Quick answer: Reverse T3 is an inactive form of thyroid hormone made from T4. The body produces more reverse T3 during serious illness, chronic stress, fasting or calorie restriction, which may help conserve energy. Reverse T3 is not a standard routine screening test, and its value outside these situations is debated, so it is interpreted cautiously alongside TSH, free T4 and free T3.
Key takeaways
Reverse T3 is an inactive form of thyroid hormone that the body makes from T4 instead of the active T3.
It tends to rise during serious illness, chronic stress, fasting or heavy calorie restriction, possibly as a way to conserve energy.
Its meaning outside those situations is debated, so it is read alongside TSH, free T4 and free T3 rather than on its own.
What is Reverse T3?
Reverse T3 is made when an enzyme removes a different iodine from T4 than the one that makes active T3. It does not activate thyroid receptors.
Why Reverse T3 matters
Reverse T3 reflects how T4 is being processed, and it rises with illness, stress and calorie restriction. It is sometimes used to explore persistent symptoms.
Understanding your result
Reference ranges vary by lab, age and sex. Your provider interprets your result against the lab’s range and your overall picture, rather than chasing a single number. Functional labs are optional. They can be helpful in more complex cases.
What can raise reverse T3
Reverse T3 can rise with serious illness or recovery from surgery, chronic stress or high cortisol, and fasting or very low-calorie diets. Some medications such as beta blockers and steroids can raise it, as can high-dose T4 medication.
What affects Reverse T3
Reverse T3 is affected by calories and protein, stress and sleep, illness, and medications.
Ways to support healthy levels
Eat enough to fuel your activity, and manage stress and sleep (see cortisol). Address any underlying illness, and discuss persistent symptoms with your provider rather than adjusting medication on your own.
What does the research show about reverse T3?
Researchers studied blood, liver and muscle samples from 80 intensive care patients. Reverse T3 was higher and T3 was lower than in healthy people, and the T3-to-reverse-T3 ratio tracked with lower activity of the enzyme that activates thyroid hormone and higher activity of the enzyme that inactivates it. The samples came from critically ill patients who had died, so the findings describe severe illness. (Peeters et al., 2003)
In 8 healthy men who fasted for 30 hours, reverse T3 rose while T3 fell from an average of 2.30 to 1.84 nmol/L, and the changes had not reversed by the evening after an 800-calorie meal. With only 8 participants, the study shows the direction of change rather than how large it is for most people. (Hugues et al., 1984)
Honestly, the research on reverse T3 is thin. Most studies are small or involve serious illness, and there are no good trials showing that adjusting care based on reverse T3 improves how people feel, which is why it is usually read alongside TSH, free T4 and free T3.
Reverse T3 and healthy aging
Thyroid hormones set the pace of metabolism in nearly every cell and directly influence mitochondria, the energy factories of the cell. Thyroid function and autoimmunity both change with age, so keeping an eye on thyroid health supports energy, heart health and healthy weight over time.
Related thyroid tests: TSH, free T4, free T3, TPO antibodies, thyroglobulin antibodies.
When to see a naturopathic doctor
It may be time to see a naturopathic doctor if you have fatigue, weight change, feeling cold or hot, hair loss or constipation, or anxiety, palpitations or tremor. The same is true if you have irregular periods or trouble conceiving, a family history of thyroid disease, or abnormal thyroid results you want explained.
See a medical provider promptly for a racing or irregular heartbeat, severe confusion or drowsiness, a rapidly growing neck lump, or trouble swallowing or breathing.
Reverse T3 at a glance
| Topic | What the article explains |
|---|---|
| Why it matters | Reflects how T4 is being processed; rises with illness, stress and calorie restriction; and sometimes used to explore persistent symptoms |
| What can affect it | Calories and protein; stress and sleep; illness; and medications |
| Supportive habits discussed | Eat enough to fuel your activity; manage stress and sleep; address underlying illness; and discuss persistent symptoms with your provider rather than adjusting medication on your own |
Questions to bring to your clinician
These questions can help you get the most out of your next visit.
Which of my other results would help put my Reverse T3 result in context?
What might my reverse T3 be telling us, given my health history and symptoms?
Which other labs would be worth pairing with my reverse T3 to get a clearer picture?
When would it make sense to recheck my reverse T3, and what could I work on in the meantime?
Frequently asked questions
Is reverse T3 a standard test?
No. It is a specialty test with limited, debated use.
What raises reverse T3?
Illness, stress, fasting, calorie restriction and some medications.
Should I take T3 if my reverse T3 is high?
Not on your own. Discuss with your doctor.
Does dieting affect thyroid hormones?
Yes. Fasting and severe calorie restriction can lower T3 and raise reverse T3.
Sources
- Bianco AC, et al. Paradigms of Dynamic Control of Thyroid Hormone Signaling. Endocr Rev 2019. PMID: 31033998. https://pubmed.ncbi.nlm.nih.gov/31033998/
- Hugues JN, et al. Rapid adaptations of serum thyrotrophin, triiodothyronine and reverse triiodothyronine levels to short-term starvation and refeeding. Acta Endocrinol (Copenh) 1984. PMID: 6695550. https://pubmed.ncbi.nlm.nih.gov/6695550/
- Peeters RP, et al. Reduced activation and increased inactivation of thyroid hormone in tissues of critically ill patients. J Clin Endocrinol Metab 2003. PMID: 12843166. https://pubmed.ncbi.nlm.nih.gov/12843166/
- Spaulding SW, et al. Effect of caloric restriction and dietary composition of serum T3 and reverse T3 in man. J Clin Endocrinol Metab 1976. PMID: 1249190. https://pubmed.ncbi.nlm.nih.gov/1249190/
- Van den Berghe G. Non-thyroidal illness in the ICU: a syndrome with different faces. Thyroid 2014. PMID: 24845024. https://pubmed.ncbi.nlm.nih.gov/24845024/
- MedlinePlus: Triiodothyronine (T3) Tests
- MedlinePlus: Thyroid Tests
This article is for educational purposes only and is not medical advice. It does not replace an individual evaluation. Talk with your healthcare provider before starting supplements or changing medications, especially if you are pregnant, nursing, or managing a health condition.
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