Background

Iodine is an essential trace element that is needed in small amounts for synthesis of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3). Insufficient intake of iodine leads to hypothyroidism – an impaired production of these thyroid hormones. Iodine deficiency can cause a range of adverse conditions, referred to as iodine deficiency disorders, ranging from goiter (enlarged thyroid gland) to reduced growth and severe intellectual disability.

Indicators to assess iodine status in populations are important for determining the magnitude and distribution of iodine deficiency as a public health problem and for monitoring and evaluating the impact of implemented public health programmes to address iodine deficiency. The World Health Organization, in collaboration with partners, has been providing guidance on indicators for assessing iodine status and the control of iodine deficiency disorders since the early 1990s. A summary of the guidance developed over time, including the latest guidance from 2007, can be found in Urinary iodine concentrations for determining iodine status.

As new research has been published related to these indicators and more evidence is available for vulnerable population groups as pregnant women, lactating women and infants, there is a need to revisit the indicators and thresholds recommended for defining iodine adequacy and status. A scoping review and a consultation will be required to fully determine the indicators to be included in this update; however, it is anticipated that this work will include the following: breast milk iodine concentration, thyroglobulin, thyroid stimulating hormone, thyroid size and urinary iodine concentrations.

Based on median urinary iodine concentrations, remarkable progress has been made by countries in the last decades for achieving sufficient intakes to prevent iodine deficiency disorders. However, progress has recently stalled with 21 countries estimated to have insufficient iodine intakes (<100 µg/L) in 2020, compared to previous trends of insufficient iodine intakes in 19 countries in 2017, 25 in 2015, 32 in 2012, 47 in 2007, 54 in 2003, and 110 in 1993. These data are based primarily on school-age children, and the global situation of iodine nutrition status among other population groups, such as pregnant women who are particularly vulnerable to iodine deficiency, is uncertain.

The preferred strategy for the control of iodine deficiency disorders remains universal salt iodization. It is recommended that iodine concentrations in salt be adjusted in countries based on their own dietary salt intake data. As strategies for salt reduction are implemented, careful monitoring of both sodium and iodine intake at the country level is needed to ensure that individuals consume iodine at levels that are effective and safe. In order to monitor the progress toward the elimination of iodine deficiency disorders in all population groups, it requires the appropriate selection and measurement of indicators.

The overall objective for updating guidance on the use and interpretation of iodine status indicators is to utilize new knowledge obtained since 2007 to describe:

• the indicators used in assessing iodine intake, inadequacy and excess;
• thresholds for defining iodine status of populations for each indicator of iodine intake and thyroid function, and their use in the monitoring and evaluation of public health programmes (e.g. salt iodization programmes); and
• how to use and apply these indicators in practice, including:
  • indicators to use for which purpose (e.g. assessment of intakes, deficiency, excess, response to an intervention), and in which population groups; and
  • defining the public health significance of iodine deficiency/excess based on the available indicators.

This update of the guidance on indicators for assessing iodine status will follow the procedures outlined in the WHO Handbook for the development of normative products for ‘standards’ type products. Upon completion of this work, the results will then be combined with existing updated guidance on monitoring of salt iodization programmes to provide one manual that is an update to the 3^rd edition of Assessment of iodine deficiency disorders and monitoring their elimination.