Thyroxine (T4), Total
The main thyroid hormone in circulation, paired with an index that corrects for binding-protein swings.
Consider this test if:
- Fatigue, weight change, heat or cold intolerance, palpitations, or mood changes suggesting a thyroid problem
- An abnormal TSH that needs hormone-level confirmation
- Pregnancy, estrogen therapy, or oral contraceptive use that can shift binding proteins and distort total T4
- Tracking a known thyroid condition or response to thyroid medication
- Building a baseline thyroid profile alongside TSH
- HSA/FSA eligible
- Typical results in 1 day · 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 immunoassay thyroid results, so stop high-dose biotin for a few days before testing. Note any thyroid medication, estrogen, oral contraceptives, or pregnancy, since these affect binding proteins and the interpretation.
Video consult with your Care Team
A 1:1 call to discuss your health.
What members say on Trustpilot
What this test is for
Thyroxine (T4) is the principal hormone the thyroid releases, and total T4 measures all of it in the blood, both the small free fraction and the much larger portion bound to carrier proteins. Low total T4 points toward hypothyroidism, while high total T4 reflects hyperthyroidism, early subacute thyroiditis, or thyrotoxicosis, conditions tied to fatigue, weight change, heat or cold intolerance, palpitations, hair changes, and mood shifts. Because most T4 is protein-bound, total T4 alone can read high or low simply from changes in thyroxine-binding globulin (from pregnancy, estrogen, or oral contraceptives), so this test also reports the Free Thyroxine Index (T7), a calculation that corrects for those binding shifts and better reflects true hormone activity.
This pairing is the classic indirect read on thyroid hormone. For most people screening thyroid function, TSH is the first marker, and the separate direct Free T4 test measures the active fraction without the binding-protein correction; total T4 with the free index is most useful when binding-protein effects are in question or when building a fuller thyroid picture.
Biomarkers tested
Includes 2 biomarkers
The free T4 index is a calculated estimate of active, unbound thyroxine, correcting for the binding proteins that can distort total T4 readings and give a cleaner read on how much thyroid hormone is actually available to your tissues. Doctors use it as a backup or alternative to direct free T4 when protein levels are shifting, from pregnancy, oral estrogen, liver disease, or acute illness. Low values point toward hypothyroidism, with fatigue, weight gain, cold intolerance, hair thinning, and constipation, while high values point toward hyperthyroidism, with anxiety, palpitations, heat intolerance, and unintended weight loss, and either result is best interpreted alongside TSH.
- Specimen
- Serum or plasma
- Method
- Calculated
- Measures
- Mass concentration
Total T4 measures all the thyroxine circulating in your blood, both the small free fraction that acts on cells and the larger portion bound to carrier proteins. Because binding proteins shift with pregnancy, estrogen, and liver or kidney conditions, total T4 can rise or fall without your actual thyroid function changing, which is why it's usually read alongside TSH and free T4 rather than alone. It's most useful for catching those binding-protein effects and rounding out the picture when TSH looks off but the cause isn't yet clear.
- Specimen
- Serum or plasma
- Measures
- Mass concentration
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 day
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
-
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.
-
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.
-
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.
Mito Membership
$9 /mo
cancel anytime
Without membership
$0
pay as you go
At-cost pricing on labs, scans, and more
Priority turnaround on your results
On-demand clinician consults when you need guidance
Personalized action plans across supplements, exercise, nutrition, and sleep
AI health coaching to help you act on your results
Year-round medical support, with in-chat clinician escalation
All your health records in one personal vault, with trends and biological age tracking
Early access to new diagnostics and product releases
More tests to consider
Loading essentials…
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.
Recently viewed
Loading essentials…