There are two types of T4 (Thyroxine), some are bound to a protein and inactive and some are free and active.
Now we can measure either total T4 or free T4.
This is done to measure the function of thyroid gland. But the ratio of free and total T4 has is not influenced by the thyroid gland function. So if at all we already know T4, is there any rationale in measuring free T4. What extra useful information do we get from free T4 levels? But it is a common medical practice to order free T4 even though generally it is more pricey.
What purpose does it serve?