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.
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.
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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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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 — never a profit on the test itself; Mito makes its money on the $9 membership, not on marking up your tests. Membership is $9/mo, and you still pay the lab’s order fee. Prices are itemized before you pay, with no hidden fees.
Where do I get tested?
Choose a partner lab (Quest, Labcorp, or BioReference) at checkout. If your cart spans multiple tests, we consolidate the whole order onto a single lab so you only make one visit.
Is this eligible for HSA/FSA?
Yes. This test is HSA/FSA eligible, and you can pay with your HSA/FSA card at checkout.
When will I get my results?
Your results post to your dashboard once your lab completes them, then a clinician reviews them and your full analysis and personalized action plan (with clear next steps) follow. Turnaround varies by test: specialty assays and at-home kits take longer, and each test shows its expected turnaround before you buy.
Do I need a doctor’s order?
No. Mito provides the lab order for you, so you can book and get tested without a separate doctor visit.
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