Free T4 Blood Test

Free T4 (Thyroxine) Blood Test

The Free T4 Test measures unbound thyroxine, produced by the thyroid and converted in tissues to the more active T3. Only a very small portion of circulating T4 is free, but it is a crucial input to thyroid signaling. Free T4 is typically interpreted with TSH (and often Free T3).

Why Measure Free T4?

• Thyroid screening & diagnosis: Differentiate hypo- vs. hyperthyroid patterns with TSH.
• Therapy monitoring: Trend values on thyroid medication under clinician guidance.

Preparation & Notes

High-dose biotin may interfere with some assays—confirm any hold with your clinician. Collect at similar times for trend consistency.

How the PlexusDx Test Works

• At-home collection; prepaid return;
• ~2-day turnaround; direct order;
• HSA/FSA eligible. 

FAQ

Can illness affect results? Yes - acute illness and certain medications can transiently alter thyroid labs; interpret with clinical context.