[T3-hyperthyroidism under the aspects of secretion and peripheral production of thyroid hormones (author's transl)]

In contrast to T4/T3-hyperthyroidism, where T4, T3, and r-T3 serum concentrations (SC) are elevated, in T3-hyperthyroidism the T3-SC alone is increased, whereas T4-SC is normal and r-T3 may be decreased. Endogenous labelling of T4 with 131I is increased in T3-hyperthyroidism similarly as in T4/T3-hyperthyroidism, clearly reflecting an increased thyroidal formation of T4 in T3-hyperthyroidism. The contradiction of a normal SC to T4 in the presence of enhanced thyroidal formation of T4 may be explained by increased peripheral T4-deiodination. Since in T3-hyperthyroidism T3-SC are elevated whereas r-T3-SC are decreased, there is good evidence for a shift in T4 conversion.