Reverse T3 (rT3) Blood Test
Uncover whether reverse T3 is blocking your thyroid hormones.
Reverse T3 (rT3) is a biologically inactive form of thyroid hormone produced when the body diverts thyroxine (T4) away from conversion into active triiodothyronine (T3). This test measures the level of rT3 in a simple blood sample and is used alongside other thyroid function tests to investigate whether excess rT3 may be interfering with thyroid hormone action at the cellular level.
It is particularly relevant for people who have symptoms of an underactive thyroid (such as fatigue, weight gain, brain fog, cold intolerance, or low mood) despite normal or near-normal standard thyroid results, and for those who remain symptomatic on thyroid hormone replacement. It can also be useful in the context of prolonged physical or emotional stress, acute or chronic illness, calorie restriction, and certain medications, all of which can shift thyroid hormone metabolism towards increased rT3 production.
Biomarkers included
- Reverse T3 (rT3): The inactive thyroid hormone metabolite measured to assess whether T4 is being preferentially converted to rT3 rather than to active T3.
Why it matters
Standard thyroid tests can look reassuring while symptoms persist. Measuring rT3 gives a more complete picture of thyroid hormone metabolism and can help identify a pattern of impaired T4-to-T3 conversion, often described as a low T3 state. This insight supports more informed discussions with your doctor about whether further thyroid evaluation or adjustments to management are appropriate.
- Reverse T3 (rT3)
No special preparation required.
For blood tests, we recommend drinking plenty of water beforehand to ensure smooth sample collection. If you are taking any regular medication, continue to do so unless advised otherwise by your doctor.
Important Clinical Information:
This test is a screening tool to monitor and support your overall health. It does not replace your GP or consultant.
Always discuss your test results, especially any abnormal results, with your GP, who can interpret them in the context of your medical history and overall health.