figures in circulation — 20, or 1.53–2.80, or 10–15 — come from laboratory marketing and biomarker directories, are expressed in mutually incompatible units, and none of them traces to a derivation study.
Why does my ratio look completely different somewhere else?
Almost certainly the units. Free T3 4.6 pmol/L with a reverse T3 of 18 ng/dL is 0.0166 as a molar ratio, 0.166 as pg/mL over ng/dL, 1.66 as pg/dL over pg/mL and 16.6 as pg/mL over ng/dL times a hundred. This page converts both hormones to pmol/L first and shows the other conventions beside the answer so the two can be matched up.
Do free T3 and reverse T3 use the same conversion factor?
Yes, because they are isomers — the same molecular formula, C₁₅H₁₂I₃NO₄, with the iodine atoms in different positions — so both convert on a formula mass of 650.97 g/mol. That gives 1 pg/mL of free T3 as 1.53617 pmol/L, and 1 ng/dL of reverse T3 as 0.0153617 nmol/L, or 15.3617 pmol/L.
Do the guidelines recommend measuring reverse T3?
No. The ATA-commissioned 2023 review states there is “no evidence to support” using it to guide T4 or T3 therapy and that outside three uncommon situations “there is no need to measure rT3 in routine clinical practice”. The ATA’s own patient material calls it “not clinically useful” in healthy, non-hospitalised people.
Is a low ratio evidence of poor T4 to T3 conversion?
It is consistent with the shift that happens in nonthyroidal illness, starvation, trauma and with drugs such as amiodarone, propranolol and glucocorticoids. It is not evidence that a patient with a normal TSH and free T4 needs T3 therapy, and no trial has shown that treating the ratio improves anything.
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References
- Van Uytfanghe K, Ehrenkranz J, Halsall D, et al. Thyroid stimulating hormone and thyroid hormones (triiodothyronine and thyroxine): an American Thyroid Association-commissioned review of current clinical and laboratory status. Thyroid. 2023;33(9).
- American Thyroid Association. Thyroid Function Tests — “in healthy, non-hospitalized people, measurement of reverse T3 does not help determine whether hypothyroidism exists or not, and is not clinically useful”.
- Schmidt RL, LoPresti JS, McDermott MT, Zick SM, Straseski JA. Does reverse triiodothyronine testing have clinical utility? An analysis of practice variation based on order data from a national reference laboratory. Thyroid. 2018;28(7):842–8.
- Garber JR, Cobin RH, Gharib H, et al. Clinical practice guidelines for hypothyroidism in adults: cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Thyroid. 2012;22(12):1200–35.
- Mayo Clinic Laboratories. Test ID RT3 — T3 (triiodothyronine), reverse, serum, by liquid chromatography tandem mass spectrometry. Reported in ng/dL, reference value 10–24 ng/dL, conversion to nmol/L by multiplying ng/dL by 0.01536.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.