In the 1980s the lab I worked at used TSH and FT3 as a front line screen for a couple of years. I can't think of any cases where the FT3 was unexpectedly low. The main problem with using FT3 was that FT3 levels were often mantained within the reference range despite high FT4 and (as a second line test) low FT4. I think true low T3 syndrome must be very rare.
There do appear patients who respond better to T3 replacement than T4, I don't know how these patients can be recognised. Also in our area we have quite a few patients on "natural thyroid", Armour thyroid etc. These always have high FT3 level and suppressed TSH - and low normal FT4 - difficult to interpret the results in a consistent way
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