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Iodine deficiency and urine iodide - fact sheet
Facts about iodine deficiency
Iodine is essential for normal production of thyroid hormones. The earliest and most sensitive index of an iodine-deficient state is a rise in the level of TSH.
Iodine deficiency is diagnosed by examination of the thyroid gland for goitre and measurement of thyroid hormones. Patients with normal TSH and fT4 levels are not iodine deficient. The only reliable test for iodine deficiency in an individual is to show the normalisation of a raised TSH following iodine supplementation.
Iodine deficiency with resulting goitre and/or hypothyroidism is now uncommon in New Zealand, owing to the supplementation of salt with iodine.
Iodine deficiency has NO known effects other than on thyroid hormones. There is no good evidence for a "subclinical" iodine deficiency state.
The urine iodide test is not a reliable way to diagnose iodine deficiency in individuals. Using it to test for iodine deficiency is of no benefit to patients, and may give misleading results.
The use of iodised salt is recommended to prevent iodine deficiency. Iodine supplementation is recommended in pregnancy (1). Excessive iodine supplementation (e.g. with Lugol's iodine) can cause hyperthyroidism and may increase the risk of autoimmune thyroid disease.
Facts about urine iodide levels:
Urine iodide levels are highly variable from day to day in a given patient, and have a very low specificity and predictive value for iodine deficiency. Even an accurate 24-hour collection cannot be reliably used to assess iodine status in an individual (2). This is because urine iodide reflects only recent iodine intake, whereas the thyroid gland can store large amounts of iodine.
Measurement of urinary iodide in individual patients has only two indications: (a) assessment of iodine status at the time of therapeutic radioiodine administration, and (b) as part of the investigation of unusual cases hyperthyroidism to exclude iodine-induced hyperthyroidism.
Median urine iodide levels can used in epidemiology to assess the iodine status of populations. Although some laboratories quote urine iodide reference ranges which are based on population medians, this is incorrect (3,4) . These ranges are meant for population studies, and should not be applied to individuals; if such reference ranges are used in this way iodine deficiency will be greatly overdiagnosed.
Dr. James Davidson
Department of Chemical Pathology
Auckland City Hospital
1 BPAC guideline. http://www.bpac.org.nz/magazine/2008/december/pregnancy.asp
2 Rasmussen et al . Day-to-day variation in urinary iodine excretion. Eur J Clin Nutr 1999;53:401
3 Iodine status worldwide. de Benoist et al. (eds). WHO 2004.
Last updated at 13:39:57 27/05/2010