Different labs use different ranges for Free T4 and Free T3 tests.
My local lab uses a range of 0.58-1.64 for a Free T4 test.
The lab my Endo uses (in a different city) uses a similar range, but it is not exactly the same.
It's also possible the second test and range you listed is a Total T4 test rather than a Free T4 test.
A Total T4 test is a different test than a Free T4 test.
I have seen Free T4 tests that use a range similar to the second example you gave, though.
The first test result and range you list is a Free T4, without doubt.
The reasons there are different ranges used are several; type of machine used, how the test is processed (on the equipment being used), and a little bit of "what range shall we use?" in meetings, which throws a bit of arbitrariness into the mix. Different manufacturers of the machines calibrate them in different ways, and I imagine there are factory recommendations on what range should be used with their equipment, as well.
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