Reply: Antithyroid Drugs and Radioiodine and the Absence of Evidence

REPLY: We thank Professor Walter et al. for their kind remarks about our review on the treatment of thyrotoxicosis (1). Their comments concerning the effect of methimazole on radioiodine therapy are acknowledged. They point out their new data opposing their previously published data showing that methimazole did not increase resistance to ¹³¹I (2,3). We are not sure who should receive attribution for the statement that 50% of medical dogma is wrong, the corollary being that if we knew which half we would be in excellent shape.

The ability of antithyroid medications to increase resistance to radiation has been recognized for several decades, but so has the importance of that ability been debated (4). There are those who support that both methimazole and propylthiouracil cause a higher likelihood of failure of ¹³¹I and an increased need to retreat the patient. However, at the time of submission and resubmission of our article (October 30, 2006, and January 2, 2007, respectively), the weight of evidence indicated that propylthiouracil and not methimazole was the culprit (5,6). We hope that Walter et al. do not think less of us for failing to include the reference to their metaanalysis. However, their article was published on March 10, 2007 (3). Were we now writing the review, we would most certainly be including their observations. We would also like to reiterate that we recommend stopping methimazole for 5 d rather than 3 d. The paper that we did cite by Walter et al., from 2006, dealt with the shorter stoppage (2). We also believe that administration of 5.92 MBq (160 μCi) of ¹³¹I per gram to be retained in the gland is sufficient for a single-dose cure even in patients who were pretreated with antithyroid medications. Physicians using smaller doses might consider an upward adjustment.

We would like to take this opportunity to correct an omission in our review. In discussing the use of lithium as an adjuvant to radioiodine therapy, we quoted the results of a study by Bal et al. but did not include the citation. We are sorry for that oversight and have listed the reference below (7).