Lab result explainedThyroidLast updated November 10, 2026

Reverse T3 (rT3): normal range, what high reverse T3 means, and the free T3 to reverse T3 ratio

Reverse T3 (reverse triiodothyronine) (rT3) · Ordered separately; not part of a standard thyroid panel · reported in ng/dL (some labs pg/mL, which is 10x the ng/dL value; SI: nmol/L, ng/dL x 0.0154 = nmol/L)

Reverse T3 (rT3, reverse triiodothyronine) is an inactive by-product made when the body breaks down the thyroid hormone T4 by removing an iodine atom from the 'wrong' side of the molecule. A normal adult level is roughly 9 to 24 ng/dL (some labs 8 to 25 ng/dL, or 90 to 250 pg/mL; 0.14 to 0.37 nmol/L in SI units), with ranges differing by lab. Unlike T3, reverse T3 barely binds thyroid hormone receptors and has no meaningful hormone action; its level mainly tells you how much T4 is being inactivated rather than activated.

Reverse T3 rises whenever the body deliberately slows its metabolism: during serious illness, after surgery, with fasting or very low-calorie diets, in uncontrolled diabetes, and with drugs such as amiodarone, high-dose steroids and propranolol. In those situations free T3 falls at the same time because the same enzyme shift is responsible. It also rises when a person takes more T4 (levothyroxine) than they need, since the excess is shunted into rT3, and it falls in untreated hypothyroidism because there is less T4 to convert.

Reverse T3 has become a popular test in some functional and integrative clinics, where a high value or a low free T3 to reverse T3 ratio is described as 'rT3 dominance' or 'thyroid resistance' and used to justify T3 or desiccated thyroid prescriptions. Mainstream endocrinology does not support this: the American Thyroid Association and the American Association of Clinical Endocrinology do not recommend measuring reverse T3 in diagnosing or managing hypothyroidism, no validated ratio cut-off exists, and the idea that rT3 blocks T3 from its receptor has not held up in human studies. This page explains what the number does and does not mean.

rT3 at a glance

What it is: an inactive form of thyroid hormone made from T4 by the type 3 deiodinase enzyme; it has essentially no hormone activity of its own.
Normal adult range: about 9-24 ng/dL (8-25 in some labs; 90-250 pg/mL; 0.14-0.37 nmol/L). Labs differ; newborns run much higher.
High reverse T3: illness, surgery, fasting or crash dieting, uncontrolled diabetes, liver or kidney disease, amiodarone, steroids, propranolol, or a levothyroxine dose that is too high. Nearly always accompanied by a low or low-normal free T3 with a normal TSH.
Low reverse T3: untreated hypothyroidism (less T4 to convert) or, rarely, assay issues; it is not clinically important on its own.
Free T3 to reverse T3 ratio: FT3 (pg/mL) divided by rT3 (ng/dL), multiplied by 100; 'above 20' is a popular but unvalidated target. Guidelines do not use it, and it adds nothing to TSH and free T4.
Next step: if it was high, look for the illness, diet or drug behind it and repeat 6 to 8 weeks after recovery; do not start or change thyroid hormone on the basis of reverse T3 alone.

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What rT3 measures

T4 has four iodine atoms, two on an outer ring and two on an inner ring. Type 1 and type 2 deiodinase enzymes remove an outer-ring iodine to make T3, the active hormone. Type 3 deiodinase (and type 1 to a lesser degree) removes an inner-ring iodine instead, producing reverse T3, which is then rapidly broken down to T2. In healthy adults about 40 percent of T4 goes down the activating path and about 40 percent down the inactivating path; the balance shifts hour by hour with illness, nutrition, hormones and drugs. Reverse T3 has a half-life of only about 4 hours, so its level reflects the current rate of production rather than a store.

The lab measures reverse T3 by immunoassay or, in reference laboratories, by liquid chromatography-tandem mass spectrometry, which is more specific because the rT3 molecule is a mirror image of T3 and antibodies can cross-react. Results are reported in ng/dL in most US labs (some use pg/mL: multiply ng/dL by 10) and nmol/L elsewhere. No fasting is required, although prolonged fasting itself raises the result, and a sample taken during an acute illness will reflect that illness. Biotin interference affects some rT3 immunoassays as it does other thyroid tests.

