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Iodine-containing medicaments


See also Amiodarone ; Iodinated contrast media ; Iodopropynyl butylcarbamate ; Radioactive iodine

General information

Iodine is a non-metallic halogen element (symbol I; atomic no 53, atomic weight 127). It exists as a near-black solid but readily sublimates, giving a purple-colored vapor. It is found in nature both free (for example in large amounts in seaweeds such as kelp and in low concentrations in seawater) and in minerals such as iodyrite (silver iodide) and Chile saltpetre (sodium iodide).

Iodine must be present in the normal diet to prevent iodine-deficiency goiter or cretinism, and iodine deficiency-related disorders are still a worldwide (although preventable) group of diseases that affect about 150 million people in at least 40 countries. The WHO sponsored a program to control these disorders by the year 2000 [ , ], and since 1990 there has been a remarkable progress in prevention of iodine deficiency disorders. However, by the year 2000 one-third of the population affected by iodine deficiency disorders still did not have access to iodized salt [ ]

Scepticism about the introduction of population-wide programs to prevent iodine-deficiency disorders is occasionally encountered in regions of mild iodine deficiency, especially in Europe [ ]. The main arguments against introduction of iodized salt are a temporary rise in the incidence of hyperthyroidism [ ], a possible increase in the incidence of Graves’ disease, and the fact that the remission rates with antithyroid drug therapy will fall [ ]. The value of preventing mild iodine deficiency has been supported by a longitudinal study from Switzerland, in which 109 000 people in a defined catchment area were studied before and for 9 years after correction of mild iodine deficiency [ ]. The incidence of toxic nodular goiter increased in the first year by 27%, but thereafter there was a steady fall in the incidence of both toxic nodular goiter (− 73%) and Graves’ disease (− 33%). The range of optimal iodine intake is fairly narrow. Mild and moderate iodine excess are probably associated with higher frequency of hypothyroidism [ ].

Some drugs contain iodine in amounts that considerably exceed the optimum daily intake of inorganic iodine. Such drugs include:

  • many radiographic contrast media;

  • amiodarone and benziodarone;

  • iodopropynyl butylcarbamate;

  • iodoquinoline;

  • iodine-containing antiseptics (for example povidone-iodine).

Radioactive iodine

Different forms of radioiodine have been used at different times, including 123 I, 125 I, and 131 I. Radioactive iodine is used to scan the thyroid gland and in the treatment of thyrotoxicosis.

Potassium iodide and potassium iodate

Potassium iodide is the inorganic iodide most commonly used in high dosage for acute thyrotoxicosis. Indeed, large amounts of iodine cause reduced organification of iodine and a temporary block of thyroid hormone secretion (Wolff–Chaikoff effect) and therefore result in a more rapid thyrostatic effect than synthesis inhibitors. Potassium iodine is also used for preoperative treatment of goiter, especially to reduce preoperative bleeding. It can be used in combination with thyrostatic drugs but should never be prescribed in combination with potassium perchlorate, since each abolishes the other’s effects. The thyrostatic effects of iodide are evident even at a dose of 6 mg/day, but doses between 50 and 100 mg/day are usually recommended. In some cases of intolerance to higher doses, perchlorate can be used, for example for preoperative treatment.

Potassium iodide has been widely used in asthma and chronic bronchitis as an expectorant. There is considerable controversy about its efficacy. It should not be used in adolescent patients because of its potential to aggravate and induce acne and its effect on the thyroid gland. In view of its doubtful efficacy and definite toxicity, it would be preferable if physicians stopped prescribing it as an expectorant.

Potassium iodide and potassium iodate are commonly added to table salt to prevent iodine deficiency and associated thyroid disease.

Iodine in protecting the thyroid against radioiodine

Accidents with nuclear reactors or nuclear bombs can expose large numbers of people to several decay products of uranium, and iodine isotopes are among the most abundant compounds released in such reactions. It is therefore logical to use salts of stable isotopes of iodine to prevent the accumulation of radioiodine in a person or population at risk of such exposure. The accidents in Windscale (UK), Three Mile Island (USA), and particularly Chernobyl (Ukraine) drew attention to such problems. The major question is therefore whether the potential adverse effects of stable iodine when given indiscriminately to large groups of people would outweigh the risk of radioiodine exposure. Stable iodine needs to be rapidly available when disasters occur, since, in order to be effective, it has to be given in sufficient amounts (100 mg) within a short time before or after (− 12 to + 3 hours) radioiodine exposure. Potassium iodate (KIO 3 ) is more stable than potassium iodide (KI), since the latter readily evaporates during prolonged storage. However, the dose recommended for radioprotection of 100 mg of iodine daily over several days (138 mg iodate per day) are close to retinotoxic doses of iodate reported in cases of accidental intoxication. In these doses iodate cannot be recommended. As an additive to salt for correcting iodine deficiency, much smaller doses of iodate are used (up to 1.7 mg/day), and in these doses iodate is probably safe [ ].

The main adverse reactions to stable iodine are shown in Table 1 .

Table 1
Adverse effects of iodine given for protection against radioiodine
Adverse reaction Susceptibility factors
Iodine-induced goiter
Iodine-induced hypothyroidism Fetus and neonate
Iodine-induced hyperthyroidism People living in iodine-deficient areas; a history of hyperthyroidism
Sialadenitis Hypersensitivity (early reactions); renal insufficiency (late reactions)
Taste disturbances
Nausea and abdominal pain
Rashes
Edema (including face and glottis)
Allergic-like reactions (iodine fever, eosinophilia, serum sickness-like symptoms, vasculitis) Hypocomplementemic vasculitis

Iodine should be given to the general population if the risk of radioiodine exposure is sufficient (over 15–100 rem), but people with increased susceptibility to the adverse effects of iodine (previous thyroid disease or known serious allergies) should be excluded [ ]. In elderly people the benefit of stable iodine probably does not outweigh its potential adverse effects, while in pregnant women and infants the benefit to harm balance is not established; rapid evacuation of such people from fallout zones should be given the highest priority [ ].

Tincture of iodine

Tincture of iodine (aqueous iodine oral solution, Lugol’s solution) is a solution of iodine 5% plus potassium iodide 10% in water, which is used to reduce the vascularity of the thyroid gland in thyrotoxicosis before surgery.

Iodinated glycerol

Iodinated glycerol is used as a mucolytic agent in respiratory disorders, but its efficacy is controversial. Organically bound iodine is changed to unbound iodide after absorption.

Iodoform

Iodoform is a lemon-yellow-colored crystalline organic salt of iodine (CHI 3 ), analogous to chloroform, with a saffron-like odor, used as an antiseptic.

Iodophors

Iodophors are labile complexes of elemental iodine with macromolecular carriers that both increase the solubility and provide sustained release of iodine. Povidone-iodine is a water-soluble iodophor that is used as an antiseptic and is said to be free of the undesirable effects of iodine tincture. However, iodine can be absorbed from it through burned areas [ ], vaginal mucosa [ ], oral mucosa [ ], and in children even with normal skin [ ].

Iodinated radiocontrast media

Many radiocontrast media contain iodine. These include iodixonal, iohexol, iomeprole, iopamidole, iopanoic acid, iopitridole, iopromide, iothalamate, iotrolan, ioversol, and ioxaglate. They are covered in a separate monograph.

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