title: Longitudinal assessment of thyroid function in pregnancy.
abstract: BACKGROUND
  Trimester-specific reference intervals (RIs) for thyroid function tests are lacking for Beckman Dxl 800 analysers. We aimed to establish RIs for thyroid stimulating hormone (TSH), free thyroxine (fT4) and to track intraindividual changes in thyroid function throughout pregnancy.
  
  
  METHODS
  One hundred and thirty healthy women without antithyroid peroxidase antibodies were followed longitudinally. Thyroid function was determined at trimester-1 (T1): 9-13 weeks; trimester-2 (T2): 22-26 weeks; trimester-3 (T3): 35-39 weeks and postpartum (PP): 8-12 weeks. A subgroup (n = 47) was used to track intraindividual changes using PP as non-pregnant state (baseline).
  
  
  RESULTS
  For trimesters 1-3, TSH (median (2.5th, 5th, 95th and 97.5th percentile)) was 0.77 (0.03, 0.05, 2.33, 3.05), 1.17 (0.42, 0.47, 2.71, 3.36) and 1.35 (0.34, 0.42, 2.65, 2.83) mIU/L, respectively. Free T4 (mean (95%CI)) was 10.7 (5.9-15.5), 8.1 (4.9-11.3), 7.8 (4.5-11.0) pmol/L, respectively. In T2 and T3, 36% and 41% of the fT4 values, respectively, fell below the non-pregnancy lower normal limit. In the subgroup assessed for longitudinal changes, of the women with baseline TSH ≤ median, 71-75% remained at or below the corresponding median for trimesters 1-3. Of the women with baseline fT4 ≤ median, 69-81% also remained at or below the corresponding median for trimesters 1-3. High correlation was observed at different trimesters and baseline for TSH (Spearman's r: 0.593-0.846, P < 0.001) and for fT4 (r: 0.480-0.739, P < 0.001).
  
  
  CONCLUSIONS
  Use of trimester-specific RIs would prevent misclassification of thyroid function during pregnancy. In the majority of women, TSH and fT4 tracked on the same side of the median distribution, from a non-pregnant baseline, throughout pregnancy.
authors.affiliation: Department of Endocrinology, Austin Health, Melbourne, Australia.
authors.forename: Elif I
authors.initials: EI
authors.lastname: Ekinci
authors.forename: Zhong X
authors.initials: ZX
authors.lastname: Lu
authors.forename: Ken
authors.initials: K
authors.lastname: Sikaris
authors.forename: Intissar
authors.initials: I
authors.lastname: Bittar
authors.forename: Karey Y
authors.initials: KY
authors.lastname: Cheong
authors.forename: Que
authors.initials: Q
authors.lastname: Lam
authors.forename: Nick
authors.initials: N
authors.lastname: Crinis
authors.forename: Christine A
authors.initials: CA
authors.lastname: Houlihan
mesh_terms: D000328:Adult; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D011247:Pregnancy / Q000097:blood / Q000502:physiology*; D011264:Pregnancy Trimesters / Q000097:blood / Q000502:physiology; D012015:Reference Standards; D013960:Thyroid Function Tests / Q000592:standards*; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood
chemical_list: D013972:Thyrotropin; D013974:Thyroxine
keywords: Beckman Dxl 800 analyser; Pregnancy; free T3; free T4; thyroid function; thyroid stimulating hormone; trimester-specific reference intervals