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Iodine/iodide organification

The thyroid gland also regulates its uptake of iodide and thyroid hormone synthesis by intrathyroidal mechanisms that are independent of TSH. These mechanisms are primarily related to the level of iodine in the blood. Large doses of iodine inhibit iodide organification (Wolff-Chaikoff block, see Figure 38-1). In certain disease states (eg, Hashimoto s thyroiditis), this can inhibit thyroid hormone synthesis and result in hypothyroidism. Hyperthyroidism can result from the loss of the Wolff-Chaikoff block in susceptible individuals (eg, multinodular goiter). [Pg.857]

On the other hand, excess iodine intake may also inhibit thyroid function, by either inhibition of iodide organification (Wolff-Charkoff effect) or inhibition of Tg proteolysis with reduction in hormone secretion, and may manifest itself either as a goiter, as hypothyroidism with/without goiter, or as hyperthyroidism (0.01-0.6% in populations on iodine prophylaxis), the outcome depending on the initial and current iodine status and current thyroid dysfunction (European Commission, 2002). The comparison of iodine intake and concentrations of serum Tg at various physiological or pathophysiological conditions is shown in Table 6.1. [Pg.60]

The oxidation of intracellular iodide is catalyzed by thyroid peroxidase (located at the apical border of the thyroid acinar cell) in what may be a two-electron oxidation step forming 1 (iodinium ion), lodinium ion may react with a tyrosine residue in the protein thyroglobulin to form a tyrosine quinoid and then a 3 -monoiodotyrosine (MIT) residue. It has been suggested that a second iodide is added to the ring by similar mechanisms to form a 3,5-diiodotyrosine (DIT) residue. Because iodide is added to these organic compounds, iodination is also referred to as the organification of iodide. ... [Pg.796]

Perhaps the most ambitious early effort of this type was the model of iodine metabolism developed by Douglas Riggs (1952). He presented a theoretical model that included ingestion of iodine as iodide, its absorption into the plasma, and transfer to the thyroid and other tissues, with loss through the kidneys, sweat and expired air. Within the thyroid gland, he described the organification of iodine and production of monoiodothyronine, diiodothyronine, and thyroxine (T4) on the framework of thyroglobufln. [Pg.193]

Acute excess iodine ingestion has long been known to result in a transient decrease in iodine organification, termed the Wolff—Chaikoff effect (Wolff and Chaikoff, 1948). With sustained excess iodine exposure, however, most individuals thyroid glands escape from acute Wolff— Chaikoff effect, despite continued excess iodine exposure, and resume synthesis of normal amounts of T4 and T3. The mechanism responsible for this escape or adaptation to the iodine load probably involves a decrease in the Na /H symporter protein, resulting in a decrease in thyroid iodide content (Eng et ai, 1999). In some individuals... [Pg.758]

The risk for IIH depends on many factors. Some of them have already been discussed above. The greatest influence is due to the level of iodine supply in a population. Other environmental factors, such as smoking, have been proposed as risk factors for developing goiters due to increased thiocyanate levels in smokers exerting a competitive inhibitory effect on iodide intake and organification, especially in iodine-deficient areas (Krohn et al., 2005). [Pg.890]

Figure 101.2 Thyroid iodide transport and organification. A schematic of a thyroid follicular cell showing the key aspects of thyroid iodine transport and thyroid hormone synthesis. TSHR, TSH receptor NIS, sodium iodide symporter TPO, thyroid peroxidase Tg, thyroglobulin. Figure 101.2 Thyroid iodide transport and organification. A schematic of a thyroid follicular cell showing the key aspects of thyroid iodine transport and thyroid hormone synthesis. TSHR, TSH receptor NIS, sodium iodide symporter TPO, thyroid peroxidase Tg, thyroglobulin.
Diagnostically, perchlorate is used to assess the intrathyroidal organification of iodine. When perchlorate is administered after a dose of radioactive iodine, perchlorate washes out or discharges intrathyroidal inorganic iodide but does not affect covalently bound organic iodide. When organification is inadequate, there is a sharp decrease in intrathyroidal radioactive iodine after perchlorate administration. [Pg.1378]

Iodine test. Plasma iodine in the form of iodide is concentrated (trapped) in the thyroid cells by an energy-requiring active transport mechanism, where it is incorporated into T3 and T4 via organification. Therefore, iodine measures both trapping and organification by the thyroid gland. [Pg.1391]

The chemical steps in the biosynthesis of thyroid hormones are also well established and are shown in a simplified form in Fig. 1. There are three main phases in biosynthesis, namely trapping of iodide and its conversion into an activated form, iodination of the tyrosines (organification), and finally coupling of the iodotyrosines to form iodothyronines. Only a brief outline of the biosynthesis will be given here, as the subject has been well reviewed by others (e.g., D2, P13, S6). [Pg.105]


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See also in sourсe #XX -- [ Pg.1369 , Pg.1370 , Pg.1377 ]




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