title: Perinatal significance of isolated maternal hypothyroxinemia identified in the first half of pregnancy.
abstract: OBJECTIVE
  To establish pregnancy-specific free thyroxine thresholds and to assess perinatal effects associated with isolated maternal hypothyroxinemia identified in the first half of pregnancy.
  
  
  METHODS
  Stored serum samples from 17,298 women who previously underwent thyroid-stimulating hormone (TSH) screening in the first half of pregnancy were analyzed for free thyroxine (T(4)) concentrations and thyroid peroxidase antibodies. Women with a free T(4) below 0.86 ng/dL but a normal-range TSH were identified to have isolated maternal hypothyroxinemia. Pregnancy outcomes in these women were compared to those with a normal TSH and free T(4). Thyroid peroxidase antibody status and the relationship between TSH and free T(4) were analyzed for these women and women with subclinical hypothyroidism.
  
  
  RESULTS
  Isolated maternal hypothyroxinemia was identified in 233 women (1.3%). There were not any excessive adverse pregnancy outcomes in these women. Positive thyroid peroxidase antibody assays (greater than 50 international units/mL) were similar in normal women (4%) and those with isolated hypothyroxinemia (5%) but were greater in women with subclinical hypothyroidism (31%, P<.001). There was a negative correlation between TSH and free T(4) in normal women (r(s)=-0.19, P<.001) and those with subclinical hypothyroidism (r(s)=-0.11, P=.007). The correlation in women with isolated hypothyroxinemia was not significant.
  
  
  CONCLUSIONS
  Isolated maternal hypothyroxinemia has no adverse effects on perinatal outcome. Moreover, unlike subclinical hypothyroidism, there was a low prevalence of thyroid peroxidase antibodies and no correlation between TSH and free T(4) levels in women with hypothyroxinemia, leading us to question its biological significance.
authors.affiliation: Department of Obstetrics and Gynecology, the University of Texas Southwestern Medical Center, Dallas, Texas 75390-9032, USA. brian.casey@utsouthwestern.edu
authors.forename: Brian M
authors.initials: BM
authors.lastname: Casey
authors.forename: Jodi S
authors.initials: JS
authors.lastname: Dashe
authors.forename: Catherine Y
authors.initials: CY
authors.lastname: Spong
authors.forename: Donald D
authors.initials: DD
authors.lastname: McIntire
authors.forename: Kenneth J
authors.initials: KJ
authors.lastname: Leveno
authors.forename: Gary F
authors.initials: GF
authors.lastname: Cunningham
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D005260:Female; D005865:Gestational Age; D006801:Humans; D007453:Iodide Peroxidase / Q000097:blood; D008875:Middle Aged; D049451:Nomograms; D011247:Pregnancy; D011248:Pregnancy Complications / Q000097:blood*; D011256:Pregnancy Outcome; D011261:Pregnancy Trimester, First; D011262:Pregnancy Trimester, Second; D013959:Thyroid Diseases / Q000097:blood*; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood*
chemical_list: D013972:Thyrotropin; D007453:Iodide Peroxidase; D013974:Thyroxine