[Reference values for thyroid 24 hours 123I uptake]

To establish references for thyroid 24 h 123I uptake value (TUV), we reviewed 475 patients who had free T4 and TSH dosages, and TUV measured 24h after the administration of 123I. The patients were separated in 3 groups: hyperthyroid, euthyroid and hypothyroid, according to thyroid hormone results. The mean and standard deviation (SD) of the TUVs were calculated for each group. We used the unpaired T test to compare the means and ROC curves to evaluate the best TUV to separate the groups. The mean and SD of TUVs for the groups were: hyperthyroid (36.5% +/- 16.5%), euthyroid (14.4% +/- 5.48%) and hypothyroid (13.7% +/- 7.5%). The unpaired T test showed statistically significant difference between hyperthyroid and euthyroid groups (p < 0.01). There was no significative difference between hypothyroid and euthyroid groups (p = 0.55). The 23% TUV has the best sensitivity (80%)/specificity (93%) ratio to separate hyperthyroid from euthyroid groups. There was not a good TUV to separate euthyroid form hypothyroid groups. In conclusion, it can be established 23% as the optimal superior limit for the normal uptake values. However, it was not possible to establish a suitable inferior limit, due to the expressive overlay of TUVs of hypothyroid and euthyroid groups.