Reverse T3 (Reverse triiodothyronine: rT3) is an inactive form of the thyroid hormone T3. It is produced from T4 during periods when the body is under significant stress, chronic illness, or starvation.
rT3 can bind to thyroid hormone receptors in place of T3; however, because it is inactive, it does not stimulate them. This results in a reduction in metabolic activity and is commonly seen when the body is attempting to conserve energy during times of physiological stress.
Main thyroid hormones
The body primarily produces:
- T4 (thyroxine)
- T3 (triiodothyronine)
Under normal conditions, T4 is converted into T3 to support efficient energy use. However, in certain states, the body converts T4 into Reverse T3 (rT3) instead.
Although rT3 can bind to thyroid receptors like T3, it does not activate them and may compete with T3 for receptor binding. This can lead to reduced thyroid hormone activity at the cellular level, even when TSH and T4 levels appear normal.
Function of Reverse T3
Reverse T3 interacts with cells in a similar way to T3 by binding to thyroid hormone receptors. Normally, T3 binding influences metabolism, heart rate, and growth (especially in children). However, rT3 blocks this activation, effectively slowing these processes.
Causes of increased Reverse T3
rT3 levels typically rise during periods of:
- Stress
- Inflammation
- Systemic illness
This response is sometimes referred to as:
- Illness syndrome
- Euthyroid sick syndrome
- Low T3 syndrome
- Non-thyroidal illness syndrome
Factors that increase Reverse T3 (rT3)
These factors affect thyroid hormone balance and conversion, reducing active T3 at the cellular level:
- Chronic stress
- Inflammation
- Disrupted circadian rhythm (e.g. poor sleep, irregular sleep patterns, hormonal imbalance)
- Severe calorie restriction or fasting
- Cortisol dysregulation
- Nutrient deficiencies
- Illness or recovery phase
Why test Reverse T3?
Testing rT3 helps determine whether the body is slowing metabolism. It is useful in cases such as:
- Symptoms of hypothyroidism with normal TSH/T3
- Fatigue and low metabolic function
- Difficulty losing weight
- Chronic stress or burnout
- Assessment of thyroid hormone conversion (T4 → T3)
Reverse T3 and chronic symptoms
In some cases of hypothyroidism, TSH levels may remain within the normal range. However, a normal TSH level does not always indicate normal thyroid function. Low or normal TSH accompanied by abnormal thyroid hormone levels at the tissue level can still indicate thyroid dysfunction.
Common symptoms include:
- Fatigue
- Brain fog
- Cold intolerance
- Weight gain
- Hair thinning
- Low mood
When Reverse T3 levels increase, T3 binds less effectively to its receptors, leading to a reduction in the biological activity of T3. This may result in symptoms of hypothyroidism, even when thyroid hormone levels appear normal on routine screening tests.
In some cases, low T3 status may present as reduced serum T3 levels, low total T3, or alterations in free T3 levels. Assessing the balance between T3 and Reverse T3 provides more clinically useful information than measuring TSH alone.
How is Reverse T3 tested?
Measurement of Reverse T3 is not routinely included in standard thyroid function tests. Ordering a Reverse T3 test is typically considered when a patient is suspected of having a thyroid imbalance and continues to experience symptoms of hypothyroidism.
A comprehensive thyroid evaluation may include:
- TSH
- Free T4 (Free thyroxine)
- Free T3
- Total T3
- Reverse T3
- Thyroid antibodies
Thyroid antibodies help assess autoimmune thyroid disease, which differs from non-immune-related thyroid dysfunction. Measuring T3 and T4 levels, along with Reverse T3, may provide clearer interpretation of thyroid function.
Free T4 reflects the level of thyroxine circulating in the bloodstream, while Free T3 represents the biologically active thyroid hormone available for use by tissues. Interpretation of T3 and T4 levels should be done in the appropriate clinical context, and Reverse T3 can offer additional insight, helping healthcare providers better evaluate thyroid hormone metabolism.
Nutrients supporting thyroid hormone conversion
The conversion of T4 to T3 depends on adequate nutrition and proper enzyme function. Poor nutrition may impair this process and increase rT3.
Key nutrients include:
- Selenium
- Zinc
- Iron
- Iodine (in appropriate amounts)
- Vitamin A
- B vitamins
When the body is nutrient-deficient, T4 may not be efficiently converted into T3, and Reverse T3 levels may increase. This imbalance between T3 and rT3 can affect overall thyroid hormone levels and may lead to symptoms of hypothyroidism.
Low rT3 levels can occur in various contexts, but elevated rT3 is more commonly associated with chronic stress and illness. Reverse T3 levels should be interpreted alongside TSH, Free T4, Free T3, and Total T3 for a more accurate assessment.
Summary
Reverse T3 is an inactive thyroid hormone measured to assess:
- Stress levels
- Metabolic function
- The balance of conversion from T4 to T3
- Hypothyroid symptoms despite normal TSH
- Individuals who do not respond well to thyroid hormone therapy
Laboratory tests
- TSH
- Free T4 หรือ Free thyroxine
- Free T3
- Total T3
- Reverse T3
- Thyroid antibodies
References
- Chopra, I. J. (1974). A radioimmunoassay for measurement of 3,3′,5′-triiodothyronine (reverse T3). The Journal of Clinical Investigation, 54(3), 583–592.
- Gordon, M. L., Gregg, A. M., & Poleszak, A. B. (2026). The influence of reverse triiodothyronine on neuropsychiatric disorders: A narrative review. Military Medicine, 191(1–2), e127–e134.
- Halsall, D. J., & Oddy, S. (2021). Clinical and laboratory aspects of 3,3′,5′-triiodothyronine (reverse T3). Annals of Clinical Biochemistry, 58(1), 29–37.
- Hoang, T. D., Lee, M. L., Epstein, M., & Friedman, T. C. (2025). Reverse T3 in patients with hypothyroidism on different thyroid hormone replacement. PLOS ONE, 2(6).
