Reverse Triiodothyronine (rT3)
Thyroid marker
Reverse T3
Reverse Triiodothyronine (rT3)
An inactive isomer of T3 produced when the body converts T4 down the alternative deiodination pathway. Reverse T3 has no thyroid-hormone activity but competes with active T3 at the cellular level. It rises when the body deliberately downregulates metabolism, so it is used as a marker of non-thyroidal illness and metabolic stress rather than of primary thyroid disease.
PED Notes
Directly relevant to hard-dieting and contest-prep athletes. Aggressive caloric restriction, very low body fat, overtraining, illness, and high cortisol all shift T4 conversion away from active T3 and toward reverse T3, the classic euthyroid sick syndrome (low T3 syndrome) picture: normal or low-normal TSH, low Free T3, and a raised rT3. This is an adaptive slowdown, not primary hypothyroidism, and it typically reverses with refeeding and recovery rather than with thyroid medication. Exogenous T4 can raise rT3 (more substrate for the inactive pathway), whereas exogenous T3 (Cytomel) bypasses conversion and tends to lower endogenous rT3. A high rT3 with low Free T3 in a lean, heavily-dieting athlete signals metabolic adaptation and the need to restore energy availability. The Free T3 to reverse T3 ratio is sometimes used to gauge tissue-level thyroid status.
When high
When elevated (metabolic downregulation / euthyroid sick syndrome):
Identify the driver:
- Aggressive dieting, very low body fat, overtraining, acute illness, high cortisol, and severe caloric deficit are the usual causes
- Interpret alongside TSH, Free T4, and Free T3: the classic pattern is low Free T3 with high-normal or raised rT3 and a non-raised TSH
- Exogenous T4 loading and non-thyroidal illness both push conversion toward rT3
Correct the cause rather than the number:
- Restore energy availability: a structured refeed or diet break raises T3 and lowers rT3; this is the primary fix in dieting athletes
- Reduce training load and prioritise sleep to lower the cortisol drive
- Address illness and any nutrient deficiencies (selenium and zinc support deiodinase function)
- Selenium -- 100-200mcg/day supports the deiodinase enzymes that govern T4 to T3 conversion
- Thyroid hormone is not usually indicated for euthyroid sick syndrome; if a clinician does intervene, T3 (liothyronine) rather than T4 avoids adding rT3 substrate. Do not self-prescribe thyroid medication to chase a high rT3
When low
When low:
- A low reverse T3 is generally not a concern and simply reflects normal or upregulated metabolism
- No intervention is needed; interpret in the context of the full thyroid panel
History Chart
Reading History
Frequently Asked Questions
Reference Ranges
Standard Range
VitalMetrics Range