Iodine Intake Assessment: An Easy Way to Screen For a Preventable Cause of Thyroid Dysfunction?
Iodine deficiency is one of the top three micronutrient deficiencies targeted by the World Health Organisation for its global impact on health. Iodine is important for the normal functioning of the thyroid gland as it makes up part of the thyroid hormones. Both insufficient and excessive iodine intake can cause thyroid dysfunction: hypo- and hyperthyroidism are affected by iodine intake. Zimmermann and Boelaert write that in 2014, iodine intakes were considered to be “inadequate” in 112 countries, “deficient” in 29 countries and “excessive” in 11 countries. Even so, due to the way that the World Health Organisation defines iodine inadequacy and deficiency, a considerable proportion of the population of countries considered to have adequate iodine status may actually be deficient.
Recently, the US Preventive Services Task Force (USPSTF) issued a statement on screening for thyroid dysfunction. In the US, 0.5% of adults have clinically relevant thyroid dysfunction. Subclinical hypothyroidism is found in 3% of men and 5% of women, while 0.7% have subclinical hyperthyroidism. The USPSTF has rather cautiously stated that although screening for thyroid dysfunction in pregnant women may be beneficial, there is insufficient evidence that it is useful in adults who do not show signs of thyroid dysfunction. Even so, they name a number of risk groups that include sex, race, age, family history, and iodine intake. What the report fails to mention is the relative contribution of each risk factor in thyroid disease. This is unfortunate, for although the US is considered to be iodine-replete due to median iodine levels, iodine intakes have been falling over the past decade, and a considerable proportion of the population is at risk of thyroid dysfunction due to poor iodine intakes. Consider the conclusions from these research articles:
* Caldwell and co-workers found in 2011 that around 10% of the US population had a deficient iodine status in the years 2005-2008. Pregnant women and non-Hispanic Blacks were risk groups.
* The same group found in 2013 that median urinary iodine levels had dropped significantly between 2005-2006 and 2009-2010. Inadequate iodine intakes were found in pregnant women in their first and second trimesters, and in non-Hispanic Black women. Geographic location was also considered to be a factor.
* Zimmermann and Boelaert comment that iodine intakes in well-nourished countries have dropped over the past three decades, and the iodine levels in foods that are a source of iodine in the population are an unreliable source.
* Combet and Lean describe iodine insufficiency in Europe, where iodine fortification policy is patchy at best, to be “widespread”
The USPSTF states that current screening test for the assessment of thyroid dysfunction is actually a diagnostic test: serum thyroid stimulating hormone. This is presumably ordered in people who are symptomatic for thyroid disease, even though in many cases both overt hypo- and hyperthyroidism do not show symptoms, and when symptoms are present they are often considered to be vague and non-specific. As the commentary by Cappola and Cooper on the USPSTF’s statement note, the “screening test” is ordered only when patients visit their doctor with these non-specific symptoms and their physician suspects thyroid dysfunction: it is not actually used for screening but case finding.
Given the prevalence of iodine insufficiency and its effect on thyroid function in the US, perhaps a better screening tool could be developed? Some researchers have already looked into this issue, in pregnant women. Condo and associates developed a food frequency questionnaire that correlated well with iodine intakes assessed by 24-hour urinary iodine concentrations. A similar food frequency questionnaire was developed by Combet and Lean to estimate iodine intake. Health researchers have the data available to develop a questionnaire that considers iodine intake and other risk factors to more effectively screen individuals for thyroid disease. It is time for us to take this information further and help identify those at risk of thyroid dysfunction in an effective manner.
LeFevre ML, on behalf of the U.S. Preventive Services Task Force. Screening for Thyroid Dysfunction: U.S. Preventive Services Task Force Recommendation Statement. Ann Intern Med. 2015;162:641-650. doi:10.7326/M15-0483
Caldwell KL, Pan Y, Mortensen ME, Makhmudov A, Merrill L, Moye J. Iodine status in pregnant women in the National Children's Study and in U.S. women (15-44 years), National Health and Nutrition Examination Survey 2005-2010. Thyroid. 2013 Aug;23(8):927-37. doi: 10.1089/thy.2013.0012. Epub 2013 Jul 20. http://www.ncbi.nlm.nih.gov/pubmed/21323596
Caldwell KL, Makhmudov A, Ely E, Jones RL, Wang RY. Iodine status of the U.S. population, National Health and Nutrition Examination Survey, 2005–2006 and 2007–2008. Thyroid. 2011 Apr;21(4):419-27. doi: 10.1089/thy.2010.0077. http://www.ncbi.nlm.nih.gov/pubmed/23488982
Cappola AR, Cooper DS. Screening and Treating Subclinical Thyroid Disease: Getting Past the Impasse. Ann Intern Med. 2015;162(9):664-665. doi:10.7326/M15-0640. http://annals.org/article.aspx?articleID=2208601
Condo D, Makrides M, Skeaff S, Zhou SJ. Development and validation of an iodine-specific FFQ to estimate iodine intake in Australian pregnant women. Br J Nutr. 2015 Mar;113(6):944-52. doi: 10.1017/S0007114515000197. Epub 2015 Mar 6. http://www.ncbi.nlm.nih.gov/pubmed/25744430
Laurberg P, Jørgensen T, Perrild H, Ovesen L, Knudsen N, Pedersen IB, Rasmussen LB, Carlé A, Vejbjerg P. The Danish investigation on iodine intake and thyroid disease, DanThyr: status and perspectives. Eur J Endocrinol. 2006 Aug;155(2):219-28. http://www.ncbi.nlm.nih.gov/pubmed/16868134
Zimmermann MB, Boelaert K. Iodine deficiency and thyroid disorders. The Lancet Diabetes & Endocrinology , Volume 3 , Issue 4 , 286 - 295. http://dx.doi.org/10.1016/S2213-8587(14)70225-6