A mild deficiency can make it harder for you to stay lean. Here's where to get it and how much you need.
The Thyroid Needs Iodide
The thyroid needs iodine, either from iodized salt, certain foods, or from supplements. The body uses it to synthesize the thyroid hormones T3 and T4.
“Iodide" is the ion state of iodine, occurring when iodine bonds with another element like potassium or sodium. "Iodide” is the safe form of iodine for ingestion.
About 12% of Americans are seriously deficient in iodide as measured by urinary analysis. Many more are moderately deficient or sub-optimal. So while diet alone can provide good iodide intake, very often it doesn't. It's worth giving your food choices and iodized salt intake a quick lookover.
If most of your food is homemade and salted with a medium-to-heavy hand with iodized salt, you should have good iodine intake already. If not, then you should increase it. How much iodized salt is needed, if it's your principal source of iodide? When iodized salt is at full potency, it takes about 2 grams or about half a teaspoon of iodized salt (about 936 mg sodium) to meet your rock-bottom minimum for iodide consumption.
However, about half the time, iodized salt brands do not meet label claim for iodide, so it could take somewhat more to meet a bare minimum. Generally, fast foods, processed foods, and restaurants don't use iodized salt. If a label reads merely "salt" instead of "iodized salt," it's not iodized. And "sea salt" contains little iodide.
If you aren't consuming much iodized salt at home, increasing your intake is likely to benefit you. I recommend 300 mcg of iodide per day, but somewhat more is fine.
How Much Iodized Salt Is Too Much?
Overdosing with iodized salt is unlikely unless you use a really heavy hand. The NIH recommends 1100 mcg (1.1 mg) of iodide a day as a maximum intake, and the evidence strongly supports consuming no more than this. There's absolutely no reason to consume more than this on an ongoing basis, and a good reason not to do so.
Greater intake than this can cause hypothyroidism. It may not take even this much. A large study in China found intake of 0.8 mg/day to be associated with hypothyroidism. If using full-potency iodized salt as the sole source of sodium, 0.8 mg iodide/day wouldn't be reached until consuming about 4945 mg sodium/day. This is about a full tablespoon. I generally like seeing athletes consume about 4000 mg sodium/day, so this works out fine.
I'd prefer, however, you limit iodized salt intake to no more than about half a tablespoon per day, as I see no reason to push iodide to the extreme. In practice, some iodized salt is short of full potency, so expect some variation. The above range allows for normal variation. It does assume, however, that the salt is added after cooking, not before.
For the ladies, if pregnant I'd recommend avoiding excess iodide intake, as can happen with consumption of seaweed or over-supplementation with iodide.
What Foods Provide Plenty of Iodide?
As a way of looking at how iodine-rich some foods are, you could meet a 150 mcg/day requirement by consuming any one of these in about the following amounts:
2.5 cups of milk, yogurt, or cottage cheese per day
2.5 baked potatoes with skin
13 ounces of shrimp or of "average" saltwater fish
13 ounces of turkey
Beef, chicken, pork, rice, most wheat products, fruits, and vegetables are too low in iodine to be major sources. Seaweed or kelp are extremely rich but aren't common foods in America. Also, whey protein concentrate and micellar casein are pretty rich in iodide. It takes only about 80 grams of these proteins, give or take, to provide 150 mcg iodine.
So if your diet has several of the above things or a lot of even one of them, you won't need iodized salt or a supplement. But if your diet is strong on things like beef, chicken, vegetables, pasta, and fruits and weak on the above, then iodized salt or a supplement will probably be beneficial.
You'll find varying claims on wheat products. Some tables show a wheat product or two as good sources. But when high in iodide, it's the additives that are providing it, and additive use varies greatly between products. For example, bread slices vary from a useless 2.2 mcg to a potentially-problematic 587 mcg, with low values being more common than high.
Worse yet, bread additives may include potassium bromate, which actually works against your thyroid rather than for it. So, don't count wheat products as reliable contributors, and particularly not bromate-containing bread.