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Iodine levels and thyroid hormones in healthy pregnant women and birth weight of their offspring

¹ Centre for Research in Environmental Epidemiology-IMIM, C. Doctor Aiguader 88, 08003 Barcelona, Spain² CIBER Epidemiologia y Salud Pública (CIBERESP), Doctor Aiguader 88, 08003 Barcelona, Spain³ Hospital Parc Taulí, 08208 Sabadell, Barcelona, Spain⁴ Hospital de Terrassa Ctra. Torrebonica s/n. 08227 Terrassa, Spain⁵ Laboratorio Normativo de Salud Pública, Departamento de Sanidad Gobierno Vasco, Bilbao, Spain⁶ Miguel Hernandez University, Elche, Spain⁷ Municipal Institute of Medical Research (IMIM-Hospital del Mar)⁸ Pompeu Fabra University, Doctor Aiguader 88, 08003 Barcelona, Spain

(Correspondence should be addressed to M Alvarez-Pedrerol; Email: malvarez{at}creal.cat)

Introduction: The fetus is most vulnerable to severe iodine deficiency and hypothyroidism during pregnancy. The effects of mild iodine deficiency and subclinical hypothyroidism are poorly known. The present study assesses the association between thyroid hormones (TH)s and urinary iodine concentration (UIC) in healthy pregnant women and the birth weight of their children.

Methods: About 657 pregnant women were recruited in Sabadell and followed until delivery. The association between THs during the first trimester, UIC during the first and third trimesters, and birth weight was studied in 557, 251, and 528 mother–newborn pairs respectively, using linear and logistic regression models adjusted for potential confounders. Only 239 women had all the data available (thyroid function and UIC at the first and third trimesters). Six percent of newborns were classified as small for gestational age (SGA).

Results: The median UIC was 95 and 104 µg/l during the first and third trimesters respectively. Women with the third trimester UICs between 100 and 149 µg/l had lower risk of having an SGA newborn than women with UICs below 50 µg/l (adjusted OR (95%CI): 0.15 (0.03–0.76). There was no significant reduction in SGA among mothers with higher UICs. Lower free thyroxine and higher TSH levels during the first trimester were not associated with birth weight or SGA. Nevertheless, the analyses were repeated including only those women with all the data available, and high TSH levels became statistically significantly associated with lower birth weight and higher risk of SGA.

Conclusions: The present study suggests that iodine status during pregnancy may be related to prenatal growth in healthy women.

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