Upper limit of iodine in infant formulas.

Iodine is the rate-limiting substrate for thyroid hormone synthesis by the thyroid gland and must be available in threshold amounts to prevent goiter and hypothyroidism. Because excess iodine can inhibit thyroxine synthesis, an upper limit of iodine in infant formulas of 75 microg/100 kcal was proposed by the Food and Drug Administration in 1985. Review of recent studies of iodine physiology in infants and iodine content of milk suggests that a safe upper limit of iodine intake by normal, term infants should not exceed 100 microg/kg body wt daily. Given this limit and assuming the total intake of iodine to be derived from formula, the recommendation of 1985 still seems reasonable.