Reverse T3 is not a stand-alone diagnostic test for any condition. Its established uses are narrow: in research on non-thyroidal illness, where a high rT3 with low T3 characterises the response to critical illness, and occasionally in distinguishing non-thyroidal illness (rT3 high) from central hypothyroidism (rT3 low) when both free T4 and free T3 are low and the TSH is uninformative. Even there, the overlap between groups is large enough that most endocrinologists rely on the clinical picture and repeat testing after recovery instead.

Normal range for rT3

Reference ranges vary between labs and by age, sex and pregnancy. Use the range printed on your own report.

Group	Typical range (ng/dL (some labs pg/mL, which is 10x the ng/dL value; SI: nmol/L, ng/dL x 0.0154 = nmol/L))	Notes
Adults, typical reference range	About 9.2-24.1 ng/dL (0.14-0.37 nmol/L)	Labs differ: 8-25 ng/dL and 10-24 ng/dL are also used; some report pg/mL (90-250). Use the range on your report.
Newborns and infants	Several times the adult level in the first weeks	rT3 is high in the womb and at birth and falls to adult levels over the first months; not a sign of disease.
Older adults	Slightly higher than younger adults	A modest age-related rise in rT3, with a modest fall in T3, is normal and not treated.
Non-thyroidal illness (hospitalized, post-operative, critically ill)	Raised, often 1.5-4 times the upper limit, with low free T3 and normal TSH	Found in the majority of intensive-care patients; the degree of rise tracks illness severity. Resolves over weeks with recovery.
Fasting or very low-calorie diet	Rises 30-100 percent within 2-4 days	Returns to baseline within days of normal eating, particularly carbohydrate.
Over-replacement with levothyroxine	High rT3 with high or high-normal free T4 and low or suppressed TSH	Excess T4 is diverted to rT3; the TSH, not the rT3, is the signal to reduce the dose.
Untreated hypothyroidism	Low or low-normal	Less T4 is available to convert. rT3 rises back into range on treatment.
Free T3 to reverse T3 ratio (unvalidated)	FT3 (pg/mL) / rT3 (ng/dL) x 100; 'over 20' is the figure used in functional-medicine protocols	No clinical guideline or outcome study supports a cut-off. The ratio mainly rises or falls with illness and calorie intake.

High reverse T3

A high reverse T3 means your body is currently converting more T4 into the inactive form and less into active T3. In almost every case it is a consequence of something else rather than a thyroid problem in itself: an acute or chronic illness, recent surgery or injury, fasting or a very low-calorie diet, poorly controlled diabetes, liver or kidney disease, heart failure, depression or severe stress, or one of the drugs that block the activating enzymes (amiodarone, glucocorticoids such as prednisone, propranolol, some contrast dyes). The accompanying pattern is a low or low-normal free T3, a normal or high-normal free T4 and a normal TSH, which is the definition of non-thyroidal illness (low T3 syndrome).

This shift is thought to be protective. Reducing active thyroid hormone lowers energy expenditure and protein breakdown when the body is under strain, and randomised trials that gave T3 to critically ill patients, heart surgery patients or people on low-calorie diets to 'correct' the numbers did not improve outcomes. Reverse T3 itself does not block thyroid hormone receptors to any meaningful degree at the concentrations seen in people; the older laboratory observation that it can inhibit deiodinase enzymes in a test tube has not translated into a human condition that needs treating.

The one situation in which a high reverse T3 points to something to change is excess thyroid hormone: a levothyroxine dose that is too high, or a thyroid that is overactive, produces more T4 than can be used and the surplus is inactivated to rT3. The TSH and free T4 already show this, so the rT3 adds little. A high rT3 with a normal TSH, normal free T4 and no illness in a well person is usually a transient finding related to recent diet, exercise or a minor infection and should not prompt treatment.

