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Thyroid Function Testing: A Brief Update



Balancing It All Up - What Test to Use When, cont.

Slide 16

April 2009

So, to close this section off, we should mention some thyroid hormone tests which are either, these days, unnecessary or probably belong in the realm of voodoo. First of all there is a free thyroxin index. This test is certainly no voodoo, but it is an indirect measure of free thyroxin and requires 2 analytical measurements, a total T4 measurement and a so-called resin T3-uptake test. What those tests do, is they assist the total thyroid hormone level, plus with the resin uptake test, an indirect measurement of thyroxin-binding globulin concentration. The 2 results are then multiplied and an index is formed. This has really no advantages over FT4 measurement, while at the same time, introducing additional testing, additional costs, and, of course, additional analytical errors as the analytical errors of the 2 methods are not applicative. Another one in this category is reverse T3, which more tends towards the voodoo sides of things. It was once in vogue for distinguishing nonthyroidal illness from thyroidal illness, but was clinically really not found to be very useful. Mostly the clinical context and measurement of TSH, free T4, free T4 by dialysis, and total T4 and total T3 will give a sufficient answer, or comprehensive answer, and reverse T3 does not add much. It does, however, have occasional niche application for suspected inherited or acquired abnormalities and deiodinase activity. These are the enzymes, which in peripheral tissues convert T4, the pro-hormone, either to the active hormone T3 or the inactive hormone reverse T3. Such applications are really rare and generally most endocrinologists, let alone general internists and general practitioners, will never see patients in their lifetime who will need a reverse T3 test.

What Test to Use When, cont.

 


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