From the FDA Drug Label
Approximately 80% of the daily dose of T4 is deiodinated to yield equal amounts of T3 and reverse T3 (rT3)
- The reverse T3 (rT3) measure in a blood test is not explicitly stated as a specific value, but it is mentioned that approximately 80% of the daily dose of T4 is deiodinated to yield equal amounts of T3 and rT3 1
- The biologic potency and half-life of rT3 are not provided in the drug label.
- The pharmacokinetic parameters of thyroid hormones, including T4 and T3, are listed in Table 10, but rT3 is not included in the table 1
From the Research
The reverse T3 (rT3) measure in a blood test is an inactive form of the thyroid hormone T3, and its levels can be used to evaluate thyroid function, particularly in cases where standard thyroid tests show normal results but symptoms of thyroid dysfunction persist. The rT3 is produced when the body converts the thyroid hormone T4 into an inactive form instead of the active T3 hormone. Elevated rT3 levels may indicate conditions such as chronic stress, severe illness, malnutrition, or inflammation, where the body is conserving energy by reducing active thyroid hormone 2.
Key Points to Consider
- The test requires a simple blood draw, and normal reference ranges typically vary by laboratory but are generally between 10-24 ng/dL.
- No special preparation is needed before the test, though fasting may be recommended in some cases.
- Understanding rT3 levels can help healthcare providers determine if thyroid hormone metabolism is being affected by underlying health issues.
- The analysis of serum rT3 has been improved by mass-spectrometric methods, which are less susceptible to interference from other more abundant iodothyronines 2.
- Serum rT3 concentration is increased as part of the non-thyroidal illness syndrome, and by administration of common medications such as amiodarone which inhibit the metabolism of rT3 2.
Clinical Applications
- The rT3 test is particularly useful in diagnosing conditions like euthyroid sick syndrome or identifying reasons for thyroid hormone resistance.
- However, reverse T3 does not reliably differentiate hypothyroid sick syndrome from euthyroid sick syndrome 3.
- The relationship between serum T4 level and rT3/T3 ratio should be examined for adequate information concerning the peripheral conversion of thyroid hormones under various thyroid diseases 4.