Periods of L-T4 withdrawal and low iodine diet, which are required prior to the treatment and tracking tests that take place after a thyroidectomy, can be of a long duration and cause suffering for patients. The purpose of this study, conducted in South Korea, was to confirm if periods of L-T4 withdrawal and low iodine diet can be shortened by using TSH level prediction. By inquiring into the correlation between TSH level and serum Tg level, and measurement of the amount of iodine present in urine during the low iodine diet period, a thyroglobulin (Tg) cutoff level can be predicted.

total of 168 patients were included as research subjects. In each case, L-T4 was suspended 3-4 weeks prior to conducting radioactive iodine ablation and 131I scan, and then a low iodine diet was carried out for 2-4 weeks. Serum TSH, Tg and anti-Tg antibodies were measured on the second week of L-T4 withdrawal, and the spot urine Iodine/Creatinine ratio was measured on the second and fourth week after treatment.

Three weeks after L-T4 withdrawal, TSH levels increased to over 30μIU/ml in 97.2% of the patients, and serum thyroglobulin levels also increased with TSH level to over 30μIU/ml. There was no measured difference in the amount of iodine in the subject's urine during the low iodine diet period.

It was found that L-T4 withdrawal can be reduced to 3 weeks or less, and that the Tg cutoff level differs according to TSH level. Based on these results, we suggested that the low iodine diet period can be reduced to 1-2 weeks.