Free T4 immunoassays are flawed during pregnancy.
OBJECTIVE
The purpose of this study was to evaluate the diagnostic accuracies of 2 free thyroxine immunoassays during pregnancy.


METHODS
Serum was collected from healthy, thyroid peroxidase antibody-negative women during each trimester and nonpregnant controls. Thyrotropin, total T4 (TT4), free T4 index (FT4I), and 2 different FT4 immunoassays were studied.


RESULTS
As expected, TT4 was elevated in all 3 trimesters compared to controls (P < .001). FT4I was elevated in the 1st trimester as compared with controls (P < .05) and returned to the nonpregnant range in the 2nd and 3rd trimesters. In contrast, 1st trimester FT4 immunoassay values were either comparable or lower than controls and by the 2nd and 3rd trimesters had decreased to approximately 65% of controls.


CONCLUSIONS
Neither FT4 immunoassay accurately reflects established free T4 changes during pregnancy. TT4 and the FT4I retained an appropriate inverse relationship with TSH throughout pregnancy and appear to provide a more reliable free T4 estimate.
Lee
Richard H
RH
Department of Obstetrics and Gynecology, Women's and Children's Hospital, Los Angeles County-University of Southern California Medical Center, Los Angeles, CA, USA.
Spencer
Carole A
CA
Mestman
Jorge H
JH
Miller
Erin A
EA
Petrovic
Ivana
I
Braverman
Lewis E
LE
Goodwin
T Murphy
TM
D002624:Chemistry, Clinical / Q000592:standards*; D005260:Female; D006801:Humans; D007118:Immunoassay / Q000592:standards*; D011247:Pregnancy / Q000097:blood*; D012016:Reference Values; D015203:Reproducibility of Results; D013961:Thyroid Gland / Q000502:physiology; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000032:analysis* / Q000097:blood*
D013972:Thyrotropin; D013974:Thyroxine