Significance of multiple uptake measurements in I-123 thyroid protocols

Objectives To determine if a 6 hour standard uptake measurement for I-123 thyroid protocols correctly correlates with the standard 24 hour uptake.

Methods Percent uptake values were reviewed from 98 patient charts using the hospital’s electronic medical records. Analysis was based on 6 hour thyroid uptake alone versus the standard protocol of using both 6 and 24 hour uptakes. In standard protocol, normal 6 hour uptake values have a range between 7-15%. Percentages less than 7 demonstrate hypothyroidism; whereas uptakes greater than 15% indicate hyperthyroidism. Normal 24 hour uptake values have a range of 10-30%. Hypothyroidism is considered any percentage less than 10; whereas uptake values greater than 30 indicate hyperthyroidism.

Results Greater variability was demonstrated within the 6 hour uptake compared to the 24 hour uptake. Patient uptake measurements at 6 hours ranged between 5-28% for euthyroid, less than 1-6% indicating hypothyroidism, and 6-86% for hyperthyroidism. At 24 hours, the uptake values ranged from 11-36% for euthyroid, less than 3-7% indicating hypothyroidism and 11-90% for hyperthyroidism. At 6 hours, 24 out of the 98 patient’s uptakes did not correlate with their diagnosis when compared with the standard protocol ranges. There was a rapid turnover time for 3 of the 53 hyperthyroid patients who had a higher uptake at 6 hours than at 24.

Conclusions The 6 hour uptake did not correlate with the 24 hour standard uptake in diagnosing thyroid disease. Values best correlate with final diagnosis using an optimal measurement time of 24 hours. For patients who encountered a rapid turnover, a 6 hour uptake can be beneficial for determining I-131 treatment dose.