Free T4 (Thyroxine, Free)
What This Test Measures
Measures the unbound, active form of T4 hormone; used with TSH to assess thyroid function.
Preparation
TIMING: Take levothyroxine/NDT as usual the day before, draw first thing in the morning before that day's dose. Taking T4 right before a draw can temporarily spike Free T4 levels. Functional practitioners target mid-range to upper-half Free T4, not just 'in range.' A low-normal FT4 with low FT3 is considered suboptimal even if TSH looks normal. On NDT or T4/T3 combo, expect FT4 around mid-range with FT3 near the top. 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
More clinically useful than total T4; standard follow-up after abnormal TSH
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; generally lowest DTC prices; not available in NY, NJ, RI
Uses both Quest and Labcorp networks; competitive pricing; broad test selection
Uses Quest and Labcorp; strong panel bundling; results in 1-3 days
Uses Quest Diagnostics network; good panel pricing; established DTC provider
Uses Quest and Labcorp; no hidden draw or facility fees; moderate pricing
Labcorp consumer-direct platform; 2,000+ locations; results in 1-3 days
Quest Diagnostics consumer-direct platform; results in 1-2 days; 2,100+ locations nationwide
Uses Quest and Labcorp networks; offers both individual tests and discounted panels
Primarily uses Labcorp network; popular for hormone panels; higher individual test prices
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.