Relationship of 99mTc-Pertechnetate and 131I thyroid uptake in patient with hyperthyroidism

Abstract

Background : Measurement of the thyroid uptake is usually performed with ¹³¹I to determine therapeutic dose in patients treated hyperthyroidism with ¹³¹I. ^99mTc-pertechnetate is also trapped by the thyroid gland in a manner and to a degree very similar to iodine. This study may help identify patients who have discordance in diagnostic and radioactive iodine uptake (RAIU), for better management and treatment results. The thyroidal uptake performed by 99mTc-pertechnetate takes 30 minutes shorter than that performed by ¹³¹I for 2 days.

Objective : To study the thyroidal uptake by ^99mTc-pertechnetate in relation to RAIU in the same patients and to estimate RAIU at 24hr from ^99mTc-pertechnetate uptake.

Design : Prospective and descriptive study.

Setting : King Chulalongkorn Memorial Hospital.

Materials and Methods : Two hundred and six patients who perform RAIU were enrolled into the study. Images were acquired with LEHR collimator of gamma camera system. Images of the syringe were obtained both before and after radiopharmaceutical injection; the image on the injection site was obtained to check for subcutaneous infiltration. The images for anterior thyroid at 5 minutes and 20 minutes after injection were performed. ^99mTc- pertechnetate uptake was calculated from thyroid scan image.

Results : The study showed that RAIU at 24hr were correlation with ^99mTcpertechnetate thyroid uptakes at 5 and 20 minutes as the following equations: Estimated 24hr RAIU = 14.863 *ln (Pertechnetate Uptake at 5min) + 36.741; R² = 0.744 Estimated 24hr RAIU = 14.441 *ln (Pertechnetate Uptake at 20min) + 34.314; R² = 0.708 ^99mTc uptakes at 20min were mostly greater than uptakes at 5min but ^99mTc uptake at 5min showed better correlation than the uptake at 20min. RAIU at 3hr is used to detect early uptake especially in case of rapid turnover and can predict estimated 24-hour Iodine uptake as equation: Estimated iodine uptake at 24hr = 27.43 *ln (iodine uptake at 3hr) – 34.233; R² = 0.85

Conclusion : ^99mTc-pertechnetate uptake can be used to estimate 24hr iodine uptake but it depends on the acquisition and calculation techniques. This is an alternative method to determine RAI dose.