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.
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.
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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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