Hyperthyroidism Vs Hypothyoidism

It is named Wolff-Chaikoff effect and thought to be the result of formation of iodo-containing chemicals like iodolactones and iodolipides which inhibit thyroid peroxidase, the key enzyme of T3 formation. The effect is transitory and fully reversible but if individual has some negative preexisting factors like previous exposure of I131 or inflammation, hypothyroidosis may become permanent.

Counting that iodine is quite abundant in environment and accidental overdose may happen (kelp may contain up to 10 suggested RDAs in just one serving used for one sushi roll, and even in Japan serving just one roll is something which just never happens), and counting that hyperthyroidism is much more dangerous for survival than the opposite condition, the effect clearly makes sense.

For further reading:

Consequences of excess iodine