Iodine: a lot to swallow

Townsend Letter for Doctors and Patients, August-Sept, 2005 by Alan R. Gaby

Recently, a growing number of doctors have been using iodine supplements in fairly large doses in their practices. The treatment typically consists of 12 to 50 mg per day of a combination of iodine and iodide, which is 80 to 333 times the RDA of 150 mcg (0.15 mg) per day. Case reports (1,2) suggest that iodine therapy can improve energy levels, overall wellbeing, sleep, digestive problems, and headaches. People with hypothyroidism who experienced only partial improvement with thyroid hormone therapy are said to do better when they start taking iodine. (3) In addition, fibrocystic breast disease responds well to iodine therapy, an observation that has been documented previously. (4) The reported beneficial effects of iodine suggest that some people have a higher-than-normal requirement for this mineral, or that it favorably influences certain types of metabolic dysfunction.

While iodine therapy shows promise, I am concerned that two concepts being put forth could lead to overzealous prescribing of this potentially toxic mineral. First is the notion that the optimal dietary iodine intake for humans is around 13.8 mg per day, which is about 90 times the RDA and more than 13 times the "safe upper limit" of 1 mg per day established by the World Health Organization. Second is the claim that a newly developed iodine-load test can be used as a reliable tool to identify iodine deficiency.

Is the optimal human requirement 13.8 mg per day?

The argument, developed by one investigator, (5) that the optimal human iodine intake is around 90 times the RDA is based mainly on two points. The first point is that the average iodine intake of adults living in Japan is 13.8 mg per day, and the Japanese are among the healthiest people in the world, with low rates of cancer. The second point is in regard to the amount of oral iodine that it takes to saturate the thyroid tissues.

The idea that Japanese people consume 13.8 mg of iodine per day appears to have arisen from a misinterpretation of a 1967 paper. (6) In that paper, the average intake of seaweed in Japan was listed as 4.6 g (4,600 mg) per day, and seaweed was said to contain 0.3% iodine. The figure of 13.8 mg comes from multiplying 4,600 mg by 0.003. However, the 4.6 g of seaweed consumed per day was expressed as wet weight, whereas the 0.3%-iodine figure was based on dry weight. Since many vegetables contain at least 90% water, 13.8 mg per day is a significant overestimate of iodine intake. In studies that have specifically looked at iodine intake among Japanese people, the mean dietary intake (estimated from urinary iodine excretion) was in the range of 330 to 500 mcg per day, (7,8) which is at least 25-fold lower than 13.8 mg per day.

The other argument being proposed to support a high iodine requirement is that it takes somewhere between 6 and 14 mg of oral iodine per day to keep the thyroid gland fully saturated with iodine. Whether or not that is true, it is not clear that loading the thyroid gland or other tissues with all the iodine they can hold is necessarily a good thing. Since emerging from the iodine-rich oceans to become mammals, we have evolved in an iodine-poor environment. Our thyroid glands have developed a powerful mechanism to concentrate iodine, and some thyroid glands (or other tissues) might not function as well after a sudden 90-fold increase in the intake of this mineral. As I will explain later, relatively small increases in dietary iodine intake have been reported to cause hypothyroidism or other thyroid abnormalities in some people.

It has also been observed that iodine supplementation promotes the urinary excretion of potentially toxic halogens such as bromide and fluoride. While that effect might be beneficial for some people, it is not clear to what extent it would shift the risk-benefit ratio of megadose iodine therapy for the general population.

Is the iodine-load test valid?

For the iodine-load test, the patient ingests 50 mg of a combination of iodine and iodide and the urine is collected for the next 24 hours. The patient is considered to be iodine-deficient if less than 90% of the administered dose is excreted in the urine, on the premise that a deficient person will retain iodine in the tissues, rather than excrete it in the urine. According to one doctor who uses the test and a laboratory that offers it, 92% to 98% of patients who have taken the iodine-load test were found to be deficient in iodine.

However, the validity of the test depends on the assumption that the average person can absorb at least 90% of a 50-mg dose. It may be that people are failing to excrete 90% of the iodine in the urine not because their tissues are soaking it up, but because a lot of the iodine is coming out in the feces. There is no reason to assume that a 50-mg dose of iodine, which is at least 250 times the typical daily intake, can be almost completely absorbed by the average person. While this issue has not apparently been studied in humans, cows fed supraphysiological doses of iodine (72 to 161 mg per day) excreted approximately 50% of the administered dose in the feces. (9)