Reverse T3: what the value shows, what it does not show, and why it is so contested
Same building blocks, different arrangement. One molecule works, the other does not. About a laboratory value that two camps rub against each other over, and about the question of when a pointer is still a long way from being a target.
All guides from the thyroid cluster
I do occasionally have reverse T3 measured. And I still consider the reticence of the professional societies well founded on the substance. Saying both at once is uncomfortable. It is also the most honest thing I can say about this value.
You are holding a laboratory report. At the top the familiar lines: TSH, free T4, free T3. Further down a line you have never seen before.
Reverse T3. Next to it a number. And a small arrow pointing upwards.
Perhaps someone told you that this at last explains why you have been beside yourself for months. Perhaps also that this value says nothing at all and the money for it is gone. Most likely you have heard both, with the same conviction in the voice. So you type the term in. And you find two worlds that barely talk to each other.
Many people know exactly this pattern. In my consultations I hear it often, and it is almost never about biochemistry. It is about someone holding a number in their hand and finding nobody who explains it without an agenda.
That is exactly what this text attempts. First the biology. Then the criticism, at full strength. And only afterwards my own use of it, expressly as an observation.
What this article covers
- Why two camps argue about the same value
- What reverse T3 is biologically
- When the value rises without anything being broken
- The thinking error between amount and production
- The non-thyroidal illness syndrome
- Why a pointer is not yet a target
- The criticism of rT3, without glossing over
- What I still use the value for
- What you can look at instead
- 13 questions and 35 sources
One value, two camps arguing, and both have something right
The first world says, in essence: at last an explanation. Your TSH is unremarkable, your free T4 too, and you are still tired, cold and thinking through cotton wool. Reverse T3 is said to be the missing piece, to block the receptors, to prove a disturbed conversion. There are calculators, ratios and target values to go with it.
The second world says, in essence: you do not measure this value. It is in no guideline, it is not validated, insurance does not pay for it.
Both camps are partly right, and both leave something out. The starting observation of the first camp is real, and the European professional society acknowledges it expressly: there are people on thyroid hormone whose TSH sits in the target range and who still do not feel well. That is not an internet phenomenon, that is written in a guideline.
A task force of the European Thyroid Association worked through in 2012 why roughly 5 to 10 percent of people treated with levothyroxine still have complaints despite a normal TSH.
As explanations the task force names the awareness of a chronic illness, accompanying autoimmune conditions, autoimmunity itself, and the difficulty of producing physiological T4 and T3 concentrations in serum and tissue at the same time with levothyroxine alone.
For you that means: the underlying problem the rT3 debate circles around is acknowledged by the professional world. The dispute is only about whether one particular laboratory value is suited to capture it.
Wiersinga WM et al., Eur Thyroid J 2012;1(2):55-71. DOI: 10.1159/000339444 [Guideline]And the second camp has a point as well. A value from which no treatment with documented benefit follows is hard to justify as a routine test. Added to that are technical objections, further down in a section of their own.
This text does not take a side for you. It shows you why there are two sides and where the evidence stops on each.
That is not evasion, it is the only honest way to handle a value whose meaning we know less about than either camp claims.
This article sits in the thyroid cluster on the level of measuring. The question of this level is not what you should do, but: how good is the measurement?
Two topics I therefore only touch on: why symptoms can persist despite unremarkable values, which you can read about in Normal values, symptoms all the same, and what is meant by functional hypothyroidism, which you can read about in Functional hypothyroidism. Here it is only about this one laboratory value.
And now you know why the dispute about rT3 is not one about facts, but about the reach of a number.
What reverse T3 actually is, in plain words
Picture two keys. Same factory, same metal, the same number of teeth. Only on one of them a single tooth sits on the other side of the blade. One turns in the lock, the other goes in and nothing happens. That is exactly how T3 and reverse T3 relate to each other.
Your body mainly produces T4 in the thyroid gland, a precursor hormone with four iodine atoms. T4 is not the working hormone, it is the semi-finished product. The work is done by T3, and T3 only arises out in the tissue, when a single iodine atom is removed.
What matters is where it is removed. The T4 molecule has two rings. If the iodine is taken off the outer ring, T3 arises, that is 3,5,3'-triiodothyronine, the active hormone. If it is taken off the inner ring, 3,3',5'-triiodothyronine arises, and that is reverse T3.
Same building blocks, different arrangement. A single difference in position, and the molecule no longer does the same thing.
The three enzymes that decide this
Responsible for this decision are the deiodinases. There are three of them, and they are selenoenzymes, meaning they carry selenium in their active centre.
Deiodinase 2 is the amplifier: it cleaves at the outer ring and turns T4 into active T3. Deiodinase 3 is the off switch: it cleaves at the inner ring, turns T4 into reverse T3 and breaks existing T3 down into T2. Deiodinase 1 can do both and sits above all in liver and kidney.
A large review in Endocrine Reviews gathered how the thyroid hormone signal is controlled inside the cell. The result: the deiodinases provide the largest dynamic range of this control, larger than transporters or receptors.
For you that means: the signal does not arise in the blood, it arises in the cell. A blood value such as rT3 can therefore only ever be an indirect hint, never the finding itself.
