Inadequate dietary Intake, and exposure to toxic substances that displace iodine.
Iodine is a mineral, but one that is not abundant in the food we eat, Primarily found in very small quantities in seawater, soils are deficient in iodine, especially the further away you get from the ocean. Iodine is also fairly easily displaced from your body by toxins called toxic halides... flouride, bromine and chloride. Flouride is by far the worse culprit. Found in toothpaste and in your water supply, every time you take a shower, brush your teeth or drink from the tap, your body gets a little exposure to flouride, leeching out good iodine. Contrary to popular belief, flouridated water is actually rather poor at preventing tooth decay. Why is it in our water supply then? Basically a toxic by product of aluminium production, flouridation was sold as a way to prevent cavities because some areas with natural flouride in the water also had lower instances of tooth decay.