BACKGROUND
Most patients with thyroid cancer are low risk for recurrence of their cancer after surgery so do not require additional therapy. For patients with intermediate or high-risk thyroid cancers, radioactive iodine therapy is an option to decrease risk of recurrence. In order for radioactive iodine therapy to work, the remaining thyroid tissue needs to be “turned on” to take up the radioactive iodine and then to be destroyed. This is done by stimulating the remaining thyroid tissue with TSH, either by making the patient hypothyroid or treating with Thyrogen™ (synthetic TSH).

To make radioactive iodine therapy more effective, American Thyroid Association guidelines recommend patients go on a low iodine diet (LID) to limit iodinecontaining food for 1-2 weeks before treatment. A low iodine state is thought to increase radioactive iodine uptake into the remaining thyroid tissue. While 1-2 weeks is recommended, it is not clear what the best length of time is for a LID before radioactive iodine therapy. Some studies did not show significant difference in the effectiveness of radioactive iodine therapy between 1-week and 2-week LID courses, while another study found that 1-week of LID did not result in enough decrease in iodine level in the urine.

Urinary iodine excretion (UIE) rate is commonly used to assess iodine status after LID, since iodine is excreted through urine and UIE reflects recent iodine intake. This study compared the UIE rates at 4 days and at 7 days after starting LID to assess to best duration of a LID before radioactive iodine therapy for thyroid cancer.

THE FULL ARTICLE TITLE
Dekker BL et al 2022 Low-iodine diet of 4 days is sufficient preparation for 131I therapy in differentiated thyroid cancer patients. J Clin Endocrinol Metab 107(2):e604–e611. PMID: 34534327.

SUMMARY OF THE STUDY
A total of 65 patients with intermediate risk thyroid cancer (63% women, 62% with papillary thyroid cancer) scheduled for radioactive iodine therapy were recruited from 2 hospitals in the Netherlands. All patients were asked to follow a LID with less than 50 micrograms of iodine a day for 7 days. Urine over 24-hour was collected on days 4 and 7 of the LID to calculate UIE in micrograms of iodine a day (mcg/day).

The average UIE was 36.1mcg/day at day 4 and 36.5mcg/ day at day 7, not significantly different between the two time points. Using 50mcg/day as a target, 72.1% of patients reached the target by day 4 and 82% of patients reached the target by day 7, although this was not significantly different. The number of patients who followed a LID did not differ between the group of patients who reached UIE target of 50mcg/day and those who did not.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
The authors of this study concluded that there were no significant differences in UIE between 4-day and 7-day preparation with a LID prior to radioactive iodine therapy for thyroid cancer. Since a LID can be quite restricting for patients, a shorter duration is preferable. Indeed, this study suggests that a 4-day LID may be fine. While this data is promising, more studies with more patients are needed to establish the best duration of a LID , and its impact on the risk of thyroid cancer recurrence.

— Sun Y. Lee, MD, MSc

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