Trimester- and method-specific reference intervals for thyroid tests in pregnant Chinese women: methodology, euthyroid definition and iodine status can influence the setting of reference intervals

P>Objective The importance of diagnosis and treatment of thyroid dysfunction during pregnancy has been widely recognized. We therefore established trimester- and method-specific reference intervals for thyroid testing in pregnant women according to the NACB recommended criteria. Several factors can affect the setting of reference intervals, in particular manufacturer's methodology, euthyroid definition and iodine status. Design Cross-sectional dataset analysis. Subjects Five hundred and five normal pregnant women at different stages of gestation were rigorously selected for setting reference intervals. All were healthy, iodine sufficient, euthyroid and negative for both serum thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TgAb). Measurements Thyrotrophin (TSH), total and free thyroxine (TT4 and FT4), total and free triiodothyronine (TT3 and FT3) and anti-TPOAb and anti-TgAb were measured using the Bayer ADVIA Centaur system. Iodine content in drinking water, salt and urine was determined by national standard methods. The 2 center dot 5th and 97 center dot 5th percentiles were calculated as the reference intervals for thyroid hormone levels during each trimester. Results All participants had long-term consumption of iodized salt and median urinary iodine of 150-200 mu g/l during each three trimester. The reference intervals for the first, second and third trimesters were, respectively, TSH 0 center dot 03-4 center dot 51, 0 center dot 05-4 center dot 50 and 0 center dot 47-4 center dot 54 mIU/l and FT4 11 center dot 8-21 center dot 0, 10 center dot 6-17 center dot 6 and 9 center dot 2-16 center dot 7 pmol/l. The manufacturer's method, euthyroid definition and iodine status may influence TSH and FT4 reference intervals. Alterations in thyroid hormone concentrations during pregnancy differed at different stage of gestation and to those of a nonpregnant state. Conclusions The trimester- and method-based reference intervals for thyroid tests during pregnancy are clinically appropriate. Some variables should be controlled when establishing reference intervals.