title: An Inverse Relationship Between Weight and Free Thyroxine During Early Gestation Among Women Treated for Hypothyroidism.
abstract: BACKGROUND
  Following treatment sufficient to normalize thyrotropin (TSH), nonpregnant hypothyroid adults display higher free thyroxine (FT(4)) concentrations than a reference population. Our aim is to determine whether FT(4) concentrations are higher during pregnancy among women treated for hypothyroidism and whether their weight is associated with FT(4) levels. Weight/FT(4) relationships have not previously been reported in treated hypothyroid adults (either pregnant or nonpregnant).
  
  
  METHODS
  Thyroid-related measurements were available from over 10,000 women at two early pregnancy time periods from the FaSTER (First and Second Trimester Evaluation of Risk for Fetal aneuploidy) trial (1999-2002). All women were receiving routine prenatal care. Present analyses were restricted to 9267 reference women and 306 treated, hypothyroid women with TSH between the 2nd and 98th reference percentiles. We compared FT(4) values between those groups at 11-14 and 15-18 weeks' gestation, using linear regression to estimate FT(4)/maternal weight relationships, after accounting for treatment and other potential covariates.
  
  
  RESULTS
  In comparison to reference women, median FT(4) values and percent of FT(4) values ≥95th reference percentile were significantly higher in treated women at both 11-14 and 15-18 weeks' gestation (p<0.001) overall and after stratification by weight into tertiles. Among both treated and reference women, median FT(4) decreased monotonically with increasing weight, regardless of anti-thyroperoxidase antibody status. Maternal age, maternal weight, and treatment status were important predictors of FT(4) levels (p<0.001, defined by partial r(2) values of 1% or higher). Anti-thyroperoxidase antibody status, TSH values (after logarithmic transformation), and all interaction terms were well below an r(2) of 1%. FT(4) levels were 1.45 pmol/L higher in treated than reference women, independent of other factors. Maternal age and weight reduced FT(4) levels by 0.0694 pmol/L/y and 0.0208 pmol/L/kg, respectively.
  
  
  CONCLUSIONS
  FT(4) concentrations are higher among treated hypothyroid pregnant women than among reference women, and higher maternal weight is associated with lower FT(4) levels, regardless of treatment status. This inverse relationship is not associated with higher TSH levels. While no immediate clinical implications are attached to the current observations, increased peripheral deiodinase activity in the presence of higher weight might explain these findings. Further investigation appears worthy of attention.
authors.affiliation: 1 Women and Infants Hospital and Alpert Medical School of Brown University , Providence, Rhode Island.|2 Savjani Institute for Health Research , Windham, Maine.
authors.forename: James E
authors.initials: JE
authors.lastname: Haddow
authors.affiliation: 1 Women and Infants Hospital and Alpert Medical School of Brown University , Providence, Rhode Island.|2 Savjani Institute for Health Research , Windham, Maine.
authors.forename: Louis M
authors.initials: LM
authors.lastname: Neveux
authors.affiliation: 1 Women and Infants Hospital and Alpert Medical School of Brown University , Providence, Rhode Island.|2 Savjani Institute for Health Research , Windham, Maine.
authors.forename: Glenn E
authors.initials: GE
authors.lastname: Palomaki
authors.affiliation: 1 Women and Infants Hospital and Alpert Medical School of Brown University , Providence, Rhode Island.
authors.forename: Geralyn
authors.initials: G
authors.lastname: Lambert-Messerlian
authors.affiliation: 3 University College of Physicians and Surgeons , New York, New York.|4 Royal College of Surgeons in Ireland , Dublin, Ireland .
authors.forename: Fergal D
authors.initials: FD
authors.lastname: Malone
authors.affiliation: 3 University College of Physicians and Surgeons , New York, New York.
authors.forename: Mary E
authors.initials: ME
authors.lastname: D'Alton
mesh_terms: D000328:Adult; D001835:Body Weight; D005260:Female; D006801:Humans; D007037:Hypothyroidism / Q000097:blood*; D016014:Linear Models; D011247:Pregnancy; D011248:Pregnancy Complications; D011261:Pregnancy Trimester, First; D011262:Pregnancy Trimester, Second; D012016:Reference Values; D013960:Thyroid Function Tests; D013961:Thyroid Gland / Q000473:pathology; D013972:Thyrotropin / Q000097:blood; D013974:Thyroxine / Q000097:blood*; D055815:Young Adult
chemical_list: D013972:Thyrotropin; D013974:Thyroxine