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Thyroid Function Testing: A Brief Update



Balancing It All Up - What Test to Use When

Slide 9

April 2009

So, how do we balance it up? What tests do we use in what situations and when? Well, there are several scenarios we have to consider. First of all, the initial diagnosis. For initial diagnosis, the general recommendation of all professional bodies and of all laboratories is to measure TSH. It is equally useful for hypo- and hyperthyroidism, has the highest sensitivity and specificity for initial diagnosis, and is least likely to be disturbed by nonthyroidal illness or drugs. The reason it is so exquisitely sensitive for changes in thyroid hormone function is that its response is magnified. When we look at this graph on the right, you can see that a doubling or halving of the free thyroxin concentrations will result in a 100-fold increase or decrement in the TSH concentration. So, the response of TSH to changing thyroid hormone levels is greatly exaggerated, which facilitates the rapid feedback. This is very useful for analytical measurements as it will show even minor changes in thyroid functions very graphically.

What Test to Use When

 


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