Objective: The purpose of this study was to retrospectively examine the correlations of hormonal ratios with radioiodine I-131 therapeutic parameters and the potentiality of prescribing the therapeutic I-131 target dosage for an individual patient with Graves' disease using baseline serum levels of thyroid hormones.
Materials and methods: Serum T3, T4, and FT4 levels 6 and 12 months after I-131 therapy/baseline levels (hormonal ratios) were calculated for a total of 68 therapeutic courses in 57 patients with Graves' disease. The therapeutic parameters were absorbed dose (Gy), dose concentration (microCi/g) and oral dose (mCi). Linear regression analysis was performed for correlating hormonal ratios (X) and therapeutic parameters (Y).
Results: Significant (P<0.05) negative correlations of the hormonal ratios were observed with absorbed dose (R -0.50 for T3, -0.61 for T4, and -0.46 for FT4 at 6 months, and -0.29 for T3, -0.44 for T4 at 12 months) and dose concentration (R -0.57 for T3, -0.58 for T4, and -0.49 for FT4 at 6 months and -0.27 for T3, -0.27 for T4 at 12 months), but not with oral dose at 6 months and 12 months or the absorbed dose and dose concentration for FT4 at 12 months. The correlations were higher at 6 months than at 12 months and in serum T4 than in serum T3 and FT4. The formulae for serum T4 at 6 months were as follows: Y (Gy)=109-53X and Y (microCi/g)=109-52X.
Conclusions: These results suggest that the hormonal ratios are significantly correlated with the absorbed dose and dose concentration. The formulae for serum T4 at 6 months may serve to prescribe the individual oral dosage for Graves' disease, although the correlation coefficients are about -0.6.