Reply: Pregnancy Outcome After ¹³¹I Therapy

REPLY: We were interested to see the letter of Chrissa Sioka and Andreas Fotopoulos about our article (1). In addition to reporting results similar to ours, showing that ¹³¹I therapy is safe regarding subsequent pregnancy outcome, with no increase in the risk of miscarriage, induced abortion, or congenital malformation, they added new data showing that ¹³¹I therapy probably increases the incidence of menstrual cycle abnormalities (2).

To confirm these results, we analyzed the responses given by women in our series to similar questions. Of 2,190 women questioned about cycle abnormalities before and after their cancer and followed at least 2 y, we excluded 36 in whom another cancer had developed before thyroid cancer, 158 in whom another malignancy later developed, 263 who received external radiotherapy for thyroid cancer, and 137 who were treated with radioiodine for distant metastases. Of the remaining 1,866 women, 1,054 were diagnosed with thyroid cancer at age 45 y or younger; 287 of these reported cycle abnormalities before the diagnosis, and 767 had no cycle abnormalities before the diagnosis. Of these 767 women, 326 received at least 1 radioiodine treatment with 3.7 GBq, and 441 did not. The majority of women were interviewed more than 2 y after radioiodine treatment. The proportion of women who reported cycle abnormalities after thyroid cancer was not significantly higher among women who were treated with ¹³¹I (n = 34, 10%) than among those who were not treated with ¹³¹I (n = 41, 9%). In a multivariate logistic regression taking into account year and age at menarche, at diagnosis, and at interview; weight; and smoking habit, we did not observe any increased risk of cycle abnormalities after thyroid cancer among women who had received ¹³¹I therapy (odds ratio, 1.2 [95% confidence interval, 0.7–1.2]).

As a final note, we were not able to confirm the results of Souza Rosário et al. (3), who reported transient abnormalities after ¹³¹I therapy. However, our inability to confirm those results may be related to the long delay between treatment and interview in our patients, most of whom were not interviewed until more than 2 y after radioiodine treatment.