Cruciferous Vegetables, Iodine, And Thyroid Function
"Very high intakes of cruciferous vegetables, such as cabbage and turnips, have been found to cause hypothyroidism (insufficient thyroid hormone) in animals.The RDA for iodine for adults is 150mcg (150 micrograms/day). Seafood and sea vegetables (seaweeds like kelp) are good sources of iodine.
Two mechanisms have been identified to explain this effect:
Increased exposure to thiocyanate ions from cruciferous vegetable consumption or, more commonly, from cigarette smoking, does not appear to increase the risk of hypothyroidism unless accompanied by iodine deficiency. One study in humans found that the consumption of 150 g/day (5 oz/day) of cooked Brussels sprouts for four weeks had no adverse effects on thyroid function."
- The hydrolysis of some glucosinolates found in cruciferous vegetables (e.g., progoitrin) may yield a compound known as goitrin, which has been found to interfere with thyroid hormone synthesis.
- The hydrolysis of another class of glucosinolates, known as indole glucosinolates, results in the release of thiocyanate ions, which can compete with iodine for uptake by the thyroid gland.
So, if you eat a lot of cabbage, cole slaw, cauliflower, broccoli, Brussels sprouts, turnips, and the like, it would be a good idea to make sure you're getting 150 mcg of iodine a day, either through a supplement or sea foods. (If you use sea salt or other non-iodized salt, you need to be even more attentive. A 1/4 teaspoon of iodized salt, at 70 mcg, contributes about half of the day's requirements.)
DV = Daily Value. The Daily Value used here is 150 mcg. A food that provides 50% of the DV provides 75 mcg iodine.
According to the NIH Office of Dietary Supplements, dairy is a good source of iodine "due to the use of iodine feed supplements and iodophor sanitizing agents in the dairy industry."