Background - Ultra High Dilutions (UHD) are diluted beyond the Avogadro limit with dynamization (dilution with succussion). The process of anuran amphibian metamorphosis is controlled by thyroid hormones, including the resorption of the tadpole tail.Methods - A randomized and blinded study was performed to investigate the influence of triiodothyronine (T3) 5 · 10−24 M (10cH) on apoptosis induced by T3 100 nM in Rana catesbeiana tadpoles’ tail tips, in vitro. Explants were randomized to three groups: control: no T3 in pharmacological or UHD dose; test: T3 100 nM and challenged with T3 10cH (UHD); positive control: T3 100 nM, treated with unsuccussed ethanol. The apoptotic index and the area of explants of test and control groups at the first and final day of the experiment were compared by t-test.Results - There was no difference in tail tip area between test and control groups, but a significantly higher (p < 0.01) index of apoptosis in explants of the test group.Conclusion - This data suggest that T3 10cH modifies the effect of T3 at pharmacological dose, opening new perspectives for further studies and investigation of the dose–effect curve.