<P> Treatment of thyrotoxicosis (including Graves' disease) with 600--2,000 mg potassium perchlorate (430--1,400 mg perchlorate) daily for periods of several months or longer was once common practice, particularly in Europe, and perchlorate use at lower doses to treat thryoid problems continues to this day . Although 400 mg of potassium perchlorate divided into four or five daily doses was used initially and found effective, higher doses were introduced when 400 mg / day was discovered not to control thyrotoxicosis in all subjects . </P> <P> Current regimens for treatment of thyrotoxicosis (including Graves' disease), when a patient is exposed to additional sources of iodine, commonly include 500 mg potassium perchlorate twice per day for 18--40 days . </P> <P> Prophylaxis with perchlorate - containing water at concentrations of 17 ppm, which corresponds to 0.5 mg / kg - day personal intake, if one is 70 kg and consumes two litres of water per day, was found to reduce baseline radioiodine uptake by 67% This is equivalent to ingesting a total of just 35 mg of perchlorate ions per day . In another related study where subjects drank just 1 litre of perchlorate - containing water per day at a concentration of 10 ppm, i.e. daily 10 mg of perchlorate ions were ingested, an average 38% reduction in the uptake of iodine was observed . </P> <P> However, when the average perchlorate absorption in perchlorate plant workers subjected to the highest exposure has been estimated as approximately 0.5 mg / kg - day, as in the above paragraph, a 67% reduction of iodine uptake would be expected . Studies of chronically exposed workers though have thus far failed to detect any abnormalities of thyroid function, including the uptake of iodine . This may well be attributable to sufficient daily exposure or intake of healthy iodine - 127 among the workers and the short 8 - hr biological half life of perchlorate in the body . </P>

What is the nuclear composition of an atom of iodine-131