Free Thyroxine (Free T4)
The unbound thyroid hormone actually available to your tissues, and the usual next step after an abnormal TSH.
Consider this test if:
- A TSH result came back high or low and you want to know whether free hormone is affected
- Symptoms of an overactive thyroid (racing heart, weight loss, anxiety, heat intolerance) or underactive thyroid (fatigue, weight gain, cold intolerance, dry skin)
- Adjusting thyroid medication and wanting to assess the dose sooner than TSH can settle
- A known pituitary condition or recently treated Graves disease, where TSH alone can mislead
- Tracking a diagnosed thyroid condition over time
- HSA/FSA eligible
- Typical results in 1-2 days · Reviewed by a real clinician
- Drawn at a CLIA/CAP-accredited lab near you ·
Pre-test considerations
No fasting required. Timing of the draw is not critical, but if you take levothyroxine, keep the timing consistent relative to your dose for comparable results. Heparin can alter free T4 readings, so the sample should not be drawn during or soon after heparin administration.
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What this test is for
Free T4 measures the fraction of thyroxine that is not bound to carrier proteins and is therefore free to enter cells and drive metabolism. It is the standard companion to TSH: high free T4 points to hyperthyroidism, low free T4 to hypothyroidism, and the pattern between the two clarifies what the thyroid is doing. Because it reads only the unbound hormone, free T4 is less distorted by changes in binding proteins than total T4, which makes it the more reliable measure in pregnancy, illness, or estrogen use. It is most useful for following up an abnormal TSH, monitoring treatment, and checking thyroid status when TSH is unreliable, such as with pituitary problems or recently treated Graves disease.
This is the directly measured free T4. The Free Thyroxine Index found in the total-T4 thyroid panels is a calculated estimate of the same thing, used when only total T4 and T3 uptake are available.
- 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 1-2 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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