The Dose of Radioactive Iodine Needed to Ablate the Thyroid Remnant Left Behind After Thyroidectomy

The study participants are randomly allocated to receive either a 1110 MBq or a 3700 MBq dose of radioiodine (131I) approximately 5 weeks after thyroidectomy. Thyroxin substitution is initiated only after administration of radioactive iodine. Treatment efficacy is monitored using serum thyroglobulin measurements and whole body radioiodine scanning. The absorbed radiation dose at the thyroid remnant and the biological half-life of radioactive iodine are measured with SPECT, 131I iodine detector and a Geiger counter. Treatment related adverse events are collected using structured forms 4 to 5 days, 2 weeks and 3 months after administration of radioiodine. The need for a repeat treatment is assessed 4 to 6 months after the first administration of radioiodine. The criteria for a repeat radioiodine treatment are serum thyroglobulin \> 1 ug/L and/or presence of abnormal radioiodine uptake in a whole body radioiodine scanning, which is carried out following a 4-week interruption of thyroxin supplementation or following administration of rhTSH. Number of patients: 160 Aims of the study: * To find out weather the risk for second radioiodine treatment differs with two dose levels of radioiodine: 1110 MBq or 3700 MBq. * To study possible differences in the adverse effects in the treatment groups. Also days at hospital are counted. * To analyse the effect of absorbed radiation dose to the treatment results