Free T3 (Triiodothyronine)

Description

What is the Free T3 (Triiodothyronine) Test?

The Free T3 Test from BiomarkersLabs.com measures Free Triiodothyronine — the unbound, biologically active thyroid hormone fraction available to target tissues. This single-marker test is available as a standalone order or as a component of multi-marker thyroid panels, and is processed through CLIA-certified partner laboratories (Quest Diagnostics and LabCorp) for USA orders, with IVDR-compliant partner laboratories serving EU and UK practitioners. Results are delivered to your secure BiomarkersLabs.com practitioner portal within 1–3 business days of specimen receipt.

This test is indicated when TSH and Free T4 alone are insufficient — specifically when a patient reports persistent hypothyroid symptoms despite normalised standard markers, when T4-to-T3 conversion status needs to be assessed, or when monitoring T3-containing thyroid hormone therapy. It is one of the most commonly added markers in functional and integrative thyroid workups and is frequently ordered alongside Reverse T3 (BML-THY-006) to calculate the Free T3:RT3 ratio. For practitioners requiring a full thyroid profile in a single draw, the Complete Thyroid Panel (BML-THY-015) includes Free T3 alongside TSH, Free T4, Reverse T3, TPO and TGA antibodies.

BiomarkersLabs.com is a B2B laboratory testing platform exclusively serving licensed healthcare practitioners. All orders require a verified practitioner account. Results are never delivered to patients. CLIA-certified (USA) · IVDR-compliant (EU/UK) · HIPAA · GDPR · PIPEDA compliant.

What does this test measure?

• Free T3 (Free Triiodothyronine) — Unbound circulating triiodothyronine. Reported as a single quantitative value in pmol/L (EU/UK) or pg/mL (USA) with laboratory-specific reference ranges by age and sex. Free T3 reflects peripheral T4-to-T3 conversion adequacy and is the most direct marker of active thyroid hormone availability at the cellular level. It is clinically distinct from Total T3, which includes protein-bound fractions affected by TBG levels.

Clinical indications

• Persistent hypothyroid symptoms — fatigue, cold intolerance, weight gain, cognitive slowing — with normal TSH and Free T4
• Suboptimal clinical response to levothyroxine monotherapy despite TSH normalisation
• Monitoring patients prescribed combination T4/T3 therapy (Armour Thyroid, Nature-Throid, liothyronine) to assess active hormone adequacy and avoid T3 excess
• Assessment of T3 thyrotoxicosis where hyperthyroid symptoms are present but Free T4 remains within range
• Low T3 Syndrome investigation in chronic illness, post-critical care, or post-surgical recovery
• Evaluation of impaired conversion secondary to selenium deficiency, iron deficiency, elevated cortisol, or adrenal dysfunction
• Comprehensive thyroid optimisation workup alongside Reverse T3 for Free T3:RT3 ratio assessment
• Amiodarone or glucocorticoid therapy monitoring for drug-induced T3 suppression

Sample type and collection

Sample Type	Blood (serum)
Fasting Required	No
Collection Method	Venepuncture at an approved collection site. USA patients attend a Quest Diagnostics or LabCorp patient service centre using the requisition generated through your BiomarkersLabs.com portal. EU, UK and Canada patients are directed to partner collection networks in their region.
Specimen Timing Note	For patients on thyroid hormone medication (levothyroxine, liothyronine, or desiccated thyroid extract), collect before the morning dose to obtain a pre-dose trough level. This is the standard approach for clinically meaningful treatment monitoring and avoids post-dose T3 peaks that may misrepresent baseline status.

Turnaround time

1–3 business days from specimen receipt at the processing laboratory. Turnaround begins upon confirmed specimen receipt — not from the date of patient collection. You will receive an automated email notification when results are available in your BiomarkersLabs.com practitioner portal. Results are never sent to the patient and are not accessible outside your portal.

Availability

USA · EU · UK · Canada

Available to licensed practitioners ordering for patients in all US states. This test does not carry a New York state restriction — it can be ordered for patients in New York. For EU, UK, and Canadian practitioners, ordering and collection logistics are managed through your BiomarkersLabs.com portal on registration.

