Dietary intake of iodine

A normal rate of thyroid hormone synthesis depends on an adequate dietary intake of iodine. Iodine is naturally present in water and soil, although some soils contain very low amounts. As a result, seafood is a more reliable source of iodine than crop plants. Approximately 1.6 billion people in more than 100 countries live in areas where natural sources of dietary iodine intake are marginal or insufficient. A minimum of 60 j.g of elemental iodine is required each day for thyroid hormone synthesis, and at least 100 j.g/day is required to eliminate thyroid follicular cell hyperplasia and thyroid enlargement (i.e., iodine deficiency goiter). 

An adequate dietary intake of iodine is essential to prevent hypothyroidism. In many areas of the world, dietary iodine intake is insufficient and must be supplemented. There is another element in which a dietary intake may be insufficient that is also associated with thyroid hormone metabolism. This element is... 

Physiologic Function Testing, An example of this application is the assay of thyroid hormone levels in (lie blood winch, in turn, can aid in the assessment of thyroid function. The radioactive iodine uptake test, which involves the administration of a dose of l31l (iodine-131) to the patient, is also a valuable procedure in assessing thyroid function. At present, the technique is best reserved for problem cases rather than used as a primary screening test. The main disadvantage of this test is the effect of the dietary intake of iodine, which reacts in various ways in different individuals. 

There are many forms of hypothyroidism, differing in their cause and age of onset (see Table 31-1). Severe adult hypothyroidism (myxedema) may occur idio-pathically or may be caused by specific factors such as autoimmune lymphocytic destruction (Hashimoto disease). In the child, thyroid function may be congenitally impaired, and cretinism will result if this condition is untreated. Hypothyroidism may result at any age if the dietary intake of iodine is extremely low. Several other forms of hypothyroidism that have a genetic or familial basis also exist.54... 

The release of thyroid hormone following endocytosis of colloid is inhibited by iodide and is known to be reduced with high dietary intake of iodine. This inhibition is due to the inverse relationship between the iodide content of Tg and the digestibility of iodo-Tg by lysosomal peptidase that is, poorly iodinated Tg is more readily digested than richly iodinated Tg. 

Numerous nutritional factors influence the production of thyroid hormone (Danforth et al., 1979 Azizi, 1978 Burman et al., 1979 Serog et al., 1982) among the most important is dietary intake of iodine (Kopp, 2004). Given the adaptive role that thyroid hormone plays in determining an appropriate life-history strategy, its dependence on dietary iodine for production remains curious. 

In a substudy of MoBa, including pregnant women grouped as users or nonusers of vitamin and mineral supplements, median urinary iodine excretion per 24 h was 190p,g and 110p,g, respectively (Brantsaeter et al., 2007). In the same study, dietary iodine intake was calculated by a food-frequency questionnaire and a food diary. The dietary intake of iodine among nonsupplement users was below the recommendations, whereas the dietary iodine intake of supplement users was above the recommendations. This demonstrates that supplements may contribute considerably to the total dietary iodine intake. 

As the major sources of iodine in the diet are animal based, it is reasonable to suggest that vegans (a strict vegetarian diet of plant foods only) (Table 45.1) are likely to have a low dietary intake of iodine. So where can vegans obtain their dietary iodine ... 

In general, it is assumed that about 90% of ingested iodine is excreted in the urine, and that an equilibrium is established between dietary iodine intake and urinary iodide (UI) excretion. The measurement of UI excretion can therefore provide an accurate approximation of the very recent dietary intake of iodine (UNICEF, 1998) and, therefore, is the index of choice for evaluating the degree of iodine deficiency and its correction (Hetzel and Dunn, 1989). Since the results obtained reflect recent dietary iodine intake, the determination of UI provides little useful information on the long-term iodine status of an individual. Due to thyroidal economy considerations, a relatively high UI measurement usually reflects recent exposure. Iodine concentrations in casual urine (spot) specimens of children or adults provide an adequate assessment... 

Figure 116.1 Iodine excretion in casual urine samples of 60-65-year-old Danish subjects, who are users or nonusers of iodine supplements, living in an area with a relatively high intake of iodine (eastern part of Denmark, Copenhagen), before and after iodization. This figure indicates that even without supplements the dietary intake of iodine seems to be sufficient after iodization. The estimated 24-h excretion is the iodineicreatinine ratio multiplied by the expeoted daily creatinine excretion for the given individual (Rasmussen et aL, 2002). Unpublished data from the Danish Investigation of Iodine Intake and Thyroid Disease.

Consequences of marginal iodine deficiencies on neurological and intellectual development and functions are not well documented in industrialized countries, particularly effects upon hearing capacity. The objective of the present study was to evaluate the possibility that a relative iodine deficiency in children, as assessed by urinary concentration of iodine used as an index of the dietary intake of iodine, could result in mild hearing impairment. As a combination of iodine deficiency and increase in thiocyanate intake may cocontribute to thyroid dysfunction, iodine thiocyanate ratio (I/SCN) in urine was also assessed. 

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