Bianco AC et al., Endocr Rev 2019;40(4):1000-1047. DOI: 10.1210/er.2018-00275 [Mechanism Review]Why would a body build in an off switch at all? Because a metabolism that could only accelerate would be dangerous. Deiodinase 3 is upregulated in heart muscle and brain when oxygen becomes scarce, and it can lower energy consumption there. That is expressly described as a protective mechanism, not as a fault.
Reverse T3 is not the enemy of T3. It is the imprint of a brake that belongs to the same system. Whoever sees the imprint knows that braking happened, but not yet why.
The matter of receptor blockade
On many German language pages you read that reverse T3 settles onto the T3 receptors and blocks them. The image is catchy. The data do not carry it in that sharpness.
Halsall and Oddy gathered in the Annals of Clinical Biochemistry what is known about the biochemistry, measurement and clinical meaning of reverse T3. It is one of the most important single sources on this value.
Their finding: rT3 is the third most abundant circulating iodothyronine in the blood, binds only weakly to the nuclear T3 receptors and counts as an inactive end product that diverts T4 away from the T3 pathway. A binding to receptors outside the cell nucleus has also been described, and its clinical meaning remains open.
For you that means: the notion of a plug in a lock is an image, not a finding. Reverse T3 does not displace the active hormone from the receptor, it arises by a different route from the same starting material.
Halsall DJ, Oddy S, Ann Clin Biochem 2021;58(1):29-37. DOI: 10.1177/0004563220969150 [Review]The difference sounds academic, but it matters. If rT3 were a blockade, it would make sense to want to remove it. If it is the end product of a switch point, then the interesting question is not the product but the switch.
How this switch works in detail and why different tissues react differently is described in From T4 to T3: the conversion.
A high rT3 value is not a diagnosis. It is an observation about a switch point in the tissue, seen from a great distance, namely from the blood.
And now you know why the question of why matters more than the number itself.
When the value rises without anything being broken
There is a handful of situations in which reverse T3 reliably rises. They all have something in common: the body currently has a reason to be thrifty.
Fasting and low energy intake
One of the clearest studies on this is fifty years old.
A group around Vagenakis and Braverman had nine euthyroid volunteers with obesity fast completely for four weeks, and a second small group beforehand on suppressive doses of levothyroxine.
What was observed was a steep fall in serum T3, accompanied by a proportionally similar rise in reverse T3, in the T4 suppressed group as well. After five days of refeeding both values returned to their starting levels.
For you that means: the switching happens in the tissue, because it occurred even under an excess of T4. And it recedes within a few days, which speaks for a regulatory answer and not for damage.
Vagenakis AG et al., J Clin Endocrinol Metab 1975;41(1):191-194. DOI: 10.1210/jcem-41-1-191 [In vivo, human, n=13]What is interesting is that it is not only the amount of calories that counts, but the composition as well.
Danforth and colleagues fed volunteers of normal weight considerably above their needs over weeks to months and varied the macronutrients while doing so.
Short term overfeeding raised the T3 production rate markedly, from about 29.6 to 54.0 micrograms per day and 70 kg with carbohydrates, while T4 production stayed unchanged. Conversely, a low carbohydrate diet imitated the T3 and rT3 pattern of fasting in its direction, even at an unchanged calorie count.
For you that means: a very low carbohydrate phase can produce an rT3 pattern without anything being diseased. The body evidently reads carbohydrate intake as information about the supply situation.
Danforth E Jr et al., J Clin Invest 1979;64(5):1336-1347. DOI: 10.1172/JCI109590 [In vivo, human, overfeeding study]Why a body in deficit turns so many dials at once is described in The calorie deficit: why it is true and still not enough. The thyroid side of that adaptation is what you are looking at right here.
Your body is not doing this against you. It is doing it because it is supposed to economize right now.
An elevated rT3 after a strict dieting phase is no proof of a defect, rather one that the regulation is working.
Illness, surgery, intensive care
The second large group of triggers is illness itself. Acute infections, operations, injuries, sepsis and nearly every form of severe illness can shift the pattern in the same direction: T3 falls, rT3 rises, TSH does not rise in compensation. How strongly depends on the severity of the illness. Whether the pattern is a consequence of the illness, part of the answer to it or both, is contested to this day.
Medications
Chopra and colleagues gave fifteen people dexamethasone, 2 mg every six hours over four doses, and measured closely afterwards.
Reverse T3 rose noticeably within eight hours, with a maximum after 24 to 32 hours, and stayed elevated for about a day after stopping. T3 fell in the opposite direction. The authors ruled out altered protein binding as an explanation.
For you that means: a single messenger from the stress system can produce the pattern within hours. What matters is the limit of this study: a pharmacological dose, not everyday stress. Anyone who concludes from this that a demanding job measurably raises your rT3 is going beyond the data.
Chopra IJ et al., J Clin Endocrinol Metab 1975;41(5):911-920. DOI: 10.1210/jcem-41-5-911 [In vivo, human, n=15]The second example is amiodarone for cardiac arrhythmias. It inhibits the 5'-deiodinase: serum T4 and serum rT3 rise, serum T3 falls. Manifest functional disturbances occur in 14 to 18 percent of those treated long term.
If you take one of these medications, please change nothing about it on your own, neither the dose nor the timing, and certainly not because of a laboratory value. Changes to corticosteroids, amiodarone, levothyroxine or antithyroid drugs belong exclusively in medical care.
