Mild-to-moderate iodine deficiency

RELATION OF IODINE TO THYROID FUNCTION Normal thyroid function requires an adequate intake of iodine without this, thyroid hormone cannot be made, TSH is secreted in excess, and the thyroid becomes hyperplastic and hypertrophic. The enlarged and stimulated thyroid becomes remarkably efficient at extracting the residual traces of iodide from the blood, developing an iodide gradient that may be 10 times normal. In mild-to-moderate iodine deficiency, the thyroid usually succeeds in producing sufficient hormone, in part by preferentially secreting T. In more severe deficiency, as is common in some parts of the world, adult hypothyroidism or cretinism may occur. 

Figure 44.3 Iodine content of 12 consecutive monthly spot urine samples in each of the 15 healthy free-living men in an area with mild-to-moderate iodine deficiency. From Andersen et a ., (2001,2003), reproduced with permission.

These men had mild-to-moderate iodine deficiency, with median urinary iodine content of 50.0 jg/l. Inspection of Rgure 44.3 reveals substantial and random variations in the iodine content of spot urine samples in each of the 15 subjects. No systematic difference between individuals is apparent. 

In the smdy of variation in more than one variable, variations can be correlated. Such associations may be analyzed (Feldt-Rasmussen et al, 1989). The association between iodine excretion and serum TSH was studied in 15 healthy men in an area with mild-to-moderate iodine deficiency (Andersen et al., 2001). The association differed between individuals, and when individuals were grouped according to urinary iodine excretion levels, a negative correlation was found only in individuals with an iodine excretion below 50iig/24h (Figure 44.6). 

The obvious consequence of mild-to-moderate iodine deficiency in a population is a high rate of goiter and thyroid multinodularity that will increase with age (Knudsen et ai, 2000a). The most important factor behind this development is probably not insufficient thyroid hormone production caused by iocfine deficiency, but autoregulation of thyroid iocfide utifization (Laurberg, 2000). 

Over the years, people living with iodine deficiency tend to develop multifocal thyroid autonomy, and multinodular toxic goiter is a common cause of hyperthyroidism. The difference in relative distribution of the four most common causes of hyperthyroidism in Iceland, with high iodine intake, and Judand, Denmark, with mild-to-moderate iodine deficiency, is shown in Figure 47.3. In Iceland, Graves ... 

Figure 47.3 Nosological types of hyperthyroidism with different iodine intake levels. Relative frequency of the four most common nosological types of hyperthyroidism in Iceland, with relatively high iodine intake from consumption of fish and high iodine content of dairy products, and from East Jutland, Denmark, with mild-to-moderate iodine deficiency. MNTG, multinodular toxic goiter GD, Graves disease STA, solitary toxic thyroid adenoma SAT, subacute thyroiditis. Data from Laurberg et al., (1991).

Taken together, these studies may suggest that any increase in iodine intake of a population living with mild-to-moderate iodine deficiency may be associated with an increase in the occurrence of hypothyroidism in the population. The higher the iodine intake, the more cases of hypothyroidism will develop. 

Until the 1990s, total goiter prevalence (TGP) in school-age children was the primary indicator for the assessment of functional consequences of iodine deficiency in the population (WHO et al., 2001). Thyroid size was traditionally determined by palpation, but the reliability of this method is limited by high inter-observer and intra-observer variations. The measurement of thyroid size by ultrasound has therefore been an important step in the detection of mild-to-moderate iodine deficiency. International reference values for normal thyroid size are now available from iodine-sufficient children (Zimmermann et al., 2004). Because TGP is not a sensitive indicator of recent changes in iodine status in the... 

The World Health Organization (WHO), the United Nations International Childrens Emergency Fund (UNICEF), and the International Council for Control of Iodine Deficiency Disorders (ICCIDD) have defined three degrees of severity of iodine deficiency mild (iodine intake of 50—99p,g/day), moderate (20 9 p,g/day), and severe (<20p,g/day) (WHO/UNICEF/ICCIDD, 1994). Severe iodine deficiency was eradicated from many parts of the world, but milder forms stiU exist and may escape detection. Thirty-two European countries were still affected by mild-to-severe iodine deficiency in the late 1990s (Delange, 2002). Turkey is one of these mild-to-moderate iodine-deficient areas (Yordam et ai, 1999). 

The median urinary iodine (MUI) concentration provided a measure of the current nutritional status of iodine. MUI was performed in spot urine samples obtained from all mothers and their neonates 5 days after delivery, by using the Sandell—Kolthoff reactions. Decreased MUI excretion of both neonates with hypothyroidism and their mothers was classified as mild-to-moderate iodine deficiency according to the WHO criteria. The mean total thyroid volume (TTvol) of the neonates and their mothers was increased on ultrasonography, whereas no palpable thyroid tissue was detected on physical examination (Table 65.3). 

Data from the NHANES I—III surveys indicate that Americans are not an iodine-deficient population according to the WHO criteria. Nevertheless, some subpopulations are mild-to-moderately iodine deficient with median UI > 100p,g/l and with a UI < 50p,g/l in no more than 20% of the population. 

The assessment of iodine intake and status is important in elderly subjects. Mild-to-moderate iodine deficiency is associated with a high incidence and prevalence of goiter and nodular hyperthyroidism in middle-aged and elderly subjects (Laurberg et ai, 1991 Pedersen et ai, 2002). In populations with high iodine intake, there is an increased risk of hypothyroidism, especially among... 

Notes-. Iodine intake expressed as iodine dietary intake and urinary iodine excretion in elderly males (M) and females (F). The early Danish surveys indicate mild-to-moderate iodine deficiency, even among users of dietary supplements. The 68-year-old Danes were also compared to elderly subjects in Iceland with a long-standing relatively high iodine intake. After the mandatory fortification of salt in 2000-2001, the Danish National Survey 2002 indicates a sufficient iodine intake among elderly subjects. N, numbers 7-D R, 7-days food record FFQ, food frequency questionnaire. 

Research from the early 1990s has identified mild-to-moderate iodine deficiency in adults, pregnant women and children in New Zealand. 

Iodine administration to patients with no recognized underlying thyroid disease, especially in areas of mild to moderate iodine deficiency. 

Judged from internationally recommended levels of iodine intake Denmark is an area of mild to moderate iodine deficiency. The basic levels of urinary iodine excretion have not changed much during the last 30 years, but part of the population take individual iodine supplementation in the form of vitamin/mineral tablets containing 150 Mg iodine. The basic levels of iodine excretion in tuine found in the studies cited above corresponds well to the intake levels calculated from analyses of food and beverage (10). 

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