Reproducibility of radioactive iodine uptake (RAIU) measurements

Measurement of radioactive iodine uptake (RAIU) is an important aspect of the assessment and treatment of patients with hyperthyroidism. Its uncertainty affects how much of a true change in RAIU can be detected as well as appropriateness of the therapy dosage upon which it is based. In this study, a method of estimating the reproducibility and least significant change (LSC) values for RAIU measurements, and the implications of the values observed are discussed, with emphasis on application to quality assurance initiatives.

We prospectively studied 36 consecutive patients referred for RAIU measurements. Twenty-four hours after oral administration of 370 kBq of ¹³¹ I-NaI in capsule form, RAIU measurements were obtained in duplicate using a thyroid probe uptake system. Assessment of reproducibility was performed using root-mean-square standard deviation.

Average difference between duplicated RAIU measurements in our study cohort was -0.1 ± 1.6% and ranged from -4.8% to 3.1%. Reproducibility of probe-based RAIU measurement was calculated to be 1.1% and 95% LSC was 3.2%.

In our clinic, probe-based RAIU is a reproducible and relatively precise measurement. Using the method we have outlined, each institution can perform reproducibility assessment and compute the LSC of RAIU measurements based on its own staff, iodine isotope, equipment, protocols, and patient population. These values are useful in the assessment of serial change in RAIU, and as more experience is accumulated, can serve as benchmarks to be used in quality assurance initiatives.