With amiodarone a fixed monitoring schedule is foreseen anyway, according to the German primary care guideline TSH, fT3 and fT4 every six months. Reverse T3 is not part of it.
In pregnancy, different target ranges and a different framework of care apply to thyroid values. The hormonal balance shifts physiologically, the binding proteins change, the requirement rises.
From a reverse T3 value no conclusions at all are drawn there. Thyroid questions in pregnancy belong in medical hands, closely accompanied and without self experiments.
And now you know why an elevated rT3 asks first about the circumstances of your life and not about the thyroid.
The thinking error that often happens in this debate
Now comes the part that is usually missing from the German language advice literature. It is banal once you have heard it, and it changes everything.
A laboratory value measures an amount in the blood. It does not measure a production rate.
Picture a bathtub. The water level is rising. Two explanations are possible: the tap is running harder, or the drain is blocked. From the water level alone you cannot tell them apart. Exactly this question was measured by someone in 1978.
Eisenstein and colleagues determined kinetically, in four euthyroid people with obesity, the production rate and the metabolic clearance rate of reverse T3, before fasting and during it.
The serum concentration of rT3 rose by 69 percent. At the same time the metabolic clearance rate fell in all four people, from 96 plus minus 23 to 68 plus minus 17 litres per 70 kg and day. From this only a slight rise in the mean production rate followed arithmetically. The authors concluded that deiodination at the inner and at the outer ring can be regulated independently of each other.
For you that means: during fasting the rT3 value rose markedly although barely more rT3 was being formed. What had changed was the breakdown. The drain had become slower, not the tap stronger.
Eisenstein Z et al., J Clin Endocrinol Metab 1978;47(4):889-893. DOI: 10.1210/jcem-47-4-889 [In vivo, human, kinetics, n=4]The same principle applies with amiodarone. The value rises there not because more rT3 is formed, but because its breakdown is inhibited. And there is a third example that sits even closer to your everyday life.
An endocrinology practice retrospectively evaluated the first rT3 measurements of 976 people who complained of exhaustion while on thyroid treatment, measured by mass spectrometry.
The share of elevated values depended on the type of treatment: 20.9 percent on levothyroxine alone, that is 29 out of 139, compared with 9 percent on no hormone replacement at all, that is 31 out of 345. In the regression rT3 rose with free T4 and fell with the logarithm of TSH. The lowest values were found on T3 only preparations.
For you that means: elevated rT3 occurs more often on levothyroxine. The most obvious reason, however, is not a conversion defect but a substrate calculation. More starting material, more end product.
Wilson JB et al., PLoS One 2025;20(6):e0325046. DOI: 10.1371/journal.pone.0325046 [Cohort, n=976]The reverse conclusion is uncomfortable for both camps. That a T3 containing preparation can lower the rT3 value is no proof of efficacy. It simply reduces the starting material. The value moves without you feeling any better for it. Whether a T3 containing preparation or a desiccated thyroid extract (NDT) comes into question for someone is a medical decision after examination and conversation, and not a question that is settled by this laboratory value.
The same number, three very different readings
The concentration of reverse T3 in the blood, at one point in time, with the method of one laboratory.
So an amount. A water level.
That your body is producing masses of rT3, that the conversion is disturbed, the receptors blocked, the value in need of lowering.
So production rate, cause and treatment target in a single step.
During fasting the concentration rose by 69 percent, while clearance fell from 96 to 68 litres per 70 kg and day and production barely increased.
On amiodarone the value rises through inhibited breakdown, on levothyroxine along with the available T4.
Sources: Eisenstein 1978 (DOI: 10.1210/jcem-47-4-889), Halsall 2020 (DOI: 10.1177/0004563220969150), Wilson 2025 (DOI: 10.1371/journal.pone.0325046). The jump from the amount to the production rate is the most common error in the lay literature, and it usually happens unnoticed.
If you keep having complaints on levothyroxine, that is a topic of its own and to be taken seriously, one that does not hang on a single laboratory value. Data and options are set out in Levothyroxine and symptoms that stay.
Before you ask how high the value is, ask whether it became high through more inflow or through less outflow.
This one question separates a sensible interpretation from a story. And now you know why most rT3 texts turn off at this point.
The non-thyroidal illness syndrome, well described and poorly understood
There are three names for this pattern. That there are three says something about the embarrassment of the field.
- Non-thyroidal illness syndrome, a thyroid change during an illness outside the thyroid.
- Low T3 syndrome, named after the most conspicuous laboratory finding.
- Euthyroid sick syndrome, ill with normal thyroid function.
The laboratory picture is always similar: T3 low, reverse T3 elevated, T4 low to normal. And, most striking of all, TSH does not rise in compensation, although you would expect that in a true hypothyroidism.
Langouche, Jacobs and Van den Berghe worked through the syndrome across all age groups, from premature infants to critically ill adults.
In the acute phase the pattern arises predominantly through increased peripheral inactivation, with reduced food intake playing a part. The severity of the illness correlates strongly with how pronounced it is, and the authors expressly call the causality contested. The acute changes count today as a sensible adaptation that saves energy and supports the innate immune response.
For you that means: the professional literature does not describe this pattern in the acute phase as damage, but as an answer. Only in prolonged illness does a central suppression come on top, which may no longer be sensible.
