Assessment of thyroid function during pregnancy: first-trimester (weeks 9-13) reference intervals derived from Western Australian women.

OBJECTIVE: To establish first-trimester-specific reference intervals for thyroid function tests in pregnant Australian women.

DESIGN, SETTING AND PARTICIPANTS: Serum samples were collected from 2159 pregnant women (9-13 weeks' gestation) attending a private pathology practice for first-trimester screening during October and November 2006. Levels of serum thyrotropin (TSH), free thyroxine (fT4), free triiodothyronine (fT3), thyroid peroxidase antibodies (TPOAb), and thyroglobulin antibodies (TgAb) were measured by chemiluminescent immunoassay (Abbott ARCHITECT analyser).

MAIN OUTCOME MEASURES: Reference intervals based on 2.5th and 97.5th percentiles for TSH, fT4 and fT3, after exclusion of 338 women with positive TPOAb or TgAb tests; comparison with reference intervals for non-pregnant women (TSH, 0.40-4.0 mU/L; fT4, 9-19 pmol/L; fT3, 3.0-5.5 pmol/L).

RESULTS: Derived reference intervals for thyroid function tests during the first trimester of pregnancy were: TSH, 0.02-2.15 mU/L; fT4, 10.4-17.8 pmol/L; and fT3, 3.3-5.7 pmol/L. If the non-pregnant TSH reference range was applied to the study participants, 344 women (16.0%) whose serum TSH concentration was within the first-trimester-specific reference range would be misclassified as having subclinical hyperthyroidism, and 98 women (4.5%) with a TSH concentration above the first-trimester-specific upper reference limit would not be identified.

CONCLUSIONS: The reference interval for TSH during the first trimester of pregnancy differs substantially from that for non-pregnant women, and applying the general laboratory reference range to pregnant women results in misclassification of thyroid status for 20.5% of women. Australian pathology laboratories should adopt pregnancy-specific reference intervals for thyroid function tests.