y do tadpoles that grow in water deficient in iodine fail to develop  into adult fog

The growth and development of the animals like tadpole is controlled by the hormone called thyroxine produced by the thyroid gland. The proper function of thyroid gland and production of thyroxine needs iodine. Thus, if the water in which tadpole is living is deficient in iodine, it retards its growth and metamorphosis. Thus, it fails to develop into adult frog.