Correspondence

Excess Iodine from an Unexpected Source

N Engl J Med 2009; 360:424-426January 22, 2009

Article

To the Editor:

It is standard practice to induce a decrease in the body's iodine stores before treatment with radioiodine for thyroid cancer. Such patients should avoid an iodine-rich diet,1,2 and physicians should be aware that some over-the-counter supplements contain iodine.

A 55-year-old man had a large mass on the right side of his neck, which was identified in a specimen from a fine-needle aspiration biopsy as papillary thyroid carcinoma. He had a total thyroidectomy, with a modified dissection of the right side of the neck. Pathological examination revealed a multifocal, papillary thyroid carcinoma, the largest being 1.8 cm in diameter, with capsular and vascular invasion but no extrathyroidal extension. Metastases were found in 26 of 78 lymph nodes.

Ablation with radioiodine was recommended, but the patient had undergone computed tomographic (CT) scanning with an iodine-rich contrast agent 2 months earlier. To determine whether the level of urinary iodine was elevated, a random urinary iodine concentration was obtained, and the level was found to be elevated to 394 μg per liter (reference range, 42 to 350; Mayo Medical Laboratory, Rochester, Minnesota). The patient reported having ingested a large number of supplements (Table 1Table 1Iodine Content in the Patient's Dietary Supplements.), but he had reviewed the labels and found no obvious sources of iodine. Consequently, the elevated level of urinary iodine was believed to be due to the use of the contrast agent used during CT. A 24-hour urine collection was then obtained, and the iodine content was elevated to 363 μg (optimal level, 94 to 360) according to spectrophotometric measurement in our laboratory using a method modified by Benotti et al.3 After 10 days on furosemide (20 mg daily) and a low-iodine diet, the 24-hour urinary iodine excretion remained elevated to 370 μg. The iodine content of the 22 supplements the patient had been taking was then measured (Table 1).

The selenium supplement was the source of the excess iodine (which is found in kelp, an inactive ingredient in the supplement). The patient stopped taking all supplements for 8 weeks and maintained a regular diet, and his 24-hour urinary iodine excretion returned to a normal level (192 μg). He did not resume taking the supplements and was again placed on a low-iodine diet for 4 weeks, and the 24-hour urinary iodine excretion fell to a low level of 36 μg.

Physicians who use radioiodine are aware that excess exogenous iodine impairs the efficacy of iodine-131 therapy. In our patient, excessive iodine intake came from an unexpected source — kelp-enriched selenium. The iodine content of kelp is highly variable, and it is likely that the iodine levels in different batches of kelp-enriched selenium are also highly variable.4 Although it is not routine to assess urinary iodine concentrations in patients receiving radioiodine, we emphasize the need for physicians to review patients' medications and dietary habits carefully to prevent ingestion of hidden sources of iodine.

Seth M. Arum, M.D.
UMass Memorial Medical Center, Worcester, MA 01655

Xuemei He, M.D.
Lewis E. Braverman, M.D.
Boston University Medical Center, Boston, MA 02118
lewis.braverman@bmc.org

Supported by funding from the Endocrine–Thyroid Research Fund at Boston Medical Center.

4 References

1. 1

   Guljord L, ed. Low-iodine cookbook. 6th ed. New York: Thyroid Cancer Survivors' Association, 2007. (Accessed January 5, 2009, at http://www.thyca.org/Cookbook.pdf.)
   

2. 2

   Pearce EN, Pino S, He X, Bazrafshan HR, Lee SL, Braverman LE. Sources of dietary iodine: bread, cows' milk, and infant formula in the Boston area. J Clin Endocrinol Metab 2004;89:3421-3424
   CrossRef | Web of Science | Medline

3. 3

   Benotti J, Benotti N, Pino S, Gardyna H. Determination of total iodine in urine, stool, diets, and tissue. Clin Chem 1965;11:932-936
   Web of Science | Medline

4. 4

   Teas J, Pino S, Critchley A, Braverman LE. Variability of iodine content in common commercially available edible seaweeds. Thyroid 2004;14:836-841
   CrossRef | Web of Science | Medline

Citing Articles (1)

Citing Articles

1. 1

   Bruce Rosenzweig. 2010. Kelp and Thyroid Function. , 125-134.
   CrossRef