This study evaluated biokinetic behavior of radioiodine in the bodies of ten female adult patients, with well-differentiated thyroid cancer, treated with 131I post-near total thyroidectomy, for ablation of remnant thyroid. In vivo and in vitro bioassay analyses were performed from the first hour following radioiodine administration until minimum detection limits were reached. The retention of 131I in the body from day 1 to day 6 after the intake may be mathematically represented by an exponential decreasing curve, with an average biological half-life of approximately 0.81 d, with the exception of patients who presented thyroiditis. From day 6 to day 13, urinary excretion rates indicated an increased liberation of iodine. After 2 wk, the body retention of iodine followed an exponential decrease, with a half-life of about 15 d. The average whole-body dose for these patients was 0.27 Gy, as estimated through cytogenetic techniques.