Clinical Thyroidology February 2013 Entire Issue - American Thyroid ...

Clin Thyroidol 2013;25:40–42.ClinicalTHYROIDOLOGYSerum FT 4 Values in the Upper NormalRange in the First Trimester of PregnancyAre Associated with Lower Birth WeightJorge H. MestmanMedici M, Timmermans S, Visser W, de Muinck Keizer-Schrama SM, Jaddoe VW, Hofman A, HooijkaasH, de Rijke YB, Tiemeier H, Bongers-Schokking JJ, Visser TJ, Peeters RP, Steegers EA. Maternal thyroidhormone parameters during early pregnancy and birth weight: the Generation R Study. J Clin EndocrinolMetab 2013;98:59-66. Epub November 12, 2012; doi: 10.1210/jc.2012-2420.SUMMARYBackgroundMaternal hyperthyroidism during pregnancy isassociated with an increased risk of low birthweight, resulting in a predisposition to neonatalmorbidity and mortality. The objective of theauthors was to study the effects on birth weight ofearly pregnancy maternal serum thyroid parameterswithin the normal range, as well as the relationbetween umbilical-cord thyroid parameters andbirth weight.MethodsIn early pregnancy, serum TSH, FT 4 , and thyroid peroxidaseantibody levels were determined in 4464pregnant women. Cord serum TSH and FT 4 levelswere determined in 2724 newborns. Small size forgestational age at birth (SGA) was defined as a gestationalage–adjusted birth weight below the 2.5thpercentile. The associations between normal-rangematernal and cord thyroid parameters, birth weight,and SGA were studied using regression analyses.ResultsIn mothers with normal-range FT 4 and TSH levels,higher maternal FT 4 levels were associated withlower birth weight (P = 1.6x10–5), as well as with aslightly increased risk of SGA newborns (odds ratio,1.09; 95% confidence interval, 1.01 to 1.17; P = 0.03).Birth weight was positively associated with both cordTSH (P = 0.007) and cord FT 4 levels (P = 9.2x10–13).ConclusionsThe authors showed that maternal high-normal FT 4levels in early pregnancy are associated with lowerbirth weight and an increased risk for the deliveryof SGA newborns. In addition, birth weight is positivelyassociated with cord TSH and FT 4 levels. Theauthors postulated that even mild variation in thyroidfunction within the normal range can have importantconsequences for the fetus.ANALYSIS AND COMMENTARYIn the past two decades, several population studiesreported an association between euthyroid autoimmunedisease, mild thyroid dysfunction (includingisolated maternal hypothyroxinemia), and abnormalpregnancy outcome (mainly, but not limited to miscarriages,prematurity, and impaired neurodevelopmentin the offspring). Not all the studies are consistent,with the same type and frequency of complications.Several major factors may have contributed to this,among them, iodine status in a given population, lackof thyroid-test trimester-specific reference intervalsfor each population, and exclusion of complicationsintrinsic to pregnancy, such as gestational diabetescontinued on next pageCLINICAL THYROIDOLOGY • FEBRUARY 2013 40 VOLUME 25 • ISSUE 2 • © 2013Back to Contents