We know that some animals do metamorphosis (change from larva to adult)
This is possible because of hormones
Insect Hormones helps insects transform from Larva to Adult
Thyroxine Hormones secreted by thyroid gland help in growth of frogs.
The production of throxine hormones requires the presense of iodine in water.
If Ponds do not have sufficient iodine, thyroxine hormone doesn’t form and tadpoles are unable to metamorphize into frogs.