In some patients, residual thyroid is not completely removed by a single nail clearing treatment [58]. Most often, this is due to high residual thyroid tissue prior to nail clearing treatment, or inadequate uptake of 131I by residual thyroid tissue and DTC lesions (mostly due to the presence of high amounts of stable iodine in the body), or insufficient 131I dose used for nail clearing, or low sensitivity to 131I radiation. After 4-6 months of nail clearing treatment, an assessment of the completeness of nail clearing can be performed. If no thyroid tissue is visualized in the Dx-WBS images after TSH stimulation and the thyroid aspiration rate of 131I is <1%, it indicates complete 131I nail clearing. Serum Tg testing and thyroid ultrasonography may also assist in determining whether nail clearance is complete. If there is still residual thyroid tissue after the first nail clearance, a second nail clearance may be performed to achieve the goal of complete nail clearance. The principles of 131I dose determination are the same as for the first treatment. However, some investigators believe that if no abnormal extra-thyroidal 131I uptake is seen on Rx-WBS after the first nail clearance in such patients, and if serum Tg continues to be monitored dynamically at <1ng/mL, and if there are no significant abnormalities on neck ultrasound, then re-nail clearance is not necessary.