Langouche L, Jacobs A, Van den Berghe G, J Endocr Soc 2019;3(12):2313-2325. DOI: 10.1210/js.2019-00325 [Review]The Leuven intensive care group has put it most sharply: the syndrome has different faces. In the early phase the changes concern hormone binding, cellular uptake transport and the activity of D1 and D3, partly caused by the accompanying nutrient restriction, and that appears favourable. In protracted critical illness, by contrast, TRH expression is suppressed in the hypothalamus, which explains the low TSH release. The author's conclusion is sober: tolerating the early fasting response appears sensible.
What actually happens in the tissue
One piece of work did not only look into the blood, but directly into the tissue. It is uncomfortable to read and one of the most informative there is on this topic.
Peeters and colleagues examined tissue and blood samples from 80 people who had died in an intensive care unit, taken within minutes of death.
Serum rT3 was higher, while TSH, T4, T3 and the T3 to rT3 ratio were lower than in healthy people. In the liver D1 was downregulated, and D3 was induced in liver and skeletal muscle, although it does not occur there at all in healthy people. This was most pronounced with cardiovascular failure, kidney failure requiring dialysis and a need for catecholamines, that is, wherever tissue perfusion was poor.
For you that means: the picture in the blood is not a measurement artefact, it depicts a real event in the tissue. And above all it depicts the severity of the underlying illness, not a thyroid problem in its own right.
Peeters RP et al., J Clin Endocrinol Metab 2003;88(7):3202-3211. DOI: 10.1210/jc.2002-022013 [Cohort, n=80]Tissue concentrations of thyroid hormone do not necessarily mirror the low serum values. Depending on the organ and the type of illness they can fall, stay the same or even rise.
Paraphrased from Fliers and Boelen, J Endocrinol Invest 2021. DOI: 10.1007/s40618-020-01482-4 [Review]That is the clearest rejection of a simplifying blood value interpretation to be found in this literature, and it comes from two leading authors of the field. A second paper from the same group shows that the genes involved change in an organ dependent way in both directions, from inhibition to activation.
So in different organs different things are running at the same time. A blood value shows only an average of that, and averages tend to keep quiet about the extremes.
The German primary care guideline draws a practical consequence from this. It names the non-thyroidal illness syndrome expressly as a confounder: in acute severe illness or anorexia nervosa first a mild TSH suppression, in the recovery phase rather a TSH elevation, to be watched particularly after a hospital stay.
That is good advice, independent of any rT3 question. Whoever has thyroid values measured shortly after a severe illness is measuring the illness along with them.
A third review sums up the therapeutic side in one sentence that has not changed for years: there is still no indication for thyroid hormone therapy in the great majority of people with this syndrome, even if a small subgroup might be an exception.
And now you know why the pattern is well described and nobody can nevertheless say what it means for you personally.
A pointer is not a target
This section is the most important one in the article to me. It answers a question that reaches far beyond the thyroid: why can a value indicate something and still be a poor treatment target? Let us start with the strong side.
The value predicts something, and clearly so
Iervasi and colleagues examined 573 consecutive people with heart disease with a complete thyroid profile and followed them for one year.
173 people had a free T3 below 3.1 pmol/l, 400 had a normal value. Overall mortality was 14.4 versus 3.0 percent. In the Cox model free T3 was the strongest predictor of overall mortality, with a hazard ratio of 3.582, ahead of lipid disorder, age and pump function.
For you that means: the value is a strong pointer. This study is what set the debate going in the first place. It does not show, however, that the value is a target.
Iervasi G et al., Circulation 2003;107(5):708-713. DOI: 10.1161/01.cir.0000048124.64204.3f [Cohort, n=573]Two further cohorts confirm the direction. In 458 people with dilated cardiomyopathy the low T3 syndrome was the second strongest predictor of mortality, hazard ratio 3.147, though only 17 of 458 had the pattern. In 367 people with mild to moderate COVID-19, 7.4 percent met the criteria, and the syndrome predicted deterioration independently, adjusted odds ratio 3.19.
And raising the value still does not change the course
Now the other half, which rarely appears in the German advice literature.
Kaptein, Beale and Chan searched systematically for studies in which T3 or T4 was given against placebo, in obesity under calorie restriction and in illnesses outside the thyroid.
The result: no consistent effects on weight loss, protein breakdown, metabolic rate or heart rate. In acute kidney failure mortality under T4 therapy was 3.3 times higher. What was documented, by contrast, was that such treatment produces a subclinical hyperthyroidism.
For you that means: there is at least one setting in which the well meant correction of a laboratory value went along with a raised mortality. That is no argument against thyroid hormone in true hypothyroidism. It is an argument against treating a number.
