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.

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 2·5th and 97·5th 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 μg/l during each three trimester. The reference intervals for the first, second and third trimesters were, respectively, TSH 0·03-4·51, 0·05-4·50 and 0·47-4·54 mIU/l and FT4 11·8-21·0, 10·6-17·6 and 9·2-16·7 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.