Pathologically low serum total triiodothyronine (T3) concentrations are a characteristic finding in patients with severe non-thyroidal illnesses. No adequate explanation has yet been offered for this phenomenon. We have, therefore, investigated the serum concentrations of total thyroxine (T4), total T3 and total 3,3',5'-triiodothyronine (reverse T3)--the metabolically-inactive metabolite of thyroxine--and of TSH in 13 patients with acute myocardial infarction, in 12 patients with compensated liver cirrhosis, in 9 patients with decompensated liver cirrhosis and in 15 patients with chronic renal failure on chronic intermittent haemodialysis by radioimmunoassay. The values obtained were compared to corresponding values of a normal control group (n - 23). According to our results the decrease in serum T3 combined with normal T4 concentrations in severe non-thyroidal illnesses seems to be a consequence of an alteration in thyroxine degradation. Two different possibilities of alteration can be considered: 1. inhibition of the overall deiodinationof T4, leading to low total T3 serum concentrations with concomitant normal to low reverse T3 serum concentrations (chronic uraemia), 2. a shift in the monodeiodination of T4 towards enhanced reverse T3 production, leading also to low total T3 concentrations, but with a concomitant increase in reverse T3 serum concentrations (myocardial infarction, liver cirrhosis). The results obtained in our patients with liver cirrhosis show, moreover, that this alteration of T4 metabolism depends on the severity of the illness.