Sodium has very little effect on your thyroid -- but iodine does. Your thyroid uses iodine to produce the hormones that control your metabolism. In the 1920s iodine was added to salt, creating iodized table salt, to prevent low thyroid function. An iodine deficiency can cause hypothyroidism, but it's quite rare in the U.S. and other First World countries.
Hypothyroidism
Your thyroid uses iodine to manufacture two hormones -- T3 and T4 -- which control the speed with which your body uses oxygen and energy, called your metabolism. If you don't have sufficient iodine intake, your body can't make enough of these hormones and your metabolism will slow down. Signs of hypothyroidism include weight gain, irregular heartbeat, weak and brittle nails and hair and an increased sensitivity to cold temperatures. Typically, hypothyroidism is treated with a synthetic T4 hormone replacement called levothyroxine, sold as Synthroid, Unithyroid, Levoxyl or Tirosint.
Salt and Iodine
Salt is 40 percent sodium and 60 percent chloride. But in the 1920s, iodine was added to salt to treat "the goiter belt" -- several sections of the U.S. and Canada that had a higher-than-average percentage of people with goiters caused by an iodine deficiency. Although iodine deficiency is now very rare in the U.S., up to 40 percent of the world's population is still at risk, according to the American Thyroid Association.
Dose
The U.S. Department of Agriculture recommends that adults consume at least 150 micrograms of iodine daily. Pregnant women need more -- between 200 and 300 micrograms daily. One teaspoon of iodized table salt contains 400 micrograms of iodine. If you are following a low-sodium diet, iodine is also found in seafood, dairy, eggs and soy milk. Most daily multivitamins also contain at least your recommended daily allowance of iodine. Iodine deficiency is tested for using a urine sample.
Conclusion
Adding iodine to your diet will only help treat hypothyroidism if the cause of your low thyroid function is an iodine deficiency. Too much iodine may cause low thyroid function, as can taking certain medications, such as lithium, in combination with iodine. You may develop an enlarged thyroid gland -- called a goiter -- if you have an inadequate or excessive iodine intake. Both too little and too many thyroid hormones can create health problems.
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