The more accurate reference methods, however, show that a small increase in free T4 levels in the first trimester of pregnancy subsequently returns to prepregnancy levels without the “grossly low” values seen with immunoassays.
The free T4 immunoassays “basically shouldn't be used in pregnancy,” she said. A TSH test is an appropriate biosensor of endogenous free T4 during pregnancy. “That's what we're trying to encourage physicians to use rather than these free T4 immunoassays,” she added.
New studies show that these immunoassays are 'just glorified T4 tests and can be very misleading.' Dr. Spencer
In pregnancy, a free T4 immunoassay to confirm an abnormal TSH level is not as reliable. DR. FAIX
Measuring Free T4 Is Challenging
Only a tiny fraction of the total T4 that's present in the blood is free and not bound to plasma proteins—usually about 1 ng/DL. Accurately measuring this tiny percentage has proved challenging over the years.
Most available immunoassays use a one-step “labeled analogue” method or a one-step “labeled antibody” method, but some use a two-step “back-titration” method of measuring free T4. “There continues to be evidence of significant discrepancies between these different approaches,” Dr. Faix said.
The IFCC working group proposes a reference method against which free T4 tests should be compared—a new approach using direct equilibrium dialysis combined with tandem mass spectrometry. A pilot project that used frozen sera to compare this method with the three commercially available immunoassays found significant differences in results between the proposed reference method and the three immunoassays.
The reference method isn't practical for clinical laboratories, and operates very differently than the immunoassays, so it probably won't help improve the current assays, Dr. Spencer said. Standardizing a reference test for free T4 may be a good academic exercise, but “I'm not sure how much of an impact it's going to have.”