Reverse T3
Reverse T3 (rT3) Blood Test
This venous blood test measures Reverse T3 (rT3), the inactive form of the thyroid hormone T3.
While rT3 is not routinely checked in standard thyroid panels, it can provide valuable insight when symptoms of hypothyroidism or metabolic slowdown persist despite “normal” TSH, T4, or T3 results.
Reverse T3 increases when the body diverts active thyroid hormone into its inactive form, often in response to stress, illness, inflammation, chronic fatigue, calorie restriction, overtraining, or underlying metabolic dysfunction. Measuring rT3 can therefore help clarify why someone may continue to feel fatigued, sluggish, or unwell even when their main thyroid markers look normal.
Important Sample & Collection Information
- This test requires a venous blood draw and must be centrifuged.
- A phlebotomy appointment is not included in the price. You will need to arrange your own private phlebotomist who can take the sample and spin it before shipping.
- Results-only service:
- Your laboratory report will be sent directly to you for your own review.
- No consultation or personalised interpretation is included.
What this test measures
- Reverse Triiodothyronine (rT3) The inactive form of T3 that competes with active T3 at receptor sites.
When should you consider this test?
Consider measuring rT3 if you:
- Have persistent fatigue, low energy, brain fog, low mood, or metabolic slowdown despite normal TSH, T4, or T3.
- Suspect thyroid hormone conversion issues, especially low T3 symptoms.
- Have been under chronic stress, experienced recent illness, trauma, or overtraining.
- Have chronic inflammation or unexplained hypothyroid-type symptoms.
- Want to assess how your body may be diverting thyroid hormone into its inactive form during physiological stress.
Testing rT3 can help identify hidden patterns of thyroid dysfunction and guide further exploration into adrenal stress, inflammation, nutrient deficiencies, and metabolic health.
Preparation Instructions
Fasting for a blood test:
For the most accurate results, please fast for 8 hours before your blood draw. Morning appointments after an overnight fast are recommended.
Stay well hydrated:
Drink 2–3 litres of plain water the day before and at least 500ml 1–2 hours before your appointment. Good hydration helps improve the quality of your sample and makes the venous draw easier.
