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Thyroid Function Testing: A Brief Update

Appendix The Key Assays-FT4

Slide 19

April 2009

T4 assays come in 2 flavors. The most commonly used ones are immunoassay. They are all competitive assays, which means they have a limited dynamic range, as we mentioned before. When you have a choice or the manufacturer gives you that information, then you should select the assay with the widest dynamic range, because you want to cover both low Free T4 level, as well as substantially high Free T4 level, because at very high levels, many laboratories have an alert or alarm levels for critical values. So if your test hasn’t got enough dynamic range to cover both low levels and all the way up to the alarm levels, the critical value levels, then you end up diluting a lot of samples. For competitive assays, in general, and for free hormone assays, in particularly, dilutions are always a dicey thing in terms of accuracy. So, if you have a choice, select an assay with the widest dynamic range. Also, these assays do differ whether they are affected by TBG concentrations or not. The claim to fame, of course, is that they aren’t affected, and the way they work is really that the assay uses an analog to thyroxin as a competitor, and that analog is meant to not bind at all to TBG. But, of course, life is never perfect and they do bind to TBG and, if you know that information, then you should select the assay which has the lowest TBG binding. Again, this can be very difficult to come by. Anyway, those assays which do have the lowest TBG binding for the analog competitor do correlate most closely with the reference method, dialysis. Which brings us to the reference methodology, free thyroxin measurement by either dialysis or ultrafiltration (ie, by physical separation of the TBG-bound T4 from the free T4). This method is a little cumbersome and a little slower then the immunoassays, but in some situations, where there is highly abnormal binding protein or binding proteins that have changed very quickly, or binding proteins that are immature (this would be in very small children for example), the reference methodology is indispensible to get accurate free T4 measurements.

Appendix The Key Assays-FT4


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