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Iodine deficiency: Clinical implications

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Iodine requirements during lactation

During lactation, thyroid hormone production and renal iodine clearance return to the prepregnancy state. However, a significant amount of iodine is excreted into breast milk at a concentration 20 to 50 times greater than that in plasma.15 It is recommended that lactating women continue high iodine intake to ensure sufficient iodine in breast milk to build reserves in the newborn’s thyroid gland.

The iodine requirement during lactation is 225 to 350 μg/day.16 Breast milk containing 100 to 200 μg/L of iodine appears to provide adequate iodine to meet Institute of Medicine recommendations for infants.17 The amount of iodine excreted into breast milk depends on maternal iodine intake. In the setting of iodine sufficiency, the iodine content of breast milk is 150 to 180 μg/L, but it is much lower (9–32 μg/L) in women from iodine-deficient areas, eg, the “goiter belt,” which included the Great Lakes, the Appalachians, and northwestern states. While iodized salt has virtually eliminated the goiter belt, the risk of iodine deficiency remains for people who avoid iodized salt and dairy.15

To ensure adequate iodine intake, the American Thyroid Association recommends that women receive iodine supplementation daily during pregnancy and lactation.11 However, the iodine content of prenatal multivitamins is currently not mandated in the United States. Only half of marketed prenatal vitamins in the United States contain iodine, in the form of either potassium iodide or kelp. Though most iodine-containing products claim to contain at least 150 μg of iodine per daily dose, when measured, the actual iodine content varied between 33 and 610 μg.18

CONSEQUENCES OF IODINE DEFICIENCY

Goiter

Goiter in iodine-deficient areas is considered to be an adaptation to chronic iodine deficiency. Low iodine intake leads to reduced thyroid hormone production, which in turn stimulates TSH secretion from the pituitary. TSH increases iodine uptake by the thyroid, stimulates thyroid growth, and leads to goiter development.

Initially, goiter is characterized by diffuse thyroid enlargement, but over time it may become nodular from progressive accumulation of new thyroid follicles. Goiter in children from iodine-deficient areas is diffusely enlarged, whereas in older adults it tends to be multinodular.

Iodine deficiency and chronic TSH stimulation may play a role in TSH receptor-activating mutations of thyroid follicles. These “gain-of-function” mutations are more common in the glands of patients with goiter in areas of iodine deficiency but are relatively rare in areas of iodine sufficiency.19 Toxic multinodular goiter may eventually develop, and hyperthyroidism may occur if iodine deficiency is not severe.

Goiter generally does not cause obstructive symptoms, since the thyroid usually grows outward. However, a very large goiter may descend to the thoracic inlet and compress the trachea and esophagus. The obstructive effect of a large goiter can be demonstrated by having a patient raise the arms adjacent to the face (the Pemberton maneuver). Signs suggesting obstruction are engorged neck veins, facial plethora, increased dyspnea, and stridor during the maneuver. Computed tomography of the neck and upper thorax may provide information on the degree of tracheal compression.20

Hypothyroidism

A normal or low triiodothyronine (T3), a low serum thyroxine (T4), and a variably elevated TSH are features of thyroid function tests in iodine deficiency.11,21,22 As long as daily iodine intake exceeds 50 μg/day, the absolute uptake of iodine by the thyroid gland usually remains adequate to maintain euthyroidism. Below 50 μg/day, iodine storage in the thyroid becomes depleted, leading to hypothyroidism.1

Clinical manifestations of iodine-deficiency disorder by age group

The clinical manifestations of hypothyroidism from iodine deficiency are similar to those of hypothyroidism from other causes. Because of thyroid hormone’s role in neural and somatic development, the manifestations of hypothyroidism differ among age groups (Table 1).

Cretinism

Before the development of fetal thyroid tissue in the 10th to 12th week of gestation, the fetus is dependent on maternal thyroid hormone, which crosses the placenta to support general and neural development. Iodine deficiency leading to maternal hypothyroidism (in early gestation) or inadequate fetal thyroid hormone production (in late gestation) may result in various degrees of mental retardation or lower than expected IQ.

