Iodine is concentrated in the thyroid (synthesises thyroxine) and ovaries. It is involved in energy metabolism, Vitamin A metabolism, body temperature, growth and immune function. Seafood, kelp, eggs and free range hens are rich in iodine. Ruminant ration ranges from 0.5 - 2.0 ppm. Symptoms of deficiency include big neck in calves and enlargement of the thyroid gland. Brassicas like kale and swede can be goitrogenic (inhibit iodine uptake to the thyroid).
15 to 20 mg of iodine per cow per day may be required to compensate (or 2mg of iodine per kg of dry matter). Chloride and fluoride are halogens which can inhibit iodine.
Kelp is the best natural source of iodine.