Iodine and your thyroid gland

DEAR DOCTOR K: I cook with kosher salt, which doesn’t contain iodine. Am I getting enough iodine in my diet? Can I get iodine from other foods?

DEAR READER: Before I answer your question, I’ll need to explain a little about the thyroid gland and how it works. The thyroid is a small, butterfly-shaped gland that perches with its wings wrapped around the front of your windpipe, below your voice box. This gland influences the rate at which every cell, tissue and organ in your body functions. It does this primarily by secreting thyroid hormones.

Your thyroid needs fuel to produce thyroid hormones. This fuel is iodine. Many people think of iodine as a liquid that you put on a cut to help kill any germs. It is used that way, but it also has other medical uses.

Iodine is a natural part of our environment. It’s in seawater, rocks and other parts of our world. It gets into our body in various foods such as packaged breads and milk products. Shrimp and fish, such as cod and tuna, are also good food sources of iodine.

Iodine in foods gets absorbed into the bloodstream and carried by the blood to the thyroid gland in the neck. There, it fuels the production of thyroid hormones.

You asked about kosher salt. Most table salt is “iodized:” It contains iodine. However, iodine is not found in kosher salt. So if you use kosher salt only, could you cause yourself thyroid problems?

It’s a reasonable question, because iodine deficiency can lead to enlargement in the front of the throat, known as a goiter. Severe iodine deficiency can progress to hypothyroidism, a condition in which the thyroid cannot produce enough thyroid hormones. Hypothyroidism can cause fatigue, constipation, weight gain, slowed heart rate and depression.

Iodine deficiency is generally not a problem in the United States. Many, if not most, Americans consume iodized salt. And food manufacturers add iodine to many commercial breads and milk products. Still, iodine consumption has declined considerably since the late 1970s.

The reason for the decline may have to do with changing eating habits. For example, many specialty salts, including kosher salt and sea salt, contain no iodine. Nor do fresh bakery breads. Processed foods are high in salt, but most processed foods do not contain iodized salt.

On my website, AskDoctorK.com, I’ve put a table showing the recommended dietary allowance (RDA) for iodine for different age groups. You need to consume about two grams of iodized salt per day to meet the RDA for adults. But only if you’re not consuming other food sources of iodine — and I’d bet that you are.

So I think it’s unlikely that your use of kosher salt instead of the iodized salt most people sprinkle on their food will cause you any problems. If you are really concerned, ask your doctor. He or she can examine your thyroid gland to see if it is enlarged. Simple blood tests can tell if you have hypothyroidism.

Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.