Cretinism: Comparative features of neurologic and myxomatous subtypes

Severe iodine deficiency during gestation typically results in cretinism, characterized by severe mental retardation accompanied by other neurologic or physical defects. Cretinism is divided into two subtypes according to clinical manifestations (neurologic and myxomatous cretinism; Table 2), which may reflect the different timing of intrauterine insult to the developing fetal nervous system and whether the iodine deficiency continues into the postnatal period. Both types can be prevented by adequate maternal iodine intake before and during pregnancy.23,24

Although mild gestational iodine deficiency does not result in cretinism, it nevertheless has an adverse impact on fetal neurodevelopment and subsequent functioning. Children of mothers with mild gestational iodine deficiency were found to have reductions in spelling, grammar, and English literacy performance despite growing up in iodine-replete environments.25

Impaired cognitive development

Reduction in IQ has been noted in affected youth from regions of severe and mild iodine deficiency. A meta-analysis of studies relating iodine deficiency to cognitive development suggested that chronic moderate to severe iodine deficiency reduced expected average IQ by about 13.5 points.26

The effects of mild iodine deficiency during childhood are more difficult to quantify. The results of one study suggested that mild iodine deficiency was associated with subtle neurodevelopmental deficits and that iodine supplementation might improve cognitive function in mildly iodine-deficient children.27

In a 2009 randomized, placebo-controlled study in New Zealand, 184 children ages 10 to 13 with mild iodine deficiency (median urinary iodine concentration of 63 μg/L) received iodine supplementation (150 μg/day) or placebo for 28 weeks. Iodine supplementation increased the median urinary iodine concentration to 145 μg/L and significantly improved perceptual reasoning measures and overall cognitive score compared with placebo.28

These findings suggest that correcting mild iodine deficiency in children could improve certain components of cognition. More research is needed to understand the effects of mild iodine deficiency and iodine supplementation on cognitive function.

ASSESSING IODINE STATUS

The diagnosis of iodine deficiency is based on clinical and laboratory assessments. Clinical manifestations compatible with iodine deficiency and careful history-taking focused on the patient’s dietary iodine intake and geographic data are keys to the diagnosis.

Four main methods are used to assess iodine status at a population level: urinary iodine, serum thyroglobulin, serum TSH, and thyroid size. Urinary iodine is a sensitive marker for recent iodine intake (within days); thyroglobulin represents iodine nutrition over a period of months and thyroid size over a period of years.1

Urinary iodine

Most dietary iodine is excreted into the urine within 24 hours of ingestion, and the 24-hour urinary iodine is considered a reference standard for the measurement of individual daily iodine intake. However, the process of collection is cumbersome, and the 24-hour urinary iodine can vary from day to day in the same person, depending on the amount of iodine ingested.

A study in healthy women from an iodine-sufficient area suggested that 10 repeated 24-hour urine collections estimated the person’s iodine status at a precision of 20% because of variable daily iodine intake.29 Therefore, when necessary, several 24-hour urine iodine determinations should be performed.

A single, random, spot urinary iodine is expressed as the urinary iodine concentration and is affected by the amount of iodine and fluids the individual ingests in a day, thus resulting in high variation both within an individual person and between individuals. Expressing the urinary iodine concentration as the ratio of urine iodine to creatinine is useful in correcting for the influence of fluid intake. The ratio of urine iodine to creatinine can be used to estimate 24-hour urine iodine with the following formula: urine iodine (μg/L)/creatinine (g/L)× age- and sex-specific estimated 24-hour creatinine excretion (g/day). Another clinical use of the spot urine iodine is to screen for exposure to a large amount of iodine from a source such as radiographic contrast.30

Although individual urine iodine excretion and urine volume can vary from day to day, this variation tends to even out in a large number of samples. In study populations of at least 500, the median value of the spot urinary iodine concentration is considered a reliable measure of iodine intake in that population.30 The spot urine iodine test is convenient, making it the test of choice to study iodine status in a large cohort. The WHO recommends using the median value of the spot urine iodine to evaluate the iodine status of a population.9

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