Kaptein EM, Beale E, Chan LS, J Clin Endocrinol Metab 2009;94(10):3663-3675. DOI: 10.1210/jc.2009-0899 [Systematic Review]Two meta-analyses from paediatric cardiac surgery make the picture even clearer. There the low T3 pattern is present after an operation on the heart lung machine practically every time, so it is the ideal situation to test whether raising the value changes anything.
| What was measured | Prognostic studies: the value as a pointer | Intervention studies: the value as a target |
|---|---|---|
| Heart disease, adults | Low fT3: 14.4 versus 3.0 percent one year mortality, HR 3.582 (Iervasi 2003) | T3 administration without consistent effects on heart rate, cardiac output, resistance (Kaptein 2009) |
| Cardiomyopathy | Low T3 syndrome as second strongest predictor, HR 3.147 (Wang 2015) | no intervention study with hard endpoints available |
| COVID-19, mild to moderate | Syndrome in 7.4 percent, adjusted OR 3.19 for clinical deterioration (Lui 2021) | no intervention data |
| Paediatric cardiac surgery | Low T3 pattern after bypass almost as a rule | 9 studies, 711 children: no difference in ventilation, intensive care and hospital stay, mortality (Flores 2019) |
All figures from the papers listed in the source list. The left column shows that the value indicates something. The right one shows that raising it did not change the endpoints examined.
A newer meta-analysis at the level of individual participant data deserves a finer reading, and I deliberately do not shorten it.
Radman and colleagues pooled five randomised trials at the level of individual participant data, 767 children from birth to three years, two trials from the USA and three from Indonesia.
Overall no significant difference in time to extubation, hazard ratio 1.09. Looked at separately it was 1.31 in Indonesia with a significant effect, and 0.95 in the USA without an effect, the same pattern for hospital stay. Baseline TSH and baseline T3 were markedly lower in Indonesia, which the authors traced back to a starting weakness often linked with undernutrition.
For you that means: giving the hormone changed the course where a state of deficiency genuinely existed, and not where the low value was merely an expression of the procedure. That is the finest nuance of this topic, and it is conciliatory rather than polemical.
Radman MR et al., Pediatr Cardiol 2024;45(5):1100-1109. DOI: 10.1007/s00246-024-03465-1 [Meta-analysis, k=5, n=767]A smoke detector does not become quieter by being unscrewed.
It reliably indicates that something is burning somewhere. It is still not a place where you put out the fire. That is exactly the relationship rT3 has to your metabolism. It indicates a situation. It is not the situation.
And what if a dampened signal is sometimes even favourable?
One study calls the reflex that more thyroid hormone is always better into question even further.
The Leiden 85-plus study followed 599 people from the age of 85 onwards, for a mean of 3.7 years.
Rising TSH values went along with lower mortality, hazard ratio 0.77 per standard deviation. Rising free T4 went along with higher mortality, hazard ratio 1.16. Neither value was associated with disability in daily life, depressive symptoms or cognitive impairment.
For you that means: among the very old a lower thyroid set point was linked with a longer life. An observational study, not proof of cause and effect. But it puts the idea into perspective that a dampened thyroid signal is always a defect.
Gussekloo J et al., JAMA 2004;292(21):2591-2599. DOI: 10.1001/jama.292.21.2591 [Cohort, n=599]None of this speaks against an existing thyroid treatment. A diagnosed hypothyroidism is treated, and that is well founded.
If you take levothyroxine or an antithyroid drug, please change nothing about it by yourself. No rT3 value and no passage in this article is a reason to reduce a dose, switch a preparation or stop something. Such changes belong exclusively in medical care.
And now you know why a good marker and a good treatment target are two completely different things.
The criticism, without glossing over
Now the five objections to reverse T3 as a routine test. I have not softened them, because I consider them justified.
First: not validated
The rT3 to T3 ratio is not validated as a diagnostic criterion. No threshold has ever been tested prospectively against a clinical endpoint. The laboratory review states plainly that measuring rT3 in serum has to this day found no routine application. How strongly a definition alone can shift the result is shown vividly by a small study.
Duyu and colleagues determined the frequency of euthyroid sick syndrome in 80 newly diagnosed children with cancer according to two definitions. Criterion 1: fT3 below the reference range, fT4 normal or low, TSH normal. Criterion 2: in addition rT3 above the reference range.
Of 75 evaluable children, 14, that is 17.3 percent, met the first criterion and 8, that is 10.6 percent, the second.
For you that means: whether rT3 is part of the definition or not changes the measured frequency of the same thing in the same group from 17.3 to 10.6 percent. If the definition moves the result that much, the value is unreliable as a criterion. That is what not validated means, in numbers.
Duyu A et al., J Clin Res Pediatr Endocrinol 2018;10(3):198-205. DOI: 10.4274/jcrpe.0015 [Cohort, cross-sectional, n=80]Second: the measurement is not harmonised
Reverse T3 was long determined by competitive radioimmunoassay. These methods have meanwhile largely been replaced by mass spectrometry. Only, not every laboratory measures the same way.
And the problem does not concern rT3 alone. An international panel commissioned by the American Thyroid Association, jointly staffed with the CDC and the IFCC, found in 2023: despite fifty years of method development, routine thyroid assays remain vulnerable to analytical interferences that can produce false results. Results from different assays should therefore not yet count as equivalent. If that already holds for TSH and fT4, it holds all the more for a rarely measured value.
The same criticism also hits the values on which routine care relies. Immunoassays for free thyroid hormones are influenced by changes in the binding proteins. Several studies found falsely normal values for T3, fT3 and fT4 in the immunoassay that lay below the reference range in the mass spectrometric reference method.
The same authors propose measuring by mass spectrometry when complaints persist on levothyroxine. So the dispute does not run neatly between laboratory medicine and functional medicine. It runs straight through both.
Third: the time of day shifts the result
This point is my favourite, because it is so simple and appears in no online calculator.
