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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?

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The hypothesis is that the free hormone is the fraction which is available for uptake into the cells and interaction with nuclear receptors. The bound hormone represents actually a circulating storage that is not immediately available for such an uptake. Since some drugs and illnesses can alter concentrations of binding proteins or the binding capacity, the free and total hormone concentrations may not be concordant. Therefore, you may find an elevated T4 when there is an excess in thyroxine-binding globuline (TBG), for example estrogene-induced TBG excess, where the free T4 is normal.

References:

Best Pract Res Clin Endocrinol Metab. 2013 Oct;27(5):689-700. doi: 10.1016/j.beem.2013.05.012. Epub 2013 Jun 25.

Clin Chem. 1992;38(7):1289 (found this reference somewhere, but the full text is not available)

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