Iodine is an essential micronutrient required for structural development and optimal functional activity of the thyroid gland and central nervous system. Iodine deficiency has been shown to be associated with endemic cretinism, endemic goitre and subcretinous mental subnormalities.Despite being iodine replete, goitre continues to be prevalent in mild to moderate degree of endemicity in most states of India. There might be other goitrogens or deficiency of other micronutrients responsible for the persistence of goitre despite adequate salt iodization.Selenium as a causative factor for goitre formation has been explained by diminished activity of selenocysteine enzymes in thyroid, notably glutathione peroxidase and deiodinase type-1 in selenium deficiency.Iron deficiency state is an important cause for persistence of goitre in the iodine replete population

Treat selenium deficiency before iodine deficiency, the reverse has been hypothesized to induce autoimmune thyroiditis.

Here's a recent paper indicating more trace mineral deficiencies such as magnesium may also play a role in pathogenesis: http://press.endocrine.org/doi/full/10.1210/jcem.87.4.8421