Sun and colleagues drew blood from 110 healthy women and 49 healthy men at four centres, each time in the morning and in the afternoon, and determined all thyroid hormones by mass spectrometry.
From morning to afternoon total T3 and free T3 fell significantly, while free T4 and reverse T3 rose significantly. No differences between the sexes were found.
For you that means: if fT3 falls over the course of the day and rT3 rises at the same time, then the fT3 to rT3 ratio moves through the time of the blood draw alone. Without anything fundamental having changed in your body.
Sun Q et al., Ther Adv Endocrinol Metab 2020;11:2042018820922688. DOI: 10.1177/2042018820922688 [In vivo, human, n=159]That also answers why the cut-offs on the internet contradict each other. In one place the ratio is supposed to be greater than 2, in another greater than 20, plus conversion debates about a factor of 1000. The reason is banal: because no threshold has ever been tested against a clinical endpoint, any number can be put into circulation without being refuted.
Fourth: no professional society recommends the measurement
That is a fact, and the reasoning matters more than the result.
The American guideline on the treatment of hypothyroidism from 2014 holds levothyroxine to be the standard of treatment. Nowhere does it say that rT3 is nonsense. It says something more precise: what is still missing is a validated marker that shows, beyond TSH, how the tissue is doing. Reverse T3 is a candidate for that role, and so far it has not passed the test.
The S2k guideline of the German College of General Practitioners and Family Physicians on the elevated TSH value does not mention reverse T3 a single time in the entire guideline text. It already considers measuring fT3 in the presence of an elevated TSH to be of no additional benefit.
If fT3 is already regarded as dispensable in this constellation, it is understandable why rT3 is not covered by statutory insurance. That is not arbitrary, it is the same logic.
Fifth: who orders the test, and what that says about the evidence
Here it gets delicate, so first this: I am talking about the state of the evidence, not about colleagues.
Schmidt and colleagues evaluated all thyroid test requests of a national reference laboratory over one year, supplemented by a systematic literature review.
Of 402,386 orders, 91,767 were for reverse T3, from 60,733 requesters. Only 20 percent of all requesters ever ordered rT3 at all, and 95 percent of those did so at most twice. Conversely, 100 requesters, that is 0.1 percent, accounted for 29.5 percent of all rT3 orders. The literature review found little evidence supporting this volume.
For you that means: the use concentrates extremely strongly on a small group of practitioners, and the published literature does not carry this volume. That is no judgement about motives, it is a statement about the state of the data.
Schmidt RL et al., Thyroid 2018;28(7):842-848. DOI: 10.1089/thy.2017.0645 [Review]A small audit at an American university hospital pointed in the same direction: of 20 rT3 requests, 11 appeared inappropriate in the clinical context. The number of cases is tiny, and that belongs in the picture. The value of this paper lies in the position it formulates, not in the statistics.
That leaves the cost question. In Germany reverse T3 is as a rule not covered by statutory insurance and is billed as an individual health service. I deliberately name no concrete amounts, because they differ from laboratory to laboratory.
A structural parallel worth knowing
Functional medicine knows a second concept of almost identical construction: a real observation, a catchy story about it, and a measurement that does not carry that story. Described at length in Adrenal fatigue: what there is to the idea of a weak adrenal gland. The dispute there, as here, is rarely about the experience. It is about the question of whether a number can depict it.
The guidelines do not recommend this test, for four understandable reasons: missing validation, missing harmonisation of the measurement, missing clarity about the cause, and missing treatment with documented benefit.
What a functional view asks in addition is not: is the criterion valid? Rather: which situation stands behind this pattern? Only the first question can be answered with a cut-off.
And now you know why I largely agree with this criticism.
What I nevertheless sometimes use the value for in my consultations
Now it gets personal, and I mark that expressly for what it is.
In clinical practice I observe that a markedly elevated reverse T3 in someone who has felt drained for months almost always stands next to other findings: usually a raised inflammatory state, sometimes a long stretch of very low energy intake, sometimes a severe illness not long past, sometimes a medication list nobody has spoken about before.
That is an observation. Not a study, not controlled, expressly not validated. I have no control group, I see a selected group of people, and I do not know how often I am mistaken.
What the value can be for me is a question: is this body currently in a state of thrift or of strain? It is then answered not by the rT3, but by the inflammatory state, iron and nutrient supply, energy intake, sleep, the medication list and how long ago the last severe illness was.
What the value can do and what it cannot
What it can do: it can raise a question and open a conversation about context that would otherwise not take place. And in rare constellations it can carry a laboratory workup further.
What it cannot do: it proves no hypothyroidism, justifies no treatment, is no treatment target and replaces no basic workup.
So anyone who tells me their rT3 is elevated does not get a treatment against rT3. They get questions about the last six months of their life.
Not suited for routine use is not the same as useless. The laboratory review describes a situation in which rT3 can genuinely provide a diagnostic fingerprint: in genetic disorders of the deiodinases, hormone transporters or binding proteins.
These are rare constellations for specialised centres, and they have nothing to do with the question of whether everyday stress raises your rT3. But they explain why the value exists at all.
How honest the evidence looks on my side
Whoever takes the criticism of the other side seriously has to name their own evidence just as harshly.
