On your lab requisition: Quest “T4 Free (FT4)” (test code 866)Labcorp “Thyroxine (T4) Free, Direct” (test no. 001974)BioReference “Thyroxine, Free (FT4)” (test code 0091-9)
The short answer
Measures free (unbound) T4 — the primary hormone produced by the thyroid gland, measured from a standard blood draw. At TestWell it costs $23.99 plus a $6 lab processing fee, needs no fasting, and results post in 1–3 business days. Physician-authorized with no doctor's visit or insurance needed; draw at Quest or Labcorp.
Hospital Lab
$150
cash pay
Other Labs
$40
+ lab fees
TestWell
$23.99
+ $6 lab processing fee
Free T4 (thyroxine) is the unbound, biologically active form of the primary thyroid hormone. It is converted to T3 (the more active form) in tissues throughout the body. Free T4 is the most reliable marker of actual thyroid hormone production because it is not affected by changes in binding proteins. Elevated Free T4 suggests hyperthyroidism, while low levels indicate hypothyroidism. This test is most useful when ordered alongside TSH for a complete picture of thyroid function, but can also be ordered standalone to monitor patients on thyroid medication.
Measures the unbound (active) form of thyroxine. Pairs with TSH to confirm thyroid dysfunction and distinguish primary from central causes.
Anyone with abnormal TSH, fatigue, weight changes, hair thinning, cold intolerance, or other symptoms of thyroid disease. Monitoring patients on levothyroxine or methimazole therapy.
Sample Type
Blood draw
Fasting
Not required
Results In
1–3 business days
Performed At
Quest or Labcorp
Your Free T4 (Thyroxine) results will include values for each of the 1 biomarker tested, along with reference ranges. Results are reviewed by a licensed physician before being released to your secure portal.
If any values fall outside the normal range, your report will include guidance on what that may mean and recommended next steps. You'll receive an email notification when your results are ready, typically within 1–3 business days.
Typical reference range layout
Lab reports show where your value sits relative to established reference intervals—your physician interprets this in context for you. Already have results? Analyze them with your AI assistant against our physician-reviewed ranges.
No fasting required. If on thyroid medication, take your dose after the blood draw.
What can throw off this result
Physician-reviewed “Before your test” pages — what each factor does, for how long, and what to do about it.
A Free T4 (Thyroxine) test costs $23.99 at TestWell — $29.99 all-in when ordered alone, because the $6 lab processing fee is charged once per order, with no draw fee or physician fee added. Hospital outpatient labs typically bill around $150.00 self-pay for the same test, and often more. With insurance, what you pay depends on your deductible and your plan's negotiated rate; TestWell's price is fixed before the draw and is HSA/FSA-eligible.
| Where you order | Price | One test, all-in | Notes |
|---|---|---|---|
| TestWell (drawn at Quest or Labcorp) | $23.99 | $29.99 | Includes the one $6 lab processing fee, charged once per order |
| TestWell (drawn at BioReference — New York & New Jersey) | $24.99 | $30.99 | New York and New Jersey orders route to BioReference; Quest Health and Labcorp OnDemand don't take self-ordered draws in those states |
| Hospital outpatient lab, self-pay (typical) | $150.00 | $150.00 | Reference figure; hospital charges vary widely by facility and are often higher |
Lab test codes: Quest test code 866 · Labcorp test no. 001974 · BioReference code 0091-9. Competitor prices are read from the linked public pages and re-checked regularly; the labs' sites may add taxes or change prices without notice.
Compare more: Quest Diagnostics price list · Labcorp price list · Quest vs Labcorp · Lab test price checker
Most T4 (>99%) is protein-bound to thyroid-binding globulin and inactive. Free T4 measures the small unbound fraction available to tissues — a more accurate reflection of thyroid hormone activity, especially in pregnancy, on estrogen, or with liver disease (all of which alter binding proteins).
Most labs use a range of about 0.8-1.8 ng/dL. Low Free T4 with elevated TSH confirms primary hypothyroidism. Elevated Free T4 with suppressed TSH confirms hyperthyroidism.
With it. TSH is the screening test; Free T4 confirms the diagnosis when TSH is abnormal. The TSH + Free T4 bundle is the standard pairing for full thyroid evaluation.
On TestWell, Free T4 is $23.99. Quest Direct lists the same test at $40, walk-in hospital labs typically bill $100-150 without insurance. Same CLIA-certified labs, same biomarker — TestWell just removes the markup.
The CPT code for Free T4 (direct measurement) is 84439. Total T4 is billed as 84436. Most thyroid workups bill 84443 (TSH) + 84439 (Free T4) together.
No fasting required. Try to draw at the same time of day for serial monitoring, and at least 4 hours after your levothyroxine dose if you're on thyroid replacement therapy.
Free T4 (Thyroxine) test results are typically ready in 1–3 business days after your draw at Quest or Labcorp. TestWell emails you the moment they post, and your results dashboard shows each marker against its reference range with a plain-English explanation.
Biotin supplements, levothyroxine timing, oral contraceptives and estrogen and ashwagandha and thyroid-support supplements can change a Free T4 (Thyroxine) test result without anything being wrong. Biotin supplements can falsely raise Free T4 — stop biotin ≥72 hours before the draw. Levothyroxine timing can raise Free T4 — draw before the daily dose, then take it. Oral contraceptives and estrogen can raise Free T4 — use TSH and Free T4 — free levels are not affected. Ashwagandha and thyroid-support supplements can raise Free T4 — interpret with the supplement in mind; stop before a repeat draw only with the clinician's agreement. Each factor is covered on our physician-reviewed "Before your test" pages; when one applies, the fix is usually to remove it and retest.
Order the Free T4 (Thyroxine) test online from TestWell for $23.99 (plus a $6 per-order lab processing fee) — no doctor's visit, referral or insurance needed. A licensed physician authorizes the order, then you walk into any Quest or Labcorp patient service center near you for the draw (the lab locator on our site lists the closest ones by ZIP code), and results arrive in 1–3 business days.
The Free T4 (Thyroxine) test is Quest Diagnostics test code 866 ("T4 Free (FT4)"), Labcorp test number 001974 ("Thyroxine (T4) Free, Direct"), BioReference code 0091-9 ("Thyroxine, Free (FT4)") (CPT 84439). TestWell orders it under those codes for $23.99 at Quest, Labcorp or BioReference — no doctor's visit or insurance required, results in 1–3 business days. Codes verified against the labs' catalogs in 2026.
Medically reviewed by Youmna DiStefano, MD · Last reviewed September 2026
TestWell Price
$23.99
+ $6 lab processing fee at checkout
No insurance needed · HIPAA compliant
Better value: the Essential Thyroid Panel
Save $31.98This test is part of the Essential Thyroid Panel — 3 tests · 3 biomarkers for $49.99, $31.98 less than à la carte.
$49.99$81.97
3 tests · 3 biomarkers
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Free T4 (Thyroxine)
$23.99