Free T4

This lab test helps to evaluate thyroid function.

Free T4 Overview

Reviewed: April 22, 2014

Free T4 is the active form of thyroxine, and is thought by many to be a more accurate reflection of thyroid hormone function. In most cases, its use has replaced that of the total T4 test.

Free T4 is measured in nanogram/liter (ng/L).  The normal range for free T4 is 0.8 - 1.8 ng/L.


Blood draw


Fasting is not required before the test

What the results mean

Increased free T4 may indicate Graves' disease and hyperthyroidism.

Lower than nornal free T4 may indicate hypothyroidism and may be seen with a goiter.