What Does a Free T4 Test Measure?
Free T4 measures the unbound, biologically active form of thyroxine — the main hormone your thyroid gland produces. Only about 0.03% of T4 circulates "free" (unbound to proteins), and this free fraction is what your cells actually use. Free T4 is more accurate than total T4 because it isn't affected by changes in binding proteins from pregnancy, estrogen therapy, or liver disease.
How the Test Works
The Free T4 test is performed via a standard blood draw at any Quest Diagnostics location. Your sample is sent to a CLIA-certified lab for analysis. Results are typically available within 1-2 business days in your JustLabs account.
Conditions It Helps Diagnose
Who Should Get Tested
Anyone with abnormal TSH, people on thyroid medication, pregnant women, and those with symptoms of thyroid dysfunction when TSH alone is insufficient.
Order your Free T4 test — $8