Iodine Deficiency Workup
- Author: Stephanie L Lee, MD, PhD; Chief Editor: George T Griffing, MD more...
There has been particular interest in monitoring iodine sufficiency in pregnant women and school-aged children. These populations are important, because they are easily accessible and are particularly vulnerable to the adverse effects caused by iodine deficiency.
Surveillance techniques to monitor iodine sufficiency in a population include assessment of thyroid volume, urinary iodine concentration, dried whole-blood spot thyroglobulin (Tg) levels, and dietary questionnaires; the last method is the least reliable. No test that can reliably diagnose iodine deficiency in individual patients is available.
Results from thyroid function studies are usually within the reference range in the presence of mild iodine insufficiency. However, in patients with euthyroidism and iodine deficiency, serum TSH levels may be normal to increased, T3 levels may be normal or slightly elevated, and T4 levels may be normal or decreased. Only in very extreme iodine deficiency does hypothyroidism develop, accompanied by an elevated serum TSH value and decreased T3 and T4 levels.
Population studies have shown that newborns with iodine deficiency disorder (IDD) have elevated TSH levels at birth that normalize when evaluated again several weeks later. The extent of their transient hypothyroidism correlates with the severity of the iodine deficiency.
Measurement of a dried whole-blood spot level of Tg can be a useful indicator of the thyroid function in children and may be a more sensitive early measure of iodine repletion than serum TSH or thyroxine (T4). International reference standards have recently been established for serum thyroglobulin values in school-aged children. Current limitations to the use of dried blood spot Tg measurements include assay complexity and the unknown utility of measuring antithyroglobulin antibody levels in children.
The 24-hour radioactive iodine uptake value is increased substantially in the presence of iodine deficiency disorder because of increased TSH stimulation and reduction in the nonisotopic iodine pool. Therefore, thyroid uptake values in iodine-sufficient areas, such as the United States, are significantly lower than in areas with iodine deficiency, as in many regions of Europe.
Thyroid size estimated on ultrasonograms has been shown to reflect the iodine sufficiency of a population. When goiter appears in more than 5% of a regional population, iodine deficiency should be considered.
Urinary Iodine Evaluation
The kidneys excrete approximately 90% of ingested iodine. Therefore, the best diagnostic test to identify IDD in a population is a median 24-hour urine iodine collection. If a 24-hour urine collection is not practical, a random urinary iodine-to-creatinine ratio can be used instead. In this case, a median of 50-100 mcg of iodine per liter is consistent with mild iodine deficiency, 20-49 mcg of iodine per liter is consistent with moderate deficiency, and less than 20 mcg of iodine per liter is consistent with severe deficiency.
In young patients with iodine deficiency, the usual finding is diffuse hyperplasia of the thyroid gland. Histologically, extreme hyperplasia can be seen with little or no colloid, as shown in the image below.
With aging, the diffuse goiter of iodine deficiency becomes more nodular. Histologically, the nodular goiter develops from areas of hyperplasia separated by areas of degeneration and fibrosis. In older patients, the thyroid gland tends to be extremely heterogeneous, with colloid-containing vesicles, hyperplastic areas, degenerating areas, and fibrosis.
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|Median urine iodine, mcg/L||>100||50-99||20-49||< 20|
|Goiter prevalence||< 5%||5-20%||20-30%||>30%|
|Neonatal thyroid-stimulating hormone (TSH),
>5 IU/mL whole blood
|Adapted from the World Health Organization (WHO)/United Nations Children's Fund (UNICEF)/International Council for Control of Iodine Deficiency Disorders (ICCIDD).|