Comparison of methods for the determination of unbound triiodothyronine in pregnancy.

Objective: To study the changes in unbound T3 concentration in the first trimester of pregnancy.

Design: We measured serum unbound T3 concentration before and after termination of pregnancy.

Patients: Twenty-six clinically euthyroid women.

Measurements: We used one non-analogue assay (Sclavo) and three analogue assays (Amersham, Becton Dickinson and Diagnostic Products Corporation).

Results: Regression analysis did not show any significant agreement between the analogue and non-analogue assays. After termination of pregnancy, unbound T3 concentration as measured by Sclavo and Amersham assays did not change whereas unbound T3 concentration increased and decreased as measured by Becton Dickinson and Diagnostic Products assays respectively. Changes were not directly related to albumin or thyroid binding globulin.

Conclusion: We believe unbound T3 concentrations as measured by the analogue assays used in this study are due to a balance of errors and cannot be used to determine true physiological changes in pregnancy.