how metamorphosis in frog tadpole is delayed or improper if iodine is insufficient?

Iodine is an essential element of thyroid hormone. The thyroid hormone with its essential iodine constituent is responsible for the control of metabolism and also for metamorphosis.  If iodine is not available in the diet, the thyroid may produce an insufficient amount of hormone which will adversely effect the process of metamorphosis.

So, if frog lays egg in iodine deficient water than the tadpoles hatched from that eggs will not be able to metamorphose into adult frog.