Triiodothyronine, Free (Free T3)
The active thyroid hormone, most useful for confirming and pinpointing hyperthyroidism.
Consider this test if:
- A low TSH or symptoms of overactive thyroid: racing heart, weight loss, tremor, heat intolerance, anxiety
- Free T4 came back normal but hyperthyroidism is still suspected
- Monitoring for relapse after treatment for Graves disease or a thyroid nodule
- A binding-protein abnormality (pregnancy, estrogen therapy) is clouding other thyroid results
- 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. Biotin supplements can distort immunoassay-based thyroid results, so stop high-dose biotin for about 48 hours before the draw. If you take thyroid medication, note your dose and timing, since recent dosing affects levels.
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What this test is for
Free T3 measures the unbound, biologically active fraction of triiodothyronine, the thyroid hormone that actually drives metabolic rate at the tissue level. It is most useful alongside TSH and free T4 to diagnose and manage hyperthyroidism, particularly when TSH is low but free T4 is normal: up to 10% of people with Graves disease or an autonomous nodule show an isolated high free T3 (T3 toxicosis), often early in disease or as the first sign of relapse after treatment. Because it reports the free hormone, it also clarifies thyroid status when binding proteins are abnormal, such as in pregnancy or with estrogen therapy. Free T3 adds little for suspected hypothyroidism, since it often stays normal until well after TSH and free T4 have already shifted.
This is the free (active) T3 measurement. The Total T3 variant captures both bound and free hormone and is affected by binding-protein levels, while Reverse T3 measures the inactive isomer and is reserved for specific non-thyroidal illness questions.
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 — 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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