Correspondence

More on Iodine Content of Prenatal Vitamins

N Engl J Med 2009; 360:2582-2583June 11, 2009

Article

To the Editor:

I have several concerns about the letter to the Editor by Leung et al. (Feb. 26 issue).1 Scientific studies must provide methodologic details or else the veracity of the results presented cannot be evaluated. Leung et al. provide no details regarding sample preparation and the method used for detecting and quantitating iodine content. A titrimetric method for measuring iodine, which dates back to 1932,2 or a modern method involving inductively coupled plasma–mass spectroscopy3 may have been used. Both methods are subject to interferences. In addition, the study draws conclusions about the iodine content in marketed products; these conclusions are not supported by a sampling plan and thus may not be justified.4 Tools for assessing the accuracy and precision of chemical measurements do exist; for instance, Standard Reference Material 3280 can be used to assign a certified value to the iodine content of multivitamins and multielement tablets.5 Although we support the need for ensuring nutritional adequacy, the lack of rigor in sampling and reporting casts doubt on the accuracy of the results of the study by Leung and colleagues.

Joseph M. Betz, Ph.D.
National Institutes of Health, Bethesda, MD 20892
betzj@od.nih.gov

Stephen A. Wise, Ph.D.
National Institute of Standards and Technology, Gaithersburg, MD 20899

5 References

1. 1

   Leung AM, Pearce EN, Braverman LE. Iodine content of prenatal multivitamins in the United States. N Engl J Med 2009;360:939-940
   Full Text | Web of Science | Medline

2. 2

   Iodine in drugs: method 932.21. In: Official methods of analysis. 18th ed. Gaithersburg, MD: AOAC International, 2005.
   

3. 3

   Reid HJ, Bashammakh AA, Goodall PS, Landon MR, O'Connor C, Sharp BL. Determination of iodine and molybdenum in milk by quadrupole ICP-MS. Talanta 2008;75:189-197
   Web of Science | Medline

4. 4

   NIST/SEMATECH e-handbook of statistical methods. Gaithersburg, MD: National Institute of Standards and Technology, 2006. (Accessed May 21, 2009, at http://www.itl.nist.gov/div898/handbook/.)
   

5. 5

   Certificate of analysis: standard reference material 3280: multivitamin/multielement tablets. Gaithersburg, MD: National Institute of Standards and Technology, 2009. (Accessed May 21, 2009, at https://www-s.nist.gov/srmors/certificates/view_cert2gif.cfm?certificate=3280.)
   

Author/Editor Response

With regard to the comments of Betz and Wise: our description of laboratory methods in our letter was limited because of space constraints. We measured iodine in prenatal multivitamin tablets, using a modification of the Sandell–Kolthoff spectrophotometric method originally described by Pino, from our laboratory, in 1965.1 Our laboratory also measures iodine concentrations by means of mass spectrometry, and the results of the two methods are generally similar. Our laboratory has been certified annually by the Ensuring the Quality of Urinary Iodine Procedures (EQUIP) program of the Centers for Disease Control and Prevention, and we have measured iodine in urine and other substances for multiple studies over the past four decades.2-5

Our intent was to provide a broad overview of the supplements available to pregnant and lactating women, who may be susceptible to iodine deficiency. Among the observations we noted in our letter to the Editor was that 49% of types of prenatal multivitamins marketed in the United States contain no iodine; this finding is not dependent on laboratory or sampling methods and has important implications for public health.

Angela M. Leung, M.D.
Elizabeth N. Pearce, M.D.
Lewis E. Braverman, M.D.
Boston University Medical Center, Boston, MA 02118
elizabeth.pearce@bmc.org

5 References

1. 1

   Benotti J, Benotti N, Pino S, Gardyna H. Determination of total iodine in urine, stool, diets and tissue. Clin Chem 1965;11:932-936
   Web of Science | Medline

2. 2

   Hollowell JG, Staehling NW, Hannon WH, et al. Iodine nutrition in the United States: trends and public health implications: iodine excretion data from National Health and Nutrition Examination Surveys I and III (1971-1974 and 1988-1994). J Clin Endocrinol Metab 1998;83:3401-3408
   CrossRef | Web of Science | Medline

3. 3

   Pearce EN, Pino S, He X, Bazrafshan HR, Lee SL, Braverman LE. Sources of dietary iodine: bread, cows' milk, and infant formula in the Boston area. J Clin Endocrinol Metab 2004;89:3421-3424
   CrossRef | Web of Science | Medline

4. 4

   Pearce EN, Leung AM, Blount BC, et al. Breast milk iodine and perchlorate concentrations in lactating Boston-area women. J Clin Endocrinol Metab 2007;92:1673-1677
   CrossRef | Web of Science | Medline

5. 5

   Teas J, Pino S, Critchley A, Braverman LE. Variability of iodine content in common commercially available edible seaweeds. Thyroid 2004;14:836-841
   CrossRef | Web of Science | Medline