Free T4 (Thyroxine), Dialysis
The most accurate measure of free thyroxine when binding-protein abnormalities throw off standard thyroid panels.
Consider this test if:
- TSH and standard free T4 results disagree or do not fit the clinical picture
- Total T4 is high or low but thyroid function appears normal
- Pregnancy, estrogen therapy, or another condition that alters thyroid-binding proteins
- Investigating fatigue, weight change, palpitations, or heat or cold intolerance when routine thyroid testing was ambiguous
- Confirming a borderline result before starting or adjusting thyroid treatment
- HSA/FSA eligible
- Typical results in 6-11 days · Reviewed by a real clinician
- Drawn at a CLIA/CAP-accredited lab near you ·
Pre-test considerations
No fasting required. Biotin supplements can interfere with many thyroid assays, so stop high-dose biotin for about 48 hours before the draw unless your clinician advises otherwise. Note any thyroid medication, estrogen, or recent iodinated contrast, since these affect interpretation. Timing during the day does not materially change the result.
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What this test is for
Free T4 measures the unbound, active fraction of thyroxine, the main hormone your thyroid releases and the one available to your tissues. This version uses equilibrium dialysis with mass spectrometry, the reference method that physically separates free hormone from protein-bound hormone before measuring it, so it stays accurate when binding proteins are abnormal. That matters for sorting out confusing results: euthyroid hyperthyroxinemia (high total T4 with normal thyroid function) from true hyperthyroidism, and euthyroid hypothyroxinemia from genuine hypothyroidism. Standard immunoassay free T4 can read falsely high or low in pregnancy, with abnormal binding proteins, in critical illness, or with certain medications, which is exactly where the dialysis method earns its keep. It is usually ordered to resolve a discrepancy, not as a first-line thyroid screen.
- Specimen
- Serum or plasma
- Method
- Equilibrium dialysis
- 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 6-11 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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