Chlorine dioxide has been shown to inhibit thyroxin formation at doses of 9mg/kg which was reversible (far more than the Miracle-Mineral-Supplement doses)
It doesnt damage the thyroid however and the reason is not known, also it doesnt cause Iodine deficiency
You shouldn't use Miracle-Mineral-Supplement at high doses long term but if you do use over 15 drops or so longterm you would best get blood tests to check for changes especially if you were feeling tired. There is a chance that thyroxin could be reduced with long term use but not below 3mg/kg doses according to animal trials which is about 20 drops
There is no chlorine gas formed internally with Miracle-Mineral-Supplement or other acidified sodium chlorite