TSH and FT4 Reference Intervals in Pregnancy
Large between-study methodological differences were identified, 11 of 102 included studies were in accordance with current guidelines; 22 cohorts involving 63198 participants were included in the meta-analysis. Not excluding thyroid peroxidase antibody–positive participants led to a rise in the upper limits of TSH in all cohorts, especially in the first (mean +17.4%; range +1.6 to +30.3%) and second trimester (mean +9.8%; range +0.6 to +32.3%). The use of the 95th percentile led to considerable changes in upper limits, varying from –10.8% to –21.8% for TSH and –1.2% to –13.2% for FT4. All other additional exclusion criteria changed reference interval cut-offs by a maximum of 3.5%. Applying these findings to the 102 studies included in the systematic review, 48 studies could be used in a clinical setting.

Source: https://academic.oup.com/jcem/advance-article/doi/10.1210/clinem/dgac425/6647985?rss=1