In some patients, the residual thyroid gland is not completely removed by a single nail clearing treatment. This is usually 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 large 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 can also assist in determining whether nail clearance is complete. For those who have residual thyroid tissue after the first nail clearance, a second nail clearance can 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 have concluded that if no abnormal extra-thyroidal 131I uptake is seen on Rx-WBS after the first nail clearance in such patients, if serum Tg is continuously monitored at <1ng/mL on ambulatory monitoring, and if there are no significant abnormalities on neck ultrasound, then re-clearing is not necessary.