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 gestation-specific 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 (T-4), free and total trnodothyronine (T-3) 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 inter-individual variations (Individuality index range 0.38-0.71). Maternal height was positively associated with free T-4 (FT4) (b=0.003, P=0.031) and pre-pregnancy body mass index with T-3 and free T-3 (b=0.017; < 0.001 and b=0.007; P < 0.001). Smoking was positively associated with T-4 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.