Diagnostic and Therapeutic Radioactive Iodine131

Postoperative total body radioactive iodine scanning is recommended for all high-risk patients with DTC as this modality decreases the local recurrence and death rates (Table 6.5). Most low risk patients are also candidates but, selective use is acceptable in low-risk node negative patients with small, occult, minimal, or incidentally discovered differentiated thyroid carcinoma. Figure 6.1 illustrates the protocol for performing a total body radioiodine scan 6-12 weeks after thyroidec-tomy that begins by withholding thyroid hormone medication (LT4) from the patient for approximately 4 weeks. The use of a LT3 hormone preparation such as Cytomel for the first two of these four weeks can alleviate the problems of symptomatic hypothyroidism. The serum TSH level is checked at the end of four weeks and should be greater than 30-50 IU prior to radioisotope administration.2 This is also the best time to establish an accurate baseline for serum thyroglobulin levels for use as a future surveillance tool for these patients. Two to five mCi of 131I is administered and a total-body scan is performed 48 hours later. Ablative doses of 131I, ranging from 100-200 mCi depending on the location and magnitude of uptake, are administered for uptake greater than 2% of the administered dose to destroy any residual or metastatic thyroid carcinoma. The scan is repeated four to seven days after administration of therapeutic radioiodine and thyroid hormone supplementation for TSH suppression is restarted. The total body radioiodine scan is usually repeated in treated patients in 6-12 months after treatment to confirm total ablation of all thyroid tissue. Additional treatment doses of 131I can be administered to a total dose of 500-600 mCi in patients with persistent isotope concentration.9 Recent experience with recombinant TSH stimulating radioiodine uptake and the use of Sestamibi scanning after thyroidectomy for DTC, suggest that cessation of thyroid hormone therapy prior to radioiodine imaging and treatment may be unnecessary in the future.

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