What exists in published reports on T3 based treatment with elevated rT3 are small case series without a comparison group. The best known describes nine people with mould exposure and exhaustion, all of whom reported an improvement. No control arm, no blinding, no predefined endpoints, several things changed at once. Nothing can be derived from that for you. I cite the paper only so that you can see what this side rests on.
No reverse T3 value is a reason to change, reduce or stop levothyroxine or an antithyroid drug on your own. Not even a very high one.
If your treatment does not match how you feel, the next step is a conversation with the practice treating you. Bring the report, bring the questions. But change nothing alone.
- A newly felt nodule on the neck or a swelling that is growing quickly
- Hoarseness that does not go away, or difficulty swallowing
- Racing heart, skipped beats or an irregular pulse
- Marked inner restlessness with weight loss, tremor and heavy sweating
- Marked slowing with cold intolerance, increasing swelling and drowsiness
That needs prompt medical assessment. In more depth: Graves disease and hyperthyroidism and Nodules and goiter: getting them checked.
And now you know how I use a value that I myself consider not validated, and why that is no contradiction as long as one says the limit out loud.
What you can look at instead, or in addition
No protocol, no dosages, no order to work through. Instead the questions that yield more than a second rT3 measurement.
The timing and the context of the measurement
Before a number is interpreted, three pieces of information belong with it: how long ago was the last severe illness or operation? At what time of day was the blood drawn? And which nutritional phase were you in beforehand?
The German primary care guideline expressly advises against measuring thyroid values immediately after a hospital stay. An afternoon draw yields a different ratio than a morning draw. And a strict dieting phase can produce the pattern all by itself.
The inflammatory state
When an rT3 pattern stands out, a look at inflammation is usually worth more than a second look at the thyroid. This is one of the few reproducible accompanying findings.
A Brazilian group compared 52 people with type 2 diabetes and 52 matched controls and measured, alongside all hormones, high sensitivity CRP.
CRP correlated positively with rT3 and negatively with the ratios fT4 to rT3 and fT3 to rT3. In a second, larger investigation with 258 people, interleukin 6 additionally correlated positively with rT3 in both diabetes groups, independently of sex, age, weight and long term blood sugar.
For you that means: the link between silent inflammation and the rT3 pattern has been described several times. These are observational data, not evidence of causation. As a direction to search in, they are nonetheless more productive than a target number for a hormone ratio.
Moura Neto A et al., Endocr J 2013;60(7):877-884. DOI: 10.1507/endocrj.ej13-0030 [Case Control, n=104] · Moura Neto A et al., Endocrine 2016;51(1):63-71. DOI: 10.1007/s12020-015-0651-5 [Cohort, cross-sectional, n=258]What silent inflammation is and how it can be recognised is described in Silent inflammation and weight. Why inflammation also shifts iron utilisation and produces a similar thinking error in doing so is described in Iron, inflammation and the hepcidin blockade.
Energy intake and carbohydrate share
If you have been eating markedly below your needs for months or living on very few carbohydrates, a shifted T3 to rT3 picture is to be expected rather than surprising. That is not a call to give up a way of eating, it is an invitation to read the finding in that light. The larger context is set out in The calorie deficit: why it is true and still not enough.
Iron and ferritin
Iron deficiency is one of the most common companions of exhaustion and has its own points of contact with the thyroid and with sleep, in detail in Iron deficiency, thyroid and sleep. All that belongs here is the note that ferritin and inflammatory markers should be read together.
Selenium, and why I deliberately slow down here
The most obvious short circuit runs: the deiodinases are selenoenzymes, so more selenium ought to improve the conversion. According to the data it is not that simple.
A review on selenium and the thyroid records that even very small amounts of selenium appear to suffice for the activity of the deiodinases. Selenium status, by contrast, appears to influence the development of thyroid diseases, which is a different topic. After long term supplementation a raised diabetes risk has also been reported.
So no dose and no recommendation in this article. The data on that are set out in Selenium, zinc, iron and vitamin D in thyroid topics.
The medication list, without changing anything
Write down once everything you take regularly, food supplements included. Not in order to leave something out, but in order to be able to put it on the table in the consultation.
Reading the basic values properly
And finally: before a special value is measured, the standard values should have been read cleanly. Which values there are, what they say and where reference ranges come from is set out in Thyroid blood values: which ones really count. With antibodies and the question of an autoimmune condition, the path leads to Hashimoto: causes in the immune system.
Questions that matter more than the next measurement
- What is supposed to happen with the result? Without a clear answer the measurement is not urgent.
- How long ago was the last severe illness? Shortly afterwards you are measuring the illness along with it.
- What was the energy intake of the past weeks? A strict phase can produce the pattern.
- What is the inflammatory state? CRP is better standardised as a measurement than rT3.
- Which medications are on the list? Corticosteroids and amiodarone are the best known influences.
- Have the basic values been discussed calmly? Special values come last, not first.
Reverse T3 is not a diagnostic value. Whoever makes a diagnosis out of it overstretches a number that is neither measured in a standardised way nor tested against an endpoint. As a question it is still worth something. Only, you answer it elsewhere.
And now you know why the more interesting move is not to change a number, but to understand the situation it indicates.
Frequently asked questions about reverse T3
What is reverse T3 in simple terms?
