Free T3 (Triiodothyronine, Free)
What This Test Measures
Measures the active, unbound T3 hormone; useful in hyperthyroidism and T3 toxicosis.
Preparation
CRITICAL TIMING: Draw 12+ hours after your last T3 dose. T3 (liothyronine/Cytomel) peaks 2–4 hours after taking it, so drawing too soon gives a falsely high reading. STTM recommends: take meds as usual the day before, draw first thing next morning before your first dose. If splitting doses 3x daily, draw before the morning dose. Free T3 in the upper third of range is considered optimal by functional practitioners (e.g., ~3.5–4.2 pg/mL on a Quest range of 2.0–4.4). These notes reflect common practices in thyroid/functional medicine communities and may differ from standard lab guidelines.
Reflects common practices in thyroid, BHRT, and functional medicine communities. This may differ from standard clinical guidelines. Not medical advice — always discuss with your provider.
General guidelines only. Requirements vary by lab and individual. Confirm with your provider before adjusting any medications.
Turnaround Time
Varies by lab
Clinical Notes
Helpful in suspected hyperthyroidism with normal T4; T3 toxicosis diagnosis
ICD-10 Codes
Common diagnosis codes associated with this test. Codes are assigned by your provider.
Self-Pay Pricing
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Uses Labcorp network; known for supplement company; competitive lab test pricing
Uses Quest Diagnostics network; good panel pricing; established DTC provider
Uses Quest Diagnostics network; generally lowest DTC prices; not available in NY, NJ, RI
Uses Quest and Labcorp; no hidden draw or facility fees; moderate pricing
Labcorp consumer-direct platform; 2,000+ locations; results in 1-3 days
Uses both Quest and Labcorp networks; competitive pricing; broad test selection
Quest Diagnostics consumer-direct platform; results in 1-2 days; 2,100+ locations nationwide
Primarily uses Labcorp network; popular for hormone panels; higher individual test prices
Uses Quest and Labcorp networks; offers both individual tests and discounted panels
Uses Quest and Labcorp; strong panel bundling; results in 1-3 days
Lab Codes
Proprietary codes used by each lab for this test.
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For informational purposes only
This page is for educational and research purposes only and does not constitute medical advice, diagnosis, or treatment recommendations. ICD-10 and CPT codes are assigned by your healthcare provider. Always consult a qualified healthcare provider before ordering any lab test or making decisions about your health.