Differences between measurements of T4 and T3 in pregnant and nonpregnant women using isotope dilution tandem mass spectrometry and immunoassays: are there clinical implications?

BACKGROUND

It has been established that triiodothyronine (T3) and thyroxine (T4) measurements by tandem mass spectrometry (MS/MS) are more specific and are significantly different from immunoassay (IA) measurements (all p< or =0.05) throughout pregnancy. In this study, we examined the clinical implications of these discrepancies.

METHODS

Kappa statistics were used to determine the degree to which IA and MS/MS agreed in their identification of out-of-reference interval observations of circulating T4 and T3 from 52 normal, iodine-sufficient women during each trimester of pregnancy.

RESULTS

After taking chance agreement into account, the two methods had poor agreement on classification of T3 values at the first (kappa=0.185) and second (kappa=0.183) trimesters, with extremely poor agreement for the third trimester and 1-year postpartum. Agreement on T4 was poor only for the third trimester (kappa=0.183). The two methods agreed on out-of-range values in only 0-25% of T3 cases and 25-66.7% of T4 cases.

CONCLUSIONS

The areas of disagreement suggest that women at risk (i.e., with analyte values outside of the 5-95% range) will not be detected using IA. Based on this cohort, our preliminary estimates are that 25-100% of such women would be missed if IA were used to assay the analyte.