From the Guidelines
Reverse T3 (rT3) is an inactive form of thyroid hormone that is not directly addressed in the provided evidence, but it is known to be produced when the body converts thyroxine (T4) to an alternate form instead of the active triiodothyronine (T3). The provided study 1 focuses on the care of children and adolescents with type 1 diabetes and recommends monitoring thyroid function with a TSH measurement, followed by free T4 and total T3 if indicated. However, in the context of real-life clinical medicine, it is essential to consider the role of rT3 in thyroid function. Some key points to consider about rT3 include:
- It is produced as an alternate form of T4 conversion, with about 20% of T4 being converted to rT3 during normal metabolism
- Elevated rT3 levels can occur in conditions like severe illness, chronic stress, inflammation, liver disease, and certain medications
- Mainstream medicine generally does not consider isolated rT3 testing clinically useful for diagnosing thyroid disorders, instead relying on standard thyroid function tests (TSH, free T4, free T3)
- The body naturally clears excess rT3 when underlying stressors resolve, so treatment typically focuses on addressing the root causes rather than directly targeting rT3 levels. Given the lack of direct evidence on rT3 in the provided study 1, the primary approach to thyroid function testing should remain focused on TSH, free T4, and free T3 measurements, as recommended in the study.
From the Research
Definition and Production of Reverse T3
- Reverse T3 (3',5'-triiodothyronine or rT3) is the third most abundant iodothyronine circulating in human blood 2.
- It is produced by the inner ring deiodination of the pro-hormone thyroxine (T4) 2.
Clinical Aspects of Reverse T3
- The measurement of serum rT3 is yet to find a routine clinical application 2.
- Reverse T3 binds weakly to the T3 thyroid nuclear hormone receptors and is thought to represent an inactive end-product of thyroid hormone metabolism, diverting T4 away from T3 production 2.
- Serum rT3 concentration is increased as part of the non-thyroidal illness syndrome 2.
- Administration of common medications such as amiodarone can inhibit the metabolism of rT3, leading to increased serum rT3 concentration 2.
Diagnostic Value of Reverse T3
- Analysis of rT3 can provide a useful diagnostic fingerprint for genetic conditions that affect the iodothyronine deiodinases, as well as thyroid transporters and transport proteins 2.
- However, reverse T3 is not reliable in distinguishing between the hypothyroid sick patient and the euthyroid sick patient due to drug and disease effects on thyroid hormone metabolism 3.
- Patients with hypothyroidism plus illness may have a normal reverse T3, and patients with euthyroidism may have a low reverse T3 3.