Common causes

Acute and chronic illness (non-thyroidal illness syndrome)
Infection, sepsis, heart attack, heart failure, major surgery, trauma, burns, cancer, liver cirrhosis, kidney failure and severe COPD all raise rT3 and lower T3 within 24 to 48 hours through cytokines and cortisol; the magnitude tracks illness severity and predicts outcome in intensive care.
Fasting, very low-calorie dieting and eating disorders
Within 2 to 4 days of eating under about 800 kcal a day, rT3 rises 30 to 100 percent and T3 falls; carbohydrate restriction in particular drives the shift. Anorexia nervosa produces the same pattern chronically.
Glucocorticoids and other medications
Prednisone, dexamethasone and high-dose inhaled or injected steroids, amiodarone (which contains iodine and blocks deiodinases), propranolol above about 160 mg a day, propylthiouracil, iodinated contrast agents and some antiepileptics raise rT3.
Too much levothyroxine or hyperthyroidism
Excess T4 is partly disposed of as rT3. The TSH will be suppressed and free T4 high; the dose, not the rT3, is what is adjusted.
Uncontrolled diabetes and diabetic ketoacidosis
Insulin deficiency and ketosis reduce T4-to-T3 conversion; rT3 normalizes when glucose control is restored.
Liver and kidney disease
The liver is the main site of type 1 deiodinase, so cirrhosis lowers T3 and raises rT3; chronic kidney disease has a similar though smaller effect.
Severe stress, depression and sleep deprivation
Sustained cortisol elevation shifts conversion toward rT3, which is one reason rT3 is modestly higher in major depression and after prolonged sleep loss; the change is small compared with illness.
Newborn period and older age
rT3 is physiologically high in the fetus and newborn and drifts up slightly in old age; neither is a disorder.
Selenium deficiency
Deiodinases are selenium-dependent; severe deficiency (seen in some regions and in long-term parenteral nutrition, rarely in the US) impairs T4-to-T3 conversion and raises rT3.

Symptoms that can go with it

Usually none that are attributable to the reverse T3 itself; symptoms belong to the illness, diet or medication behind it
Tiredness, low energy and feeling cold during or after an illness or a period of under-eating
Weight loss, loss of appetite or muscle wasting with the underlying illness
Low mood, poor concentration and slowed thinking
Hair shedding 2 to 3 months after a serious illness, crash diet or surgery (telogen effluvium), which is often blamed on the thyroid
If rT3 is high because of too much levothyroxine: palpitations, anxiety, heat intolerance, insomnia and weight loss

Low reverse T3

A low reverse T3 is seldom a problem and is rarely searched for or acted on. It most often simply reflects that there is less T4 around to convert: untreated hypothyroidism, including Hashimoto's thyroiditis and hypothyroidism after thyroid surgery or radioactive iodine, lowers both T4 and rT3, and the rT3 rises back into range once levothyroxine is started. Central hypothyroidism from pituitary disease does the same, and in that setting a low rT3 alongside low free T4 and T3 can help distinguish it from non-thyroidal illness, where rT3 would be high.

In a well person with a normal TSH and free T4, a reverse T3 at or below the bottom of the range means T4 is being converted efficiently and is, if anything, reassuring. It needs no treatment and no follow-up. The 'optimal' low rT3 targets promoted in some protocols have no basis in outcome studies.

Common causes

Untreated primary hypothyroidism
Hashimoto's thyroiditis, thyroidectomy, radioactive iodine, iodine deficiency and some drugs (lithium, amiodarone-induced hypothyroidism) reduce T4 output, so there is less substrate for rT3. TSH is high and free T4 low.
Central (pituitary or hypothalamic) hypothyroidism
Low TSH drive reduces T4 and therefore rT3; low rT3 with low free T4 and a low or normal TSH points to the pituitary rather than to illness.
Recovery from illness or refeeding
As deiodinase activity normalizes after an illness or a period of fasting, rT3 can transiently dip below baseline before settling.
Efficient conversion in healthy, well-fed people
Young, healthy adults eating adequate calories often have rT3 in the lower part of the range; this is the normal state.
Assay method differences
Mass spectrometry methods read lower than some immunoassays because they do not cross-react with T3; a 'low' value on one platform may be mid-range on another.
Hyperthyroidism treated with antithyroid drugs
Propylthiouracil in particular blocks peripheral conversion and, together with falling T4, can push rT3 low during treatment.

Symptoms that can go with it

None from a low reverse T3 itself
Tiredness, feeling cold, constipation, dry skin, weight gain and slowed thinking if the cause is untreated hypothyroidism
Headache, visual field changes or other hormone deficiencies (low cortisol, missed periods, low libido) if the cause is pituitary disease

Read it alongside

TSH — the primary test; a normal TSH with a high rT3 means non-thyroidal illness or diet, a suppressed TSH means too much thyroid hormone, a high TSH means hypothyroidism.
Free T4 — high-normal or high with a high rT3 suggests over-replacement or hyperthyroidism; low with a low rT3 suggests hypothyroidism (primary if TSH is high, central if not).
Free T3 — low free T3 with high rT3 is the signature of non-thyroidal illness; the two move in opposite directions because the same enzyme shift causes both.
CRP, white cell count, albumin and liver and kidney tests — identify the illness that is usually behind a high rT3.
Glucose and A1C — uncontrolled diabetes raises rT3 and lowers T3.
Cortisol — high cortisol (Cushing's syndrome, steroid treatment, severe stress) raises rT3; the pituitary is checked if several hormones are low.
Thyroid antibodies (TPO, TRAb) — establish whether an abnormal TSH is due to Hashimoto's or Graves' disease; they, not rT3, direct treatment.

