title: [Reference values and variability study of thyroid hormones in pregnant women from El Bierzo].
abstract: OBJECTIVE
  El Bierzo area is characterized by low urinary iodine levels in the pregnant population. Guidelines recommend that local reference values are established for the diagnosis of thyroid dysfunction in pregnancy. Our objectives were to establish reference values for thyroid-stimulating hormone (TSH), free thyroxine (FT4) and free triiodothyronine (FT3) in women in the first trimester of pregnancy and to explore the factors influencing variability in these hormones.
  
  
  METHODS
  A retrospective study of 412 women in the first trimester of pregnancy who were measured serum levels of TSH, FT4, and FT3; 163 women with conditions with a potential influence on thyroid function were excluded. Thirty smoking pregnant women were also excluded from the study of reference values. Factors examined in the variability study included age, body mass index (BMI), and smoking. A multifactorial analysis of covariance was performed.
  
  
  RESULTS
  Reference values in first-trimester pregnant women were: TSH: 0.497-3.595 mIU/L; FT4: 0.90-1.42 ng/dL; FT3: 2.49-3.56 pg/mL. TSH levels depended on mother age and on interaction between age and smoking. FT3 levels depended on the mother's BMI and smoking, and there was also an interaction between both factors.
  
  
  CONCLUSIONS
  The reference values found may be used to assess thyroid dysfunction in pregnant women from El Bierzo. TSH and FT3 levels are influenced by age and BMI of the mother and by smoking, in addition to the interaction of these factors.
authors.affiliation: Servicio de Análisis Clínicos, Hospital del Bierzo, Ponferrada (León), España. Electronic address: mlombardo@saludcastillayleon.es.
authors.forename: Manuel
authors.initials: M
authors.lastname: Lombardo Grifol
authors.forename: María Luisa
authors.initials: ML
authors.lastname: Gutiérrez Menéndez
authors.forename: Luís
authors.initials: L
authors.lastname: García Menéndez
authors.forename: María Vega
authors.initials: MV
authors.lastname: Valdazo Revenga
mesh_terms: D000293:Adolescent; D000328:Adult; D005260:Female; D006801:Humans; D011247:Pregnancy; D011261:Pregnancy Trimester, First / Q000097:blood; D012016:Reference Values; D012189:Retrospective Studies; D013030:Spain; D013972:Thyrotropin / Q000097:blood*; D013974:Thyroxine / Q000097:blood*; D014284:Triiodothyronine / Q000097:blood*; D055815:Young Adult
chemical_list: D014284:Triiodothyronine; D013972:Thyrotropin; D013974:Thyroxine
keywords: Age; Body mass index; Edad; Embarazo; Free thyroxine; Free triiodothyronine; Fumar; Pregnancy; Smoking; Thyroid-stimulating hormone; Tirotropina; Tiroxina libre; Triyodotironina libre; Índice de masa corporal