Answer to Question #2883 Submitted to "Ask the Experts"

MIRD Report 5 (Berman et al. 1988) suggests some intestinal uptake of administered sodium iodide (1.6 % at 24 hours), but it is assumed that all iodine is absorbed eventually and that the kidneys are the organs of excretion.

In W.H. Blahd's text Nuclear Medicine (Blahd 1971), some data on nonradioactive iodine loss through feces is listed on page 187. The authors say, "Recent estimates of the daily fecal iodine loss in normal individuals are 10 to 57 micrograms, representing more than 50% of the total amount of T-4 lost from the serum daily."

In the 6th edition of Werner and Ingbar's The Thyroid (Baverman and Utiger 1991), it is stated on page 370 that "the fecal excretion of iodine is negligible (5 microgram/day)." The reference for the citation is listed as Suzuki H, Higuchi T, Sawa K, et al.: Endemic coast goitre in Hokkaido, Japan. Acta Endocrinol (Copenh) 1965;50:161.

M.T. Hays (1993) reports that "the fraction of fecal radioactivity attributable to iodide was 0.55 +/- 0.35" in seven normal male subjects given repeated daily oral doses of ¹²⁵I iodide. This experiment, however, was with ¹²⁵I labeled thyroid hormone, not ¹²⁵I iodide, which is a significant difference. With organic iodine you will see liver uptake and biliary secretion, but not so with iodide. The fecal route is probably not of any significance with ¹³¹I NaI.

The fecal excretion of iodine comes from the liver breakdown of thyroid hormone and the dumping of the metabolic products into the bile and then into the intestine. When you give ¹³¹I NaI, there is nothing for the liver to break down, which is why virtually all of it is excreted by the kidneys. In post-op thyroid cancer patients in whom there is a remnant of thyroid tissue remaining, it is common to see a hazy "ghost" image of the liver in images performed a week after a therapy dose is administered. This is ¹³¹I that had been organified by the remnant and picked up by the liver for metabolism. However, the half-life of thyroid hormone in the thyroid tissue is long, and by the time most of it comes out, most of the ¹³¹I has decayed away. So very little ¹³¹I gets to the liver at all, and probably most of that iodine is reabsorbed in the gut.

Occasionally, in a very hyperthyroid patient, the uptake of ¹³¹I in the thyroid stays constant over three days after about six hours post-¹³¹I administration. In these patients, organic iodine comes out of the gland and goes to the liver, but the gut reabsorbs all the ¹³¹I which goes back to the thyroid for re-organification.

The only situation in which any significant quantity of ¹³¹I would be excreted in the feces is if the patient has severe renal insufficiency or renal failure and is given ¹³¹I and is NOT given dialysis.

Carol Marcus, PhD, MD

Editor's Note: In the United States, the retention of patient waste is not required by the U.S. Nuclear Regulatory Commission or appropriate state regulations.

• Berman M, Braverman LE, Burke J, de Groot L, McCormack KR, Oddie TH, Rohrer RH, Wellman HN, Smith EM. Report 5. I-123, I-124, I-125, I-126, I-130, I-131, and I-132 as sodium iodide. In: Loevinger R, Budinger TE, Watson EE, et al. MIRD primer for absorbed dose calculation. Society of Nuclear Medicine; 1988.
   
• Blahd WH. Nuclear medicine. 2nd ed. New York, NY: McGraw-Hill Inc.; 1971.
   
• Baverman LE, Utiger RD. Werner and Ingbar's the thyroid: A fundamental and clinical text, 6th ed. J.B. Lippincott; 1991.
   
• Hays MT. Colonic excretion of iodide in normal human subjects. Thyroid, Spring 3(1):31-35; 1993.