[Excessive peripheral conversion of thyroxine (T4) to triiodothyronine (T3) in the pathogenesis of T3-hyperthyroidism (author's transl)].

In a 41-year-old woman and a 3-year-old girl, both of them with T3-thyrotoxicosis, serum levels of total and free T4 and T3 were measured serially during anti-thyroid drug treatment. Attempts to substitute thyroxine during the antithyroid treatment had to be interrupted because the patients became hyperthyroid again with excessive increases in total and free serum T3, even when concentrations of total and free T4 were brought to subnormal levels. The increased conversion of administered thyroxine ceased later on and higher amounts of oral T4 were tolerated after one year of treatment. In both patients there was an extremely low serum T4/T3 ratio, differing in this respect significantly from six other patients with T3-thyrotoxicosis and 41 patients with "conventional" T3/T4-hyperthyroidism. It is concluded that, in patients with T3-thyrotoxicosis due to excessive peripheral T4 to T3 conversion, substitution during antithyroid drug treatment should be either with very low doses of thyroxine or with triiodothyronine in divided daily doses. In such cases the level of serum T3 represents the most reliable biochemical measurement for the control of treatment, serum T4 levels being irrelevant.