Table 2.
The 95% reference interval, double-sided, determined by the non-parametric percentile method (CLSI C28-A3) for thyrotropin and free thyroxine in the 3 trimesters of pregnancy, measured by immunoassays, and free thyroxine, measured by liquid chromatography/tandem mass spectrometry (in the first trimester).
| First Trimester | Second Trimester | Third Trimester | |
|---|---|---|---|
| (n = 270) | (n = 212) | (n = 211) | |
| TSH Architect® | |||
| Lower limit (90% CI) | 0.03 (0.02 to 0.11) a | 0.51 (0.34 to 0.62) b | 0.50 (0.38 to 0.67) |
| Upper limit (90% CI) | 3.78 (3.26 to 4.71) | 3.53 (3.26 to 4.06) | 4.32 (3.52 to 4.72) |
| FT4 Architect® | |||
| Lower limit (90% CI) | 10.42 (10.04 to 10.55) a | 8.37 (8.37 to 8.88) b 12.74 (12.3.6 to 13.13) | 8.24 (7.85 to 8.62) |
| Upper limit (90% CI) | 15.96 (15.32 to 16.6) | 12.49 (12.23 to 13.0) | |
| FT4 Cobas® | |||
| Lower limit (90% CI) | 11.46 (10.94 to 11.58) a,* | 9.65 (8.88 to 9.91) c | 8.88 (8.11 to 9.27) |
| Upper limit (90% CI) | 19.05 (18.28 to 20.47) | 14.67 (14.16 to 15.83) | 14.54 (14.16 to 15.32) |
| FT4 ID-LC/MS/MS | - | - | |
| Lower limit (90% CI) | 8.75 (8.75 to 9.27) | ||
| Upper limit (90% CI) | 18.27(17.12 to 19.44) |
TSH: thyrotropin (µUI/mL); FT4: free thyroxine (pmol/L); ID-LC/MS/MS: isotope dilution-liquid chromatography/tandem mass spectrometry. CI: confidence interval. a p < 0.0001 compared with the second trimester; b p < 0.0001 compared with the third trimester; c p < 0.0067 compared with the third trimester; * p < 0.0001 compared with FT4 Architect® and with FT4 ID-LC/MS/MS.