MATERIALS AND METHODS: Twenty consecutive patients with various thyroid pathologies were included (19 females, 19 to 63 years of age, mean 41.1 years). Images of the thyroid in the anterior position of the neck were obtained 20 minutes after intravenous administration of 370 MBq (10 mCi) of ^99mTc-pertecnetate and 2 and 24 hours after oral administration of 30 MBq (800 µCi) of ¹²³I–iodide, using a scintillation camera equipped with a low energy, high resolution parallel hole collimator. Images of the syringe (^99mTc-pertecnetate) and the plastic cup (¹²³I–iodide) were also obtained before and after the administration of the radiopharmaceutical. Calculation of thyroid uptake was described elsewhere, using images counts. The Pearson correlation coefficient was used to compare the ^99mTc-pertecnetate and ¹²³I–iodide thyroid uptake values. Simple linear regression was used to estimate a numeric model for correlation between the two sets of uptakes.

RESULTS: The Pearson correlation coefficients between ^99mTc-pertecnetate at 20 minutes and ¹²³I-iodide at 2 and 24 hours were 0.90 and 0.81, respectively, and the linear regressions were: ¹²³I(log₁₀)= 0.32 + 0.88x^99mTc(log₁₀) and ¹²³I(log₁₀)= 0.93 + 0.56x^99mTc(log₁₀), respectively.

CONCLUSION: Thyroid uptake of ^99mTc-pertecnetate at 20 minutes and ¹²³I-iodide at 2 and 24 hours correlated well. These data suggest the possibility to estimate ¹²³I–iodide thyroid uptake at 2 and 24 hours from the ^99mTc-pertecnetate thyroid uptake at 20 minutes.