Narrow intra-individual variation of maternal thyroid function in pregnancy based on a longitudinal study on 132 women

Background: Adaptive alterations in maternal physiology cause changes in thyroid hormone levels throughout pregnancy, and precise biochemical evaluation is thus highly dependent on gestationspecific reference intervals and expected intra-individual variation. Objective: The aim of the study was the assessment of the intra-individual variation as well as the longitudinal course of thyroid hormones during normal pregnancy and factors that influence the normal reference range for thyroid function. For this purpose, a longitudinal statistical model was applied. Design: In a cohort of 132 pregnant women, serial blood samples were obtained and ultrasound scans were performed throughout pregnancy. Methods: Serum levels of TSH, free and total thyroxine (T4), free and total triiodothyronine (T3) as well as autoantibodies against thyroid peroxidase and thyroglobulin were measured in 979 serum samples. Results: Intra-individual variations of thyroid hormone concentrations were smaller than interindividual variations (individuality index range: 0.38–0.71). Maternal height was positively associated with free T4 (FT4 )( bZ0.003; PZ0.031) and pre-pregnancy body mass index with T3 and free T3 (bZ0.017;!0.001 andbZ0.007; P!0.001). Smoking was positively associated with T4 and FT4, but it was modulated by gestational age. Gestation-specific reference intervals for thyroid function variables from autoantibody-negative participants are presented. Conclusions: In accordance with the data from nonpregnant adults, intra-individual variations of thyroid hormones were smaller than inter-individual variations also during pregnancy. In the evaluation of thyroid function in pregnancy, the individual longitudinal course of thyroid hormones rather than absolute values should be considered. We present a longitudinal model for the prediction of maternal thyroid function tests in pregnant women.