Reverse T3 is a close relative of the active thyroid hormone T3. Both arise from the same precursor T4 and carry three iodine atoms. The only difference lies in the ring position of the third iodine: in T3 it was removed from the outer ring, in reverse T3 from the inner one. The result binds only weakly to the nuclear receptor and counts as a largely inactive end product. It is no exotic rarity, though: rT3 is the third most abundant iodothyronine in the blood.
What does an elevated reverse T3 value mean?
At first only that more of it was circulating at the moment of the blood draw than the reference range allows for. Elevated values typically show up with fasting, low energy intake, acute illness, after surgery and under certain medications. It is a pointer to context, not a diagnosis. A threshold above which treatment should follow has never been tested against a clinical endpoint.
How does reverse T3 arise in the body?
From the precursor hormone T4, through enzymes that remove iodine. Deiodinase 2 cleaves at the outer ring and makes active T3, deiodinase 3 cleaves at the inner ring and makes reverse T3, deiodinase 1 can do both. D3 is the off switch of the system. It is upregulated in heart muscle and brain when oxygen runs short, and in severe illness also in liver and skeletal muscle, where it does not occur at all in healthy people.
Does reverse T3 really block the T3 receptors?
The central laboratory review by Halsall and Oddy says: rT3 binds only weakly to the nuclear T3 receptors and counts as an inactive end product that diverts T4 away from the T3 pathway. The popular image of a plug in a lock is not carried by the data. A binding to receptors outside the cell nucleus has also been described, and its clinical meaning remains open.
What can cause a high rT3?
Best documented are longer fasting and marked energy shortage, very low carbohydrate phases, acute infections, surgery, injuries, sepsis, intensive care and medications, above all glucocorticoids and amiodarone. The time of the blood draw plays into it as well, because rT3 rises over the course of the day. On levothyroxine alone the value lies above the reference range more often, because more starting material is available.
How high should the fT3 to rT3 ratio be?
There is no dependable answer to that so far. No threshold of this ratio has ever been tested against a clinical endpoint. On top of that, laboratories use different units and different measurement methods, and the ratio shifts with the time of the blood draw alone. Anyone who gives you a fixed target number is naming a convention, not a study result.
Why do the cut-offs for the rT3 ratio on the internet differ so much?
Because none of these numbers is documented, and therefore none can be refuted either. In the same forums you read once that the ratio should be greater than 2, and once greater than 20, plus conversion debates about a factor of 1000. As long as no threshold has been tested against a hard outcome, any number can be put into circulation. That is precisely what not validated means.
Can fasting or a diet raise the rT3 value?
Yes, and that is one of the oldest and cleanest observations about this value. In an experiment from 1975, four weeks without calories made T3 fall sharply while rT3 rose proportionally. After five days of refeeding both values returned to their starting levels. A very low carbohydrate diet can shift the pattern in the same direction too, even at an unchanged calorie count.
Why is my rT3 elevated on levothyroxine?
An analysis of 976 people with fatigue symptoms under thyroid treatment found elevated rT3 values in 20.9 percent of those on levothyroxine alone, compared with 9 percent of those on no hormone replacement. rT3 rose along with free T4. The simplest explanation is a substrate calculation: more starting material, more end product. That is no proof of a conversion disorder, and expressly no reason to change anything about your medication. That belongs in a medical consultation.
Why does my doctor not want to measure reverse T3?
Usually not out of disinterest, but for four reasons: the value is not validated as a diagnostic criterion, the measurement methods are not interchangeable between laboratories, a high value cannot be assigned to one single cause, and no treatment with documented benefit follows from the result. The German primary care guideline does not mention reverse T3 a single time.
Does statutory health insurance pay for the reverse T3 test?
As a rule, no. In Germany reverse T3 is usually billed as an individual health service. What the test costs differs from laboratory to laboratory, which is why I deliberately name no figure. The more important question is not the price anyway, but what is supposed to happen with the result. Without a clear answer to that, the measurement is not the most urgent expense.
Can reverse T3 be lowered, and does that achieve anything?
This article deliberately gives no instructions for that, and the reason is a matter of substance: there is no study showing that a lowered rT3 means a better outcome for you. In intensive care medicine exactly this was attempted with thyroid hormone. A systematic review found no consistent effects, and in acute kidney failure mortality under T4 was even 3.3 times higher. The more useful question is why the body is economizing right now. And independently of that: no rT3 value is a reason to change or stop levothyroxine, an antithyroid drug or any other medication on your own. Such decisions belong in medical care.
What is low T3 syndrome or euthyroid sick syndrome?
Three names for the same pattern: non-thyroidal illness syndrome, low T3 syndrome, euthyroid sick syndrome. What is meant is a constellation in people who are not primarily thyroid patients: T3 low, reverse T3 elevated, TSH not raised in compensation, T4 low to normal. In the acute phase it counts today as a sensible adaptation that saves energy and supports the innate immune response. Over a long course the picture is more complicated.
Reverse T3 and the rest of the body
This value does not stand on its own. It hangs on the stress axis, on the inflammatory state and on iron utilisation.
Reverse T3
What the value shows and what it does not
this articleCortisol and the HPA axis
The stress axis the rT3 story hangs on
Silent inflammation
Better standardised than rT3
Iron and hepcidin
The same thinking error at another laboratory value
Which cofactors the energy metabolism needs is set out in From calorie to energy: the cofactors.
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