Radioiodine dilution due to levothyroxine when using recombinant human thyroid-stimulating hormone: case report and discussion.

Following thyroidectomy, a patient with papillary thyroid carcinoma was found to have widespread radioiodine-avid metastatic lesions and was treated with I-131. When follow-up scanning was anticipated, his 24-hour urinary iodine was found to be 254 µg. Because of the interim development of moderate renal failure, he was considered to be a candidate for preparation using recombinant human thyroid-stimulating hormone (rhTSH), with continued administration of thyroid hormone. To decrease the dilution effect of nonradioactive iodine, levothyroxine was replaced with liothyronine, with a resultant decrease in 24-hour urinary iodine to less than 110 µg, followed by radioiodine imaging, dosimetry, and retreatment with I-131 for persistent, though improved disease. The dilution of radioiodine with nonradioactive iodine from any source may degrade image quality and reduce the effectiveness of therapy. The use of rhTSH has advantages in the evaluation and I-131 treatment of differentiated thyroid cancer. There is evidence that this approach results in a longer effective half-time of radioiodine in remnants, improved bone marrow dosimetry, and comparable remnant ablation efficacy as compared with hormone withdrawal. However, it entails continued administration of thyroid hormone, which is a source of nonradioactive iodine. Reduction of the nonradioactive iodine burden of levothyroxine by conversion to liothyronine warrants investigation as possibly enhancing the advantages of rhTSH.

I-123 whole body scanning: case report and discussion.

PURPOSE: In managing differentiated thyroid carcinoma, concern over the stunning that may occur with traditional whole body imaging using I-131 has fostered an interest in using I-123 for this purpose. A case is presented as a focus for discussion of the issues, with special reference to accuracy and the effect thereon of technical factors. CASE REPORT: A 43-year-old man with papillary carcinoma of the thyroid was scanned after thyroidectomy using 10 mCi I-123. Images at 24 and 48 hours demonstrated foci including multiple regional metastases. They were less well demonstrated in images 7 days after therapeutic I-131, and negative imaging 1 year later implied therapeutic success. DISCUSSION: I-131 has the advantages of a longer half-life, facilitating delayed imaging and lower price. I-123 has the advantages of favorable gamma-ray energy and less useless radiation to thyroid tissue and the whole body. Despite its short half-life, with a sufficient amount, images at 48 hours are practical. In the case presented, there are several possible reasons why superior lesion detection occurred in diagnostic I-123 images. In any case, the outcome demonstrates that I-131 posttherapy images are not an absolute "gold standard" for accuracy. Investigations comparing I-123 with I-131 imaging have been subject to biases, including insufficient dose of I-123, lack of delayed I-123 imaging, suboptimal collimation for I-131, and the relatively high dose of I-131 in posttherapy scanning. CONCLUSIONS: The evidence, although not definitive, strongly suggests that I-123 is at least as accurate for diagnostic whole body imaging as I-131.