Iodine content of plants is at a maximum in autumn and winter. Plants grown in the same soil take up varying amounts of iodine, and uptake varies with water availability. Most seafood and seaweed are rich in iodine. Seawater is from 17 to 50 ug iodine. Most leaves are more iodine rich than root vegetables.
Iodine content is higher in onions and asparagus.
Goitrogens - some vegetables act to block iodine absorption. Some goitrogens are kale, cabbage, broccoli, rutabaga, and peas, soybean, and peanuts. They should be used in moderation if there is any sign of iodine deficiency in the diet. Deficient cobalt or manganese interferes with iodine absorption, and excess calcium or potassium also interferes. Iodine deficiency is widespread Iodine deficiency is a widespread problem, with the world health organization estimating a 200 million people are affected worldwide.