Iodine is one of the trickier trace elements, as deficiency can cause the same main symptom (goiter) as excess. Plenty of people turn to iodine supplements in hopes of helping a "sluggish thyroid" which is usually not caused by a lack of iodine, but various metabolic dysfunctions. Iodine is needed in such small amounts that your entire lifetime's supply would fit into a teaspoon. The RDA is set at 150 mcg per day, and iodine (so far as we know) is only required by the body for one process, which is the manufacture of thyroid hormones. Interestingly enough, many people are switching away from iodized salt, and therefore not getting very much iodine, but that's more due to a lack of eating foods rich in iodine (which the Standard American Diet tends to be very poor in unless supplemented).

Functions

  • Synthesis of thyroid hormones (which maintain proper metabolism)

Primary deficiency symptoms and diseases

  • Goiter
  • Lowered metabolic rate (due to low thyroid hormone synthesis)

Food sources

You'll mostly find iodine from sea vegetables and salt water fish, but some root vegetables can contain decent amounts depending on the soil they were grown in. An easy way to ensure adequate iodine consumption is to sprinkle dulse flakes or kelp powder onto food several times per week. However, some seaweed can contain very high amounts of iodine that would not be safe for daily consumption (i.e. kelp). Land animals will also store iodine (as they require it too), but may not be a totally reliable source unless organs are consumed.

  • Various sea vegetables
  • Salt water fish
  • Root vegetables
  • Liver
  • Eggs
  • Dairy

Supplementation

Iodine supplementation is very nuanced. It is easy to get too much and either cause goiter or an overabundance of thyroid hormone, or worse...damage to the thyroid gland itself. Be wary of taking high amounts of iodine in supplement form, as you should almost never need more than 150 mcg per day unless instructed to take more by a physician who is doing routine blood work on you. By the way, iodine and iodide are interchangeable terms. Iodine just refers to the element itself, and iodide is the term for when it is chemically bound to something. You'll never find just a free iodine supplement not labeled as iodide, so don't get discouraged if you look and don't find one.

  • Iodized salt (most common)
  • Potassium iodide
  • Kelp tablets (very easy to overdo, use sparingly)

Who needs it most?

  • Those on salt restricted diets or who do not use iodized salt

Other tips

  • It is quite easy to get adequate iodine from food so long as you at least occasionally consume seafood or sea vegetables
  • Taking iodine supplements will likely not do much for weight loss (beyond correcting a deficiency)
  • Excessive iodine from food or supplements can be highly detrimental to your health and metabolic functioning