To the Editor:
We find the study from Rebagliato et al1 very interesting, and we would like to comment on some aspects we consider important. Free thyroxine levels fluctuate during pregnancy; these levels increase during the first trimester of pregnancy, and then decline during the 2nd and 3rd trimesters. This pattern is presumably due to more than the availability of iodine. In the study by Rebagliato and colleagues, the blood samples were not collected in a standardized way regarding pregnancy week. Only 1% of samples from Guipúzcoa, but 19% of samples from Valencia were obtained before 12 weeks of gestation, and 41% of samples from Guipúzcoa but only 2% from Valencia were collected during the 2nd trimester (after 14 weeks). Although the authors have adjusted regression models for gestational age, the probability of a type-I error in cross-sectional studies is very high.
The conclusions are prudent and consistent with the results. However, urinary iodine concentrations below 150 mcg/L were found in 54% of pregnant women, and only 44% of women consumed iodized salt. In these circumstances, the use of iodine supplements to achieve the recommended intake during pregnancy (250 mcg/d)2 should still be pursued. WHO recommends iodine supplements for pregnant women who live in regions where iodized salt is used in fewer than 90% of households and iodine nutrition status is inadequate.2 All studies carried out thus far regarding potassium iodide supplementation in pregnancy have shown this strategy is safe. A clinical trial conducted by our group recently found that, even though women who received 300 mcg IK/d did not have higher FT4 levels than control women, their offspring had better neurocognitive scores.3
In addition to gestational age, other factors may influence TSH and FT4 levels in pregnancy: age, parity, body mass index, smoking, chorionic gonadotropin, thyroid autoimmune antibodies status, type of iodine supplement, variability of the methods, and time elapsed since the beginning of supplementation.4,5 All these factors complicate the explanation of thyroid function differences among population samples.
Finally, we agree with Rebagliato et al1 that more controlled and prospective clinical trials are needed to help establish the most appropriate iodine supplementation doses and methods of administration for pregnant women.
Hospital de la Merced
Obstetricia y Ginecología
Service of Endocrinology and Nutrition
Hospital Universitario Carlos Haya (Fundacion Imabis)
1. Rebagliato M, Murcia M, Espada M, et al. Iodine intake and maternal thyroid function during pregnancy. Epidemiology
2. Andersson M, Benoist B, Delange F, Zupan J; WHO Secretariat. Prevention and control of iodine deficiency in pregnant and lactating women and in children less than 2-years-old: conclusions and recommendations of the Technical Consultation [Erratum in: Public Health Nutr
2008;11:327]. Public Health Nutr
3. Velasco I, Carreira M, Santiago P, et al. Effect of iodine prophylaxis during pregnancy on neurocognitive development of children during the first two years of life. J Clin Endocrinol Metab
4. Haddow JE, McClain MR, Lambert-Messerlian G, et al; First and Second Trimester Evaluation of Risk for Fetal Aneuploidy Research Consortium. Variability in thyroid-stimulating hormone suppression by human chorionic [corrected] gonadotropin during early pregnancy [Erratum in: J Clin Endocrinol Metab.
2008;93:4552]. J Clin Endocrinol Metab
5. Pearce EN, Oken E, Gillman MW, et al. Association of first-trimester thyroid function values with thyroperoxidase antibody test status, smoking, and multivitamin use. Endocr Pract