Thyroid uptake measurements with I-123: problems and pitfalls: concise communication.

The measurement of radioiodine uptake is generally considered to be straight-forward and accurate. However, during the past two decades, discrepancies in "normal" thyroid uptake values have been noticed between Montefiore Hospital and Medical Center and the Hospital of Albert Einstein College of Medicine. These differences were attributed to differences in patient population. Further investigation revealed that the persisting uptake discrepancies arose from neck-phantom differences and variations in procedure. Differences in presumably standardized neck-phantom attenuation characteristics have been shown to cause large variations in count rates from I-123 and I-131 standard capsules. The effect of high-voltage fluctuation on phantom count rates is more pronounced with I-123 than with I-131. In constant levels of high-energy contaminants in I-123 also affect the uptake measurements. large errors in the measurements of thyroid uptake values may result from seemingly unimportant variations in technique. A stable high-voltage power supply, precise high-voltage adjustment, careful selection of energy window, and the use of a standardized neck phantom with generally accepted attenuation characteristics are absolutely essential if RAIU values are to be compared and appropriate therapeutic doses are to be administered based on these measurements.