Reverse Triiodothyronine (rT3)
The inactive thyroid hormone that rises when severe illness shifts the body away from active T3.
Consider this test if:
- A thyroid panel shows a low T3 but normal TSH and free T4 during or after a serious illness
- Investigating fatigue, low performance, or poor recovery in the setting of chronic illness or significant stress on the body
- Sorting out whether abnormal thyroid numbers reflect true thyroid disease or an adaptation to illness
- Following thyroid hormone shifts in a critically or chronically ill patient over time
- HSA/FSA eligible
- Typical results in 4-6 days · Reviewed by a real clinician
- Drawn at a CLIA/CAP-accredited lab near you ·
Pre-test considerations
No fasting is required. Recent or ongoing acute illness substantially raises rT3, with maximal changes 24 to 36 hours after symptom onset, so timing relative to illness matters when interpreting the result. Note any thyroid medication or supplements, since they affect the broader thyroid picture this test is read against.
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What this test is for
Reverse T3 (rT3) is a biologically inactive isomer of T3, produced when the body converts T4 down the inactivating pathway instead of toward active T3. It rises in nonthyroidal illness syndrome (also called euthyroid sick syndrome), where critical illness, infection, cancer, heart failure, or starvation drives a low T3 with a high rT3 even though the thyroid gland itself is working normally. In acutely ill and elderly populations, an elevated rT3 has been linked to reduced physical performance and worse survival, which is why it is used mainly to interpret confusing thyroid panels in the context of systemic illness rather than as a screen for thyroid disease. It is most informative read alongside TSH, free T4, and total or free T3, since rT3 in isolation rarely changes management.
- Specimen
- Serum or plasma
- Measures
- Mass concentration
What you'll learn
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Insights that connect the dots. A clinician reads your whole picture, not one result in isolation, connecting family history, past results, and lifestyle into clear next steps.
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Your personalized action plan. Nutrition, supplements, and training in one clinician-reviewed protocol, updated as your results change.
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Track your trends. Upload results from any lab and see each biomarker trend over time, so you know what is working.
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Never miss a recheck. Mito tells you what is worth retesting and when, then reminds you so nothing slips.
What to expect
- 1 Book instantly
Click, book, done. Choose a convenient lab location near you. Transparent, up-front pricing.
- 2 Quick lab visit
Testing to fit your busy schedule, usually 15 minutes or less. Walk-in and appointments available.
- 3 Typical results in 4-6 days
Your results post straight to your dashboard as soon as the lab completes them.
- 4 Expert guidance
Included with Mito membership. A clinician reviews your results and your personalized action plan follows, with clear next steps.
Everything your health needs,
in one membership
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Every test at our cost
Members pay our cost on every test, with lab fees passed straight through. The full receipt is itemized, never padded.
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Continuous tracking, all in one place
Upload past labs and watch your trends over time. Every marker and visit lives in one longitudinal record, so all your care stays together.
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Year-round clinician support
Text anytime and get clinician-reviewed answers. When you want to go deeper, 1:1 consultations are available at affordable rates.
All for $9/month
Order any test or consult without joining. For $9/month, members unlock member prices, trend tracking, and year-round clinician guidance.
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Frequently asked questions
View all FAQsHow does pricing work?
Every test shows the member price next to the standard non-member price, so you can see what membership saves you. The member price is our cost, covering the lab and what it takes to run the service. It is never a profit on the test itself. Mito makes its money on the $9 membership, not on marking up your tests. Membership is $9 per month, and any applicable lab order or draw fee is itemized before you pay.
Where do I get tested?
Choose an available partner lab at checkout. If your cart contains multiple blood tests, Mito helps consolidate the eligible tests at one lab when possible. At-home kits and scans follow the collection or appointment process shown on their product page.
Is this eligible for HSA/FSA?
Yes. Eligible tests can be paid for with an HSA or FSA card at checkout.
When will I get my results?
Your results appear in your Mito dashboard after the laboratory completes processing. A clinician then reviews them before your analysis and personalized action plan are finalized. Timing varies by test, and specialty assays and at-home kits may take longer.
Do I need a physician's order?
No. Mito provides the laboratory order when one is required, so you do not need to arrange a separate physician visit before purchasing.
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