A case of massive accidental triiodothyronine intoxication (1000-fold the usual therapeutic dose, for 8 days) is reported with important disturbances of cardiovascular and central nervous systems that required intensive care support. Serum free triiodothyronine levels were 4789 pmol L(-1) on admittance (normal values, 3.5-6.5 pmol x L(-1)). In the absence of a specific treatment, hemoperfusions were performed but failed to accelerate significantly the decay of blood levels of free triiodothyronine (apparent half-life 25.9 hours; 95% confidence interval: 19.8-37.4 hours). The patient, a young woman, made a satisfactory recovery, in spite of important clinical complications.