Iodine content of the thyroid

Astwood EB, Bissell A (1944) Effect of thiouracil on the iodine content of the thyroid gland. Endocrinol 34 282-296 Ching M (1981) Dose-related effect of growth hormone on thyroidal radioiodine uptake. Horn Res 14 234—42 Gutshall DM, Pilcher GD, Langley AE (1989) Mechanism of the serum thyroid hormone lowering effect of perfluoro-n-decanoic acid (PFDA) in rats. J Toxicol Environ Health 28 53-65... 

The supply of 10 mg I kg in the mineral mixtures of farm animals increased the iodine content of the thyroid gland, milk and eggs significantly, but did not greatly affect the iodine concentration of meat and liver (Anke et al. 1989b, c, Wenk and Heinrich 1989). 

An extensive study on the iodine content of the thyroid glands of 95 fetal and newborn calves has been conducted in the United States by Wolff et al. (1949). Measurable amounts of iodine were first detected in the fetal thyroid at 60 days of age (28 mg %). A progressive increase in the iodine concentration was found to occur during the period of gestation, reaching a value of 118 mg % at term. The mean iodine level observed for 4 adult cows, aged 6 years, was 138 mg % (88-189 mg %). 

The iodine content of the thyroid gland in lambs and in adult sheep has been reported by Marine and Lenhart (1909) and by Sigurjonsson (1938b). In the study by Marine and Lenhart, an average iodine concentration of 9 mg % was found for tissue samples from lambs aged 8-9 months, whereas a distinctly higher mean iodine level of 59 mg % N = 22) was recorded for 1- to 4-year-old animals. In contrast to this, the assays performed by Sigurjonsson. showed essentially similar values for 5-inonth-... 

Although thyroid slices of hyperthyroid patients take up iodine more quickly than normal thyroid slices, the total iodine content of the thyroid gland (1.8 mg/g of dry weight in normal individuals) is consideraWy reduced in hyperthyroidism (0.26 mg/g of dry weight). The reduction affects both inorganic and organic iodine, but when all iodinated compounds in the thyroid are analyzed, thyroxine seems to be the compound most reduced in hyperthyroidism. 

Gollnick DA, Greenfield MA. 1978. The in vivo measurement of the total iodine content of the thyroid gland by X-ray fluorescence. Radiology 126 197-200. 

Several authors reported methods for measuring the iodine content of the thyroid in vivo. Besides the complicated procedures of neutron activation analysis (7) and x-ray spectrophotometry (26,37), the fluorescent technique has been used successfully (2,29,33,39,49,51). The clinical usefulness of such iodine measurements is limited by two factors ... 

Figure 1. Relationship between the daily urinary excretion of iodine in adults and the prevalence of goiter, the hormonal iodine content of the thyroid and the thyroidal uptake of radioiodine. From DELANGE and ERMANS with permission.

We have investigated the possibility that this hypersensitivity is due at least partly to a less efficient mechanism of adaptation to iodine deficiency, namely to a reduction of the iodine content of the thyroid in iodine deficient neonates. 

We have measured the iodine content of the thyroid in fetuses, preterm and fiillterm infants who died shortly after birth in four cities with markedly different intakes in the general population, namely Toronto, Canada Brussels, Belgium Cardiff, Wales, U.K. and Leipzig, Germany (See the iodine intakes in Table 2. The iodine intake in Brussels and Cardiff are similar). 

In conclusion, iodine cannot prevent the strong antithyroid effects of a high glucosinolate intake. However, at a lower glucosinolate content iodine can block the depressed hormone release by the thyroid, the serum T4 level reaches a plateau, the thyroid weight is not increased, and the iodine content of the thyroid is higher. 

In the most simplistic physiological model, inadequate intake of iodine results in a reduction in thyroid hormone production, which stimulates increased TSH production. TSH acts directly on thyroid cells, and without the ability to increase hormone production, the gland becomes hyperplastic. In addition, iodine trapping becomes more efficient, as demonstrated by increased radioactive iodine uptake in deficient individuals. However, this simplistic model is complicated by complex adaptive mechanisms which vary depending on the age of the individual affected. In adults with mild deficiency, reduced intake causes a decrease in extrathyroidal iodine and reduced clearance, demonstrated by decreased urinary iodine excretion, but iodine concentration in the gland may remain within normal limits. With further reduction in intake, this adaptive mechanism is overwhelmed, and the iodine content of the thyroid decreases with alterations in iodination of thyroglobulin, in the ratio of DIT to MIT, and reduction in efficient thyroid hormone production. The ability to adapt appears to decrease with decreasing age, and in children the iodine pool in the thyroid is smaller, and the dynamics of iodine metabolism and peripheral use more rapid. In neonates, the effects of iodine deficiency are more directly reflected in increased TSH. Diminished thyroid iodine content and increased turnover make neonates the most vulnerable to the effects of iodine deficiency and decreased hormone production, even with mild deficiency. 

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