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Chromosome Translocation Frequency after Radioiodine Thyroid Remnant Ablation: A Comparison between Recombinant Human Thyrotropin Stimulation and Prolonged Levothyroxine Withdrawal

Departments of Surgical and Biomedical Sciences (A.F., M.V.D., P.M., G.F.), Morphological and Biomedical Sciences (E.D., F.B., M.F.), and Pathology (C.C.), University of Verona, 37129 Verona, Italy; Department of Internal Medicine (A.D., F.M.), University of Pisa, 56126 Pisa, Italy; and Health Physics Service (C.T.), S. Chiara Hospital, 56126 Pisa, Italy

Address all correspondence and requests for reprints to: Marco Ferdeghini, M.D., Department of Morphological and Biomedical Sciences, University of Verona, 37129 Verona, Italy. E-mail: marco.ferdeghini{at}univr.it.

Background: Thyroid remnant ablation of differentiated thyroid carcinoma (DTC) patients is traditionally performed after levothyroxine withdrawal. Recombinant human TSH (rhTSH) administration increases serum TSH levels without inducing hypothyroidism.

Aim: The aim of the study was to investigate the frequency of chromosome translocations in DTC patients after the first ¹³¹I therapeutic dose and compare the frequency of translocations between DTC patients off levothyroxine and those receiving rhTSH.

Patients and Methods: A total of 20 DTC patients were randomly assigned to levothyroxine withdrawal [(30 d) group A; n=10, nine women; mean age 48.5± 19.2 yr] or rhTSH injections [(0.9 mg im per 2 consecutive days) group B; n=10, eight women; mean age 50.4± 18.8 yr] before undergoing ¹³¹I activity (3.7 GBq). The frequency of translocations in peripheral lymphocytes was analyzed by tricolor fluorescence in situ hybridization with whole-chromosome-specific probes for chromosomes 1, 4, and 8. Lymphocytes were stained routinely (about 500 each time).

Results: The two groups showed similar baseline translocation frequency. After ¹³¹I administration, the total chromosomal translocation rate was significantly lower in group B than group A (P = 0.02). The frequency of translocations increased significantly in group A only (P = 0.01 vs. baseline). Rearrangement specifically involved chromosomes 4 and 8 (P = 0.02 vs. baseline).

Conclusions: Our preliminary data show that in hypothyroid status ¹³¹I ablation therapy induces a higher translocation rate, especially in chromosomes 4 and 8. This finding, in agreement with previous dosimetric reports, suggests that whereas inducing a low extrathyroid exposure, rhTSH reduces the potential risk of chromosomal aberration associated with blood irradiation.