Reference intervals for thyroid markers in early pregnancy determined by 7 different analytical systems

The aim of the study was to verify differences between reference intervals for thyroid-stimulating hormone (TSH) and free thyroxine (FT4), as well as the cut-off for anti-thyroid peroxidase antibodies (TPO Ab) in pregnant women. Additionally, to compare these reference intervals with those recommended by the manufacturers for a healthy population. Levels of TSH, FT4, and TPO Ab were determined in a group of pregnant women (n = 216) with no history of thyroid dysfunction. Simultaneously, reference intervals for thyroid function tests were established with 7 different analytical systems representing those immunoassays most often used globally. The reference intervals for FT4 were slightly different, according to the system used by each manufacturer. Those for Architect, Centaur, Modular E170, and RIA Immunotech were slightly higher than those for Immulite 2500 and AIA 2000; the lowest being for UniCel DxI (8.13-13.2 pmol/L). For TSH, a higher interval (0.25-3.86 mU/L) is recommended for Modular E170 and IRMA Immunotech; and a lower one (0.17-2.81 mU/L) for Immulite 2500 and AIA 2000. The established cut-off limits for TPO-Ab differ according to the system used and are similar to those recommended by their manufacturers.