Assuming that the fractional uptake is the same, both after the administration of a diagnostic and a therapeutic activity, 131I uptake too low to be detected with 2-5 mCi may become detectable after the administration of 100 mCi. This should be performed routinely in patients with thyroglobulin levels above approximately 5 ng/mL during L-Thyroxine (LT4) treatment or 10 ng/mL off LT4 treatment for three main reasons: 1) in 80% of these patients, a post-therapy 131i total body scan showed foci of uptake in the neck or at distant sites, whereas in the other patients, metastases emerged clinically some years later; 2) 131I is not the only treatment modality, and, for instance, lymph node metastases may warrant further surgery; and 3) from a dosimetric point of view, the relevant parameter is the concentration of 131I, i.e., the ratio between the uptake and the mass of functioning tissue: a low uptake in a small metastasis may result in a higher 131I concentration than a higher uptake in a much larger metastasis.