Figure 5. Between-Method FT4 Variability Impacts Thyroid Testing in Pregnancy. The figure shows the upper and lower FT4 reference limits (2.5–97.5%) from 43 published studies of FT4 measurements made in each trimester of pregnancy by four different methods: Abbott (1), Beckman (2), Roche (3) and Siemens (4). The data shows the expected trend for higher FT4 in the first trimester, resulting from thyroidal human chorionic gonadotropin (HCG) stimulation which is maximal in early pregnancy. The data is re-drawn with permission from reference 156.