Stability of RAIU in pre-therapy patients

Objectives Radioactive iodine uptake (RAIU) plays a central role in 131I-NaI dose calculation for hyperthyroidism. Current guidelines recommend use of recent (≤1 month) values to calculate therapy. We wish to evaluate stability of RAIU in patients with hyperthyroidism and identify factors that might predict variation of RAIU over time.

Methods 56 patients with hyperthyroidism (32 Grave’s disease, 18 toxic multinodular goiter, 6 solitary adenoma) underwent repeat RAIU measurements within 8-52 weeks without intervening radioiodide therapy. Demographic information, etiology, gland size, and history of medical and radioiodide therapy were recorded.

Results Interval between RAIU repeat measurements was 8-52 weeks (average 25.2 weeks); RAIU ranged from 6-80% (average 44.9%). Absolute and relative RAIU differences between measurements ranged from 0-38% (average 11.6%) and 0 to 1.43 (average 0.296) respectively (SEM in our lab is 1%). There was no significant correlation between relative RAIU difference and time (R2=-0.02, p=0.88). In 49 patients, relative RAIU differences were ≤0.50, an accepted variation in treatment dosing. 7 patients had relative RAIU difference >0.50; all had small glands (Wt ≤30g) and low RAIU (≤35%). Interval between RAIU measurements in these patients was 13-39 weeks (average 21.7 weeks). Using a F-test to compare standard deviation, we show that fractional Δ RAIU variation was greater in patients with small vs large thyroids (F(41,13)=7.74, p=0.0003) and in those with low vs high initial RAIU (F(20,34)=13.0, p<0.0001).

Conclusions Variation of RAIU over 8-52 weeks is relatively constant and does not correlate with intervening interval, averaging 11.6%, with greater relative variability in patients with lower uptake and smaller glands. These results suggest that in patients with large thyroids and elevated RAIU, RAIU can be considered clinically stable and usable to calculate therapy over the course of several months.