Andersen SL, Christensen PA, Knøsgaard L, et al. - In the present study, the researchers sought to determine how the reference ranges differ by a week of pregnancy, and how the classification of the disease differs by analytical method and type of thyroid function test. For the measurement of thyrotropin (TSH), total and free thyroxine (T4), total and free 3,5,3′-triiodothyronine (T3), and T-uptake using ‘Method A’ (Cobas 8000, Roche Diagnostics), serum samples from Danish pregnant women (n = 6,282) were used. According to results, TSH reference ranges displayed a gradual decline during week 4-14 of pregnancy, a gradual increase was noted for total T4, total T3, and T-uptake, while free T4 and free T3 exhibited less variation. The classification of maternal thyroid dysfunction differed considerably by analytical method and type of thyroid function test even when pregnancy week- and method-specific reference ranges were established.