Compliance and certifications

• CLIA Certified — USA specimens processed by CLIA-certified Quest Diagnostics and LabCorp partner laboratories.
• IVDR Compliant — EU and UK specimens processed in compliance with EU In Vitro Diagnostic Regulation (EU) 2017/746.
• CE Marked — All diagnostic assays used for EU and UK specimens carry CE marking.
• HIPAA Compliant — Patient health information is handled in full compliance with HIPAA. Results delivered to the ordering practitioner’s portal only — never to the patient.
• GDPR Compliant — All EU and UK practitioner and patient data processed in accordance with the General Data Protection Regulation.
• PIPEDA Compliant — Canadian data handled in compliance with the Personal Information Protection and Electronic Documents Act.

How to order

Register free at BiomarkersLabs.com — licence verification takes 1–2 business days. USA practitioners with a valid NPI number are verified automatically. Once your account is active, search for the Free T3 Test in your portal, enter patient details, and submit the order. Your patient receives a requisition to attend their nearest approved collection site. Results are returned to your portal within 1–3 business days of specimen receipt. All pricing is visible after login — no subscription required, pay per test.

This test is available exclusively to licensed healthcare practitioners. Results are delivered to the ordering practitioner’s secure portal only — never directly to patients. Licence verification required (1–2 business days). BiomarkersLabs.com does not accept patient self-referrals.

Frequently Asked Questions — Free T3 Test

What does the Free T3 test measure and how is it reported?

The Free T3 test measures unbound Free Triiodothyronine in serum. Results are reported as a single quantitative value — in pmol/L for EU and UK reports, or pg/mL for USA reports — alongside laboratory-specific reference ranges stratified by age and sex. The result reflects the biologically active thyroid hormone fraction available to target tissues and is not affected by TBG (Thyroid Binding Globulin) levels, unlike Total T3.

Which laboratory processes the Free T3 test on BiomarkersLabs.com?

For USA practitioners, Free T3 specimens are processed by Quest Diagnostics or LabCorp — both CLIA-certified national reference laboratories. Your patient attends their nearest Quest or LabCorp patient service centre using the requisition generated through your BiomarkersLabs.com portal. For EU, UK, and Canadian practitioners, specimens are processed by IVDR-compliant regional partner laboratories. All processing details are confirmed in your portal at the point of ordering.

When should I order Free T3 alongside TSH and Free T4 rather than instead of them?

Free T3 is most clinically valuable when added to — not substituted for — TSH and Free T4. Order Free T3 as an additional marker when your patient has persistent hypothyroid symptoms despite normal TSH and Free T4, when you suspect impaired T4-to-T3 conversion, when monitoring combination T4/T3 therapy, or when assessing for T3 thyrotoxicosis with normal Free T4. For a complete single-draw thyroid profile, the Complete Thyroid Panel (BML-THY-015) includes Free T3 alongside TSH, Free T4, Reverse T3, TPO antibodies, and TGA antibodies.

Is the Free T3 test available for patients in New York state?

Yes. The Free T3 test does not carry a New York state restriction. Licensed practitioners in New York state can order this test for their patients without limitation through BiomarkersLabs.com.

Should my patient fast before a Free T3 blood draw?

No fasting is required for the Free T3 test. There are no food or drink restrictions before specimen collection. The only pre-collection consideration is timing in relation to thyroid medication: if your patient takes levothyroxine, liothyronine, or desiccated thyroid extract, the specimen should be collected before the morning dose to obtain a pre-dose trough level. Post-dose collection may produce a temporarily elevated Free T3 that does not reflect true baseline status, which is particularly important when monitoring liothyronine or desiccated thyroid therapy.

How quickly will I receive the Free T3 results in my portal?

Results are available in your BiomarkersLabs.com practitioner portal within 1–3 business days from confirmed specimen receipt at the processing laboratory. You receive an automated email notification when results are ready. Results are accessible only through your secure portal — they are never sent to the patient, never emailed in plain text, and never accessible to anyone outside your verified practitioner account.

Can I order the Free T3 test as a standalone or only as part of a panel?

The Free T3 test (BML-THY-002) is available as a standalone single-marker order. It can also be ordered as part of the TSH + Free T3 + Free T4 Panel (BML-THY-014), the Complete Thyroid Panel (BML-THY-015), or other multi-marker thyroid combinations available in the Thyroid & Endocrine category at BiomarkersLabs.com. For practitioners assessing T4-to-T3 conversion status, ordering Free T3 alongside Reverse T3 (BML-THY-006) in a single draw is the most efficient approach.