Approach Considerations
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 can reliably diagnose iodine deficiency in individual patients.
Often a dietary, medical, and supplement review can be helpful to determine iodine sufficiency in an individual patient. In the United States, intake of dairy products and eggs will supply adequate iodine intake. A vegan diet is especially low in iodine (78.5 mcg/day) compared to the adequate iodine intake in laco-ovo vegetarian (178 mcg/day). [22] Not all vitamin supplements contain iodine. A review of prenatal vitamins in the United Studies showed only 54% of prescription prenatal vitamin and 29% of nonprescription vitamins contain iodine. [23]
Laboratory Studies
Urinary iodine concentration
The kidneys excrete approximately 90% of ingested iodine. When evaluated at a population level, urinary iodine concentration (UIC) from spot samples has been shown to be a reliable biomarker of recent iodine intake for the population as a whole. Therefore, the best diagnostic test to identify IDD in a population is a median 24-hour iodine urine 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.
UIC is not a reliable measure for assessing the iodine status of an individual because of very high variation in daily dietary iodine intake. It has been estimated that 10 UIC measurements from spot samples or 24-hour collections are required to establish an individual’s iodine status with 20% accuracy. [24]
Thyroid function testing
Although urinary iodine concentration is a sensitive indicator of recent iodine intake, laboratory tests that detect abnormal thyroid function may be more useful for diagnosing chronic iodine deficiency or excessive iodine intake and for monitoring the effects of iodine supplementation. [25] Thyroid function testing include measurement of serum concentrations of the following:
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Thyroid-stimulating hormone (TSH)
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Thyroxine (T 4) as total T 4 and free (ie, unbound) T 4 (FT 4)
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Thyroglobulin (Tg)
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Triiodothyronine (T 3) as total T 3 and free T 3 (FT 3)
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.
Serum Tg concentrations are positively correlated with thyroid volume in iodine-deficient regions, and mean thyroglobulin concentrations are typically elevated in regions of both iodine deficiency and excess. Tg concentrations change more rapidly than goiter rates and thus may be a better tool for gauging responses to increased iodine intake. [24]
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). [26] International reference standards have been established for serum thyroglobulin values in school-aged children. [27] 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. [28]
Imaging Studies
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. [29]
Histologic Findings
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.
Iodine Deficiency. Histologic sections from a normal thyroid and from an endemic goiter that was removed because of compressive symptoms. The normal thyroid (A) contains thyroid cells arranged in a monolayered sheet around a storage form of thyroid hormone, colloid, while the endemic goiter (B) shows intense hyperplasia with no colloid. Image courtesy of F. DeLange.
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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Iodine Deficiency. Distribution of iodine deficiency in developing countries.
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Iodine Deficiency. Typical endemic goiters in 3 women from the Himalayas, an area of severe iodine deficiency. Image courtesy of F. DeLange.
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Iodine Deficiency. A man and 3 females (age range, 17-20 y) with myxedematous cretinism from the Republic of the Congo in Africa, a region with severe iodine deficiency. Image courtesy of F. DeLange.
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Iodine Deficiency. Histologic sections from a normal thyroid and from an endemic goiter that was removed because of compressive symptoms. The normal thyroid (A) contains thyroid cells arranged in a monolayered sheet around a storage form of thyroid hormone, colloid, while the endemic goiter (B) shows intense hyperplasia with no colloid. Image courtesy of F. DeLange.
Tables
Iodine Deficiency |
None |
Mild |
Moderate |
Severe |
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 |
< 3% |
3-20% |
20-40% |
>40% |
Cretinism |
0 |
0 |
+ |
+ |
Adapted from the World Health Organization (WHO)/United Nations Children's Fund (UNICEF)/International Council for Control of Iodine Deficiency Disorders (ICCIDD). |
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