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 triiodothyronine (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) (FT(4)) (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 FT(4), 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.
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 triiodothyronine (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) (FT(4)) (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 FT(4), 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.
Authors: Lauren E Johns; Kelly K Ferguson; David E Cantonwine; Bhramar Mukherjee; John D Meeker; Thomas F McElrath Journal: J Clin Endocrinol Metab Date: 2018-04-01 Impact factor: 5.958