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Reverse T3

An inactive by-product of thyroxine (T4). Its usefulness is limited, and an American Thyroid Association review says routine testing is not needed.

AI-verified against cited sources. Awaiting expert reviewAlso called rT3, RT3LOINC 3052-8

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What it is

Reverse T3 (rT3) is a form of T3 that does not switch on thyroid receptors. Your body makes it from T4, mostly outside the thyroid, as one way to adjust how much active hormone is around. Levels tend to rise during serious illness and with some medicines. The test is popular on wellness panels, but there is little evidence that it should guide thyroid treatment.

Why it matters

The American Thyroid Association-commissioned review (2023) says reverse T3 is rarely useful. It found no evidence that it helps guide T4 or T3 treatment, and that measuring T3 is cheaper and as effective if illness is the question. Clinicians use it in a few rare situations, such as certain inherited conditions. For most people, TSH and free T4 are the usual starting points. A result outside the range often reflects illness, a medicine or the test method rather than a thyroid problem.

If it’s low

A reverse T3 below your lab's range usually has no specific meaning on its own. In a rare inherited condition of thyroid hormone transport called MCT8 deficiency, which is usually found in early childhood, reverse T3 is low along with other typical changes. Specialist testing is used for that.

Common causes

  • Often no identifiable cause or clinical significance
  • Thyroid medicine that contains T3 (liothyronine), which lowers reverse T3 (seen in a randomized crossover trial)
  • Differences between test methods
  • Rarely, an inherited condition of thyroid hormone transport such as MCT8 deficiency

What can help

  • Ask whether the result matters for your care
  • Ask whether TSH and free T4 give a clearer picture

If it’s high

A reverse T3 above your lab's range most often reflects the body's response to illness or a medicine, not a thyroid problem. It often goes along with a lower T3, and it usually settles as you recover.

Common causes

  • Serious or long illness, surgery or a hospital stay
  • Medicines that slow the conversion of T4 to T3, such as amiodarone, glucocorticoids, propranolol and propylthiouracil
  • Radiographic contrast agents
  • Fasting or a very low-calorie diet
  • An overactive thyroid, which gives the body more T4 to convert
  • Differences between test methods

What can help

  • Ask whether the result fits with your TSH, free T4 and T3
  • Ask whether an illness or medicine could explain it, and whether retesting after you recover would help
  • Ask whether this result would change your care

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

For adults: 8–25 ng/dL. Source: Quest Diagnostics Nichols Institute, via the Mayo Clinic Laboratories extended catalog, Journal of Thyroid Research, Therapeutic Advances in Endocrinology and Metabolism, Thyroid (American Thyroid Association), Annals of Clinical Biochemistry, European Thyroid Journal.

What can affect your result

  • Illness, surgery or a hospital stay
  • Fasting and very low-calorie diets
  • Medicines that contain T3 (liothyronine), which lower reverse T3
  • Medicines such as amiodarone, glucocorticoids, propranolol and propylthiouracil
  • Time of day: slightly higher in the evening in healthy volunteers
  • Test method: mass spectrometry and immunoassay give noticeably different numbers
  • Sample handling: serum should be frozen after separation
  • Illness, surgery and hospital stays raise reverse T3 while T3 falls.
  • Thyroid medicine that contains T3 (liothyronine) lowers reverse T3; in a randomized crossover trial it was much lower on liothyronine than on levothyroxine.
  • Medicines that slow the conversion of T4 to T3 (amiodarone, glucocorticoids, propranolol, propylthiouracil) and radiographic contrast agents can raise it.
  • Immunoassays can cross-react with other thyroid hormone forms, and mass spectrometry is less affected by this. Results from different methods are not interchangeable.
  • Quest asks that serum be separated within 24 hours and sent frozen, so handling after the draw matters.

In healthy volunteers, levels were higher in evening samples than morning samples, so try to have repeat tests drawn at a similar time of day. Follows illness and recovery. There is no fixed timeline.

When to talk to a clinician

No single cutoff marks an urgent reverse T3. Talk with a clinician about your result, especially if you have symptoms such as a racing heart, tremor, heat or cold intolerance, or unexplained weight change. If you have chest pain, a very fast or irregular heartbeat, fainting or confusion, call 911 or your local emergency number.

Questions you could ask

  • Is a reverse T3 test useful for me, or are TSH and free T4 enough?
  • Which method did the lab use, and should future tests use the same one?
  • Could a recent illness, diet or medicine explain this?
  • Do my TSH, free T4 and T3 suggest anything about my thyroid?
  • Would retesting after I recover change anything?
  • Does this result matter for my care?

Questions people ask

Should I get my reverse T3 tested?

The American Thyroid Association-commissioned review says it is rarely useful and only needed in a few rare situations that a specialist would decide. For most people TSH and free T4 are the usual starting points. If you are curious, ask your clinician whether the result would change anything for you.

My reverse T3 is high. Does that mean my thyroid treatment is not working?

Not necessarily. Reverse T3 often rises with illness or certain medicines, and the review found no evidence that it should guide thyroid treatment. Your clinician can look at it together with your TSH, free T4 and symptoms.

How do I convert reverse T3 units?

Multiply ng/dL by 15.36 to get pmol/L, or by 0.01536 to get nmol/L. So 15 ng/dL is about 230 pmol/L. To get pg/mL, multiply ng/dL by 10.

Show the science and sources

Units

  • 1 ng/dL = 1 ng/dL
  • 1 ng/dL = 0.01536 nmol/L
  • 1 ng/dL = 15.36 pmol/L
  • 1 ng/dL = 10 pg/mL
  • 1 ng/dL = 10 ng/L

Open the Reverse T3 unit converter

For clinicians and researchers

Reverse T3 (3,3',5'-triiodothyronine) is made by inner-ring deiodination of T4. It is inactive because it does not bind or compete with T3 at the T3 receptor. Serum rT3 typically rises as T3 falls during nonthyroidal illness, and amiodarone and other drugs that inhibit T4 to T3 conversion also raise it. The ATA-commissioned 2023 review states that rT3 measurement is rarely useful, with two very rare exceptions, infrequent genetic thyroid syndromes and consumptive hypothyroidism from unusual tumors (deiodinase 3 overexpression); its text also mentions nonthyroidal illness that confounds the diagnosis of central hypothyroidism as a third uncommon situation. It also states that measuring T3 is as effective as rT3 when nonthyroidal illness is the question, that there is no evidence for using rT3 to guide T4 or T3 treatment, and that other current uses of rT3 and the rT3 to T3 ratio are confined to rare genetic conditions and consumptive hypothyroidism. rT3 was long measured by competitive radioimmunoassay, which has been superseded by mass-spectrometric methods that are less susceptible to interference from more abundant iodothyronines. Quest lists 8 to 25 ng/dL for LC-MS/MS; a radioimmunoassay used in one trial lists 0.14 to 0.54 nmol/L (about 9 to 35 ng/dL) in women, so intervals are method-specific and a trend should stay on one method. A study using LC-MS/MS in healthy volunteers found an evening rise in rT3 and no significant sex difference. The 2024 ETA guideline lists low rT3 (with high T3, low or low-normal T4, and a TSH in the usual range or mildly raised) among the biochemical features of MCT8 deficiency. The molecular weight is 650.97 g/mol, the same as T3 (both C15H12I3NO4), so 1 ng/dL is about 15.36 pmol/L.

LOINC 3052-8 (primary), 14929-4. Specimen: serum-or-plasma.

Sources

  1. T3 Reverse, LC/MS/MS (Quest Diagnostics test FRT3S), Quest Diagnostics Nichols Institute, via the Mayo Clinic Laboratories extended catalog
  2. Thyroid Stimulating Hormone and Thyroid Hormones (Triiodothyronine and Thyroxine): An American Thyroid Association-Commissioned Review of Current Clinical and Laboratory Status, Thyroid (American Thyroid Association) (2023)
  3. Clinical and laboratory aspects of 3,3',5'-triiodothyronine (reverse T3), Annals of Clinical Biochemistry (2021)
  4. 2024 European Thyroid Association Guidelines on diagnosis and management of genetic disorders of thyroid hormone transport, metabolism and action, European Thyroid Journal (2024)
  5. Demonstration of reciprocal diurnal variation in human serum T3 and rT3 concentration demonstrated by mass spectrometric analysis and establishment of thyroid hormone reference intervals, Therapeutic Advances in Endocrinology and Metabolism (2020)
  6. Thyroid Signaling Biomarkers in Female Symptomatic Hypothyroid Patients on Liothyronine versus Levothyroxine Monotherapy: A Randomized Crossover Trial, Journal of Thyroid Research (2022)
  7. Reverse T3 (3,3',5'-triiodo-L-thyronine), PubChem CID 644280, PubChem (NIH National Library of Medicine)

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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