Short and Long Term Effects of Radioiodine and Antithyroid Drugs on T4 Binding Proteins, Free T4 and T3, during Graves' Disease Therapy

Abstract

Ninety five patients with Graves' disease were studied before and at three months intervals after antithyroid drugs (ATD) (31 cases) or radioiodine (64 cases) therapy until recovery. Before treatment, the T4 maxima binding capacity of TBPA was significantly decreased 253.5 ± 11.4 µg/100 ml)(mean+ se) (control values: 287 ± 10.4 µg/100 ml) (α = 0.04), especially in 53.7% of patients (m = 177 ± 8 µg/100 ml). The mean of TBG (m = 20.7 ± 0.9 µg/100 ml) was not different from euthyroid subjects (m = 19.7 ± 1.7 µg/100 ml) except in 51.2% of patients who had a low TBG (m = 14.3 ± 1.1 µg/100 ml). An inverse linear correlation was found between TBG-DFT4 (α = 0.05) and DF T 3 (α = 0.02), TBPA-log DF T4 (α = 0.05) but not between TBG and TBPA. The physiological relationship between DFT3, DFT4, TT3, TBG and TBPA was studied in vitro; after adding increased quantities of T4 to a pool of sera collected from eu, hypo or hyperthyroid patients, DFT4, DFT3, FT3 index increased in linear positive relationship with TT4 concentrations, the kinetic of this phenomena was inversely correlated with the T4 maximal binding capacity of TBG or TBPA for T4. Addition of T3 to the same sera did not show any effect on the previous parameters. DFT3 depended on the level of T4 in serum more than T3 concentration and was in inverse relationship with the maximal binding capacity of TBG. This data might explain the paradoxal normal or slightly increased values of DFT3 found in T3 thyrotoxicosis. In patients treated with ATD or radioiodine, TBPA but not TBG increased significantly on year after. However, in subjects with an initial very low TGB or TBPA, this phenomenon occurred on the third month after radioiodine or ATD. During the same period, DF T4 and DF T3 were inversely correlated to TBG and TBPA. In conclusion, important changes in T4 binding proteins and free fractions of thyroid hormones were observed in Graves' disease but were corrected by antithyroid therapy. All these data were in good agreement with the normalisation of thyroid function.

Key words

Graves' Disease - Antithyroid Drugs - Radio-Iodine Therapy - Free T3 and T4 - T4 Binding Proteins