Iodide in hyperthyroidism

With the use of antithyroid drugs, the role of iodide in hyperthyroidism has been relegated to that of preparation for thyroid surgery. Iodide, as Lugol s solution ... 

The selective uptake of iodide ion by the thyroid gland is the basis of radioiodine treatment in hyperthyroidism, mainly with although various other radioactive isotopes ate also used (40,41). With a half-life of eight days, the decay of this isotope produces high energy P-particles which cause selective destmction within a 2 mm sphere of their origin. The y-rays also emitted are not absorbed by the thyroid tissue and are employed for external scanning. 

III.b.1.1. Anion inhibitors. Perchlorate, periodate, pertechnetate and thiocyanate (a naturally occurring goitrogen) are classified as iodide pump inhibitors, antagonizing iodide transport through competitive inhibition. This effect can be overcome by large dose of iodides. Perchlorate is used to block reuptake of iodide in cases of amiodarone induced hyperthyroidism and for the perchlorate discharge test . 

The major clinical use for potassium perchlorate is to block thyroidal reuptake of 1 in patients with iodide-induced hyperthyroidism (eg, amiodarone-induced hyperthyroidism). However, potassium perchlorate is rarely used clinically because it is associated with aplastic anemia. 

Iodide is well absorbed from the intestine, is distributed like chloride in the body and is rapidly excreted by the kidney. It is selectively taken up and concentrated (about x 25) by the thyroid gland, but more in hyperthyroidism and less in hypothyroidism. A deficiency of iodide reduces the amount of th)T oid hormone produced, which stimulates the pituitary to secrete TSH. The result is hyperplasia and increased vascularity of the gland, with eventual goitre formation. 

Inhibition of the release of thyroid hormone by iodide is the basis for its use in hyperthyroidism. Iodide decreases the vascularity of the enlarged thyroid gland and also lowers the elevated BMR. It also has been suggested that excess iodide might change the conformation of thyroglobulin, making the protein less susceptible to thyroidal proteolysis (66). 

Blockers are usually used as adjunctive therapy with antithyroid drugs, RAI, or iodides when treating Graves disease or toxic nodules in preparation for surgery or in thyroid storm. /3-Blockers are primary therapy only for thyroiditis and iodine-induced hyperthyroidism. 

III.b.1.4. lodinated contrast media. When iodides or thioamides are contraindicated, the contrast media ipodate and iopanoic acid may be used to treat hyperthyroidism. These drugs rapidly inhibit conversion of T4 to T3 in the liver, kidney, brain and pituitary gland, and the effects are so rapid that they are sometimes helpful in the treatment of thyroid storm (see below). The drugs are non toxic and have prolonged effect. Precautions are similar to iodide,... 

The perchlorate ion of potassium perchlorate, KCIO4, is a competitive inhibitor of thyroidal 1 transport via the Sodium Iodide Symporter (NIS).This drug can cause fatal aplastic anemia and gastric ulcers and is now rarely used. If administered with careful supervision, in limited low doses and for only brief periods, serious toxic effects can be avoided. The compound is especially effective in treating iodine-induced hyperthyroidism, which may occur, for example, in patients treated with the antiar-rhythmic compound amiodarone. Perchlorate ion can also be used in a diagnostic test of 1 incorporation into Tg, the so-called perchlorate discharge test. 

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). 

Iodides have several actions on the thyroid. They inhibit organification and hormone release and decrease the size and vascularity of the hyperplastic gland. In susceptible individuals, iodides can induce hyperthyroidism (Jod-Basedow phenomenon) or precipitate hypothyroidism. 

Neonatal hypothyroidism has been reported after maternal use of antithyroid drugs (106,107). Transient neonatal hyperthyroidism in a female child born to a mother who had been treated with potassium iodide and carbimazole during pregnancy was followed by sexual precocity (108). 

It has been suggested that potassium perchlorate should be used in the treatment of type 1 hyperthyroidism and glucocorticoids in the treatment of type 2 (SEDA-21, 199). Since hypothyroidism due to amiodarone tends to occur in areas in which there is sufficient iodine in the diet, it has been hypothesized that an iodinated organic inhibitor of hormone synthesis is formed and that the formation of this inhibitor is inhibited by perchlorate to a greater extent than thyroid hormone iodination is inhibited, since the iodinated lipids that are thought to be inhibitors require about 10 times more iodide than the hormone. However, there is a high risk of recurrence after treatment with potassium perchlorate, and it can cause serious adverse effects (SED-13,1281). 

Although iodide is effective in treating hyperthyroidism for short periods, the effects of this drug begin to diminish after about 2 weeks of administration.35 Consequently, iodide is used in limited situations, such as temporary control of hyperthyroidism prior to thyroidectomy. In addition, iodide may cause a severe hypersensitive reaction in susceptible individuals. Therefore, the use of iodide has been replaced somewhat by other agents such as antithyroid drugs and beta blockers. 

C-6) Thiouracil is used in the treatment of hyperthyroidism. Thiouracil drugs inhibit the organic binding of iodide needed in the formation of mono-, di-, triCTs), and tetraiodothyronine (T4). 

In euthyroid subjects with normal glands an excess of iodide from any source can cause goitre (with or without hyperthyroidism), e.g. use of iodide-containing cough medicines, iodine-containing radio-contrast media, amiodarone, seaweed eaters. 

In patients with hyperthyroidism, potassium iodide is used in the treatment of thyrotoxicosis and to decrease the vascularity of the thyroid before the thyroid gland is surgically removed. Potassium iodide is... 

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