In 22 patients with hepatic or renal insufficiency the serