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.