What usually happens next

If reverse T3 is high and your TSH is normal, the question is what is suppressing conversion: a recent illness, surgery, dieting or fasting, uncontrolled diabetes, or a medication. Address that and repeat the thyroid panel (TSH and free T4, with free T3 if wanted) 6 to 8 weeks after recovery.
If reverse T3 is high and your TSH is suppressed with a high free T4, the issue is too much thyroid hormone; expect a dose review if you are on levothyroxine or an evaluation for hyperthyroidism if you are not.
If reverse T3 is low with a high TSH and low free T4, that is ordinary hypothyroidism and is managed on TSH and free T4; the rT3 does not need repeating.
If reverse T3 is low with a low free T4 and a low or normal TSH, ask about pituitary testing (cortisol, prolactin, sex hormones) and possibly an MRI.
Do not start, stop or change thyroid medication, or begin desiccated thyroid or T3, on the basis of a reverse T3 or ratio alone; ask what the TSH and free T4 show and what outcome the change is expected to improve.
If you are considering paying for the test yourself, know that it rarely changes management and that an endocrinologist is unlikely to act on it in isolation.

Diet, lifestyle and natural levers with evidence

What can move rT3 once the cause is known. Nothing here replaces finding the cause.

Eat enough: avoid fasting and very low-calorie diets

Moderate evidence

Calorie restriction is the best-documented controllable cause of a high rT3 and low T3. In metabolic-ward studies, fasting or diets under about 800 kcal a day raised rT3 by 30 to 100 percent and lowered T3 by 20 to 50 percent within 2 to 4 days, and restoring intake, especially carbohydrate, reversed the change within a week. Nothing 'natural' lowers reverse T3 below what adequate nutrition and health already produce.

Recover from the illness or stress behind it

Moderate evidence

Reverse T3 has a 4-hour half-life, so it falls as soon as the enzyme shift reverses. After pneumonia, surgery or a heart attack, rT3 returns to baseline over 2 to 6 weeks; the same applies to resolving uncontrolled diabetes, treating heart failure or depression and restoring sleep. Trials giving T3 to patients during this phase have not helped, which is why clinicians wait for recovery rather than treat the number.

Review medications with your prescriber

Moderate evidence

Amiodarone, systemic glucocorticoids, propranolol above about 160 mg a day, propylthiouracil and iodinated contrast reliably raise rT3, and a levothyroxine dose that suppresses TSH does the same. Adjusting these, where the prescriber judges it safe, normalizes rT3 within days to weeks. Do not stop amiodarone, steroids or beta-blockers on your own; each has serious withdrawal risks.

Steroids and beta-blockers must be tapered under supervision; amiodarone's effects on thyroid tests persist for months after stopping because it is stored in fat.

Selenium only if deficient

Preliminary evidence

The deiodinases that make and clear rT3 are selenoproteins, and in selenium-deficient populations supplementation modestly improves T4-to-T3 conversion. In selenium-replete people, including most Americans, trials of 100 to 200 mcg a day change TPO antibodies but not T3, T4, TSH or rT3, so there is no evidence it lowers reverse T3 unless you are deficient.

More than 400 mcg a day risks toxicity (hair and nail loss, nausea, nerve symptoms); long-term use was linked to more type 2 diabetes in one large trial.

When to see a clinician promptly

A very fast or irregular heartbeat, chest pain, fever with agitation or confusion in someone taking thyroid hormone or with a suppressed TSH: possible over-treatment or thyroid storm, needs emergency care.
Severe drowsiness, confusion, low body temperature or a very slow pulse with known hypothyroidism: possible myxedema coma, an emergency.
A high reverse T3 with weight loss, night sweats, persistent fever or other unexplained symptoms, because it signals an illness that has not yet been identified rather than a thyroid problem.
A low free T4 with a low or normal TSH, with or without a low rT3, which can indicate pituitary disease and needs hormone testing.
Before starting desiccated thyroid, T3 or 'thyroid support' supplements recommended on the basis of reverse T3 or a ratio, so the TSH and free T4 can be reviewed first; these products can cause atrial fibrillation and bone loss when they suppress TSH.
Persistent fatigue, cold intolerance or weight gain with normal TSH, free T4 and a high rT3, so other causes (anemia, sleep apnea, depression, vitamin D or B12 deficiency, menopause) can be looked for.

Frequently asked questions

What is a normal reverse T3 level?

About 9 to 24 ng/dL in adults on most US assays (some labs 8 to 25 ng/dL, or 90 to 250 pg/mL). Newborns run much higher and older adults a little higher than young adults. Use the reference range on your own report, since methods differ and mass-spectrometry assays read lower than some immunoassays.

What causes high reverse T3?

Anything that makes the body slow its metabolism: acute or chronic illness, surgery, fasting or a very low-calorie diet, uncontrolled diabetes, liver or kidney disease, heart failure, severe stress or depression, and drugs including amiodarone, prednisone and other steroids, high-dose propranolol and contrast dye. Taking more levothyroxine than you need also raises it, because the surplus T4 is inactivated. A high rT3 is nearly always accompanied by a low free T3 and a normal TSH.

What is the free T3 to reverse T3 ratio and what should it be?

The ratio divides free T3 (in pg/mL) by reverse T3 (in ng/dL) and multiplies by 100; for example, free T3 3.2 and rT3 16 gives 20. Functional-medicine protocols often say the ratio should be above 20, but that figure has never been validated against symptoms, outcomes or treatment response, and no thyroid society uses it. The ratio simply falls during illness or dieting and rises with recovery; it does not identify a condition that needs thyroid hormone.

Does reverse T3 block thyroid hormone from working?

Not in any clinically meaningful way. Reverse T3 binds the thyroid hormone receptor with less than 1 percent of T3's affinity, is present at far lower concentrations than T3 and is cleared within hours. The idea of 'rT3 dominance' or 'thyroid receptor blockade' comes from early test-tube findings that rT3 can inhibit deiodinase enzymes at high concentrations; it has not been demonstrated as a human disease, and giving extra T3 to lower the ratio has not been shown to help in trials.

Why do endocrinologists not test reverse T3?

Because it does not change what they would do. Guidelines from the American Thyroid Association and the American Association of Clinical Endocrinology recommend TSH and free T4 for diagnosing and monitoring hypothyroidism and do not recommend reverse T3; a high rT3 with a normal TSH means illness, diet or a drug, which the history already reveals, and treating it with T3 has not improved outcomes in randomised trials. Its only niche is helping to tell non-thyroidal illness from pituitary hypothyroidism when both T4 and T3 are low.

Can a high reverse T3 cause symptoms like fatigue, weight gain or hair loss?

The symptoms are real but they come from the illness, under-eating, stress or medication that raised the rT3, not from the rT3 itself, which is inactive. Hair shedding 2 to 3 months after a serious illness, crash diet or surgery is telogen effluvium and recovers on its own. If you have a normal TSH and free T4 and still feel unwell, the more productive searches are for anemia, sleep apnea, depression, vitamin D or B12 deficiency, perimenopause and medication effects.

Should I take T3 or desiccated thyroid to lower reverse T3?

Not on the basis of the rT3. Adding T3 does lower the ratio arithmetically, but randomised trials of T3 in people with low T3 and high rT3 from illness, surgery or dieting showed no improvement in outcomes, and extra thyroid hormone in someone with a normal TSH suppresses the TSH and raises the risk of atrial fibrillation and bone loss. The 2021 consensus of the American, European and British thyroid associations allows a monitored trial of T3 only in people with confirmed hypothyroidism who remain symptomatic on levothyroxine with a normal TSH, and it does not use rT3 to select them.

Does fasting or a low-carb diet raise reverse T3?

Yes, reliably. Within 2 to 4 days of a fast or a diet below roughly 800 kcal a day, reverse T3 rises by a third to double and T3 falls by 20 to 50 percent while TSH stays normal; carbohydrate restriction in particular triggers it, and ketogenic diets produce a milder version. It reverses within about a week of eating normally. This is a normal energy-conserving response, which is why rT3 should not be tested during or shortly after a diet.

References

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Jonklaas J, Bianco AC, Cappola AR, et al.. Evidence-based use of levothyroxine/liothyronine combinations in treating hypothyroidism: a consensus document. Thyroid (2021)View source
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Euthyroid sick syndrome. Merck Manual Professional Version (2024)

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This page is educational and is not medical advice or a diagnosis. Lab results have to be interpreted together with your history, symptoms and other results by a clinician who knows you. Do not start or stop a supplement to change a lab value before the cause is known.