Perchlorate competitively blocks
iodide from entering the thyroid by an effect on the Na+/I-
symporter thus preventing the further synthesis of
thyroid hormone but has no effect on the iodination process itself. It is concentrated by thyroid tissue in a manner similar to
iodide but is not significantly metabolized in the gland or peripherally. What is not settled is whether there are additional
perchlorate effects on
iodide transport.
Perchlorate has a fast turnover in the body and requires frequent daily doses for
therapy of
thyrotoxicosis.
Perchlorate appears to be substantially more effective against large
iodide loads than the thionamides, and, with long-term
iodide contamination, combined
therapy of
perchlorate (with < or = 1 g/day) and thionamides is recommended for the more severe cases of
thyrotoxicosis that may result from excess
iodide or
iodide-generating organic compounds, as for example with
amiodarone. After approximately 30 days, the
perchlorate dosage can be tapered or stopped, continuing with thionamides alone. This markedly increases its safe use. Despite serious side effects during its early use, lower dosages and shorter treatment periods appear to have prevented such reactions in its recent reintroduction, mostly for
amiodarone-induced thyroid dysfunction.
Perchlorate can also protect against inhibition of thyroid function and the resulting
hypothyroidism caused by excess
iodide, presumably by reducing the formation of an iodinated inhibitor. The reduction of the
iodide pool by
perchlorate thus has dual effects--reduction of excess
hormone synthesis and
hyperthyroidism, on the one hand, and reduction of thyroid inhibitor synthesis and
hypothyroidism on the other.
Perchlorate remains very useful also as a single dose application in tests measuring the discharge of radioiodide accumulated in the thyroid as a result of many different disruptions in the further metabolism of
iodide in the thyroid gland.