Biomarkers of nutrition for development--iodine review

Abstract

The objective of the Biomarkers of Nutrition for Development (BOND) project is to provide state-of-the-art information and service with regard to selection, use, and interpretation of biomarkers of nutrient exposure, status, function, and effect. Specifically, the BOND project seeks to develop consensus on accurate assessment methodologies that are applicable to researchers (laboratory/clinical/surveillance), clinicians, programmers, and policy makers (data consumers). The BOND project is also intended to develop targeted research agendas to support the discovery and development of biomarkers through improved understanding of nutrient biology within relevant biologic systems. In phase I of the BOND project, 6 nutrients (iodine, vitamin A, iron, zinc, folate, and vitamin B-12) were selected for their high public health importance because they typify the challenges faced by users in the selection, use, and interpretation of biomarkers. For each nutrient, an expert panel was constituted and charged with the development of a comprehensive review covering the respective nutrient's biology, existing biomarkers, and specific issues of use with particular reference to the needs of the individual user groups. In addition to the publication of these reviews, materials from each will be extracted to support the BOND interactive Web site (http://www.nichd.nih.gov/global_nutrition/programs/bond/pages/index.aspx). This review represents the first in the series of reviews and covers all relevant aspects of iodine biology and biomarkers. The article is organized to provide the reader with a full appreciation of iodine's background history as a public health issue, its biology, and an overview of available biomarkers and specific considerations for the use and interpretation of iodine biomarkers across a range of clinical and population-based uses. The review also includes a detailed research agenda to address priority gaps in our understanding of iodine biology and assessment.

FIGURE 1

Iodine pathway in the thyroid cell. Iodide (I⁻) is transported into the thyrocyte by NIS at the basal membrane and migrates to the apical membrane. I⁻ is oxidized by the enzymes TPO and H₂O₂ and attached to tyrosyl residues in thyroglobulin to produce the hormone precursors MIT and DIT. The residues then couple to form T4 and T3 within the thyroglobulin molecule in the follicular lumen. Thyroglobulin enters the cell by endocytosis and is digested. T4 and T3 are released into the circulation, and nonhormonal iodine on MIT and DIT is recycled within the thyrocyte. Adapted from reference with permission. DIT, diiodotyrosine; H₂O₂, hydrogen peroxidase; MIT, monoiodotyrosine; NIS, sodium/iodide symporter; TPO, thyroperoxidase; T3, triiodothyronine; T4, thyroxine.

FIGURE 2

Iodine is an essential component of the thyroid hormones triiodothyronine and thyroxine. Reproduced from reference with permission.

FIGURE 3

The physiologic stages of iodine status. The graph shows a simplified model of human iodine and thyroid status at different stages (left to right) of iodine intake: sufficient iodine intake, low iodine intake without thyroid dysfunction, and finally, low iodine intake with hypothyroidism. The 3 stages are separated by vertical dashed bars. The scientific evidence is limited with regard to the absolute levels of habitual daily iodine intake at which thyroid stores decrease and thyroid dysfunction occurs; this likely varies strongly among individuals. Reproduced from reference with permission. TSH, thyroid-stimulating hormone.

FIGURE 4

Percentage of households consuming iodized salt. With exception of New Zealand, this indicator is not formally measured in industrialized nations. However, examples of effective iodized salt regulation in the Western world include Austria, Czech Republic, Belgium, Canada, Denmark, The Netherlands, and Switzerland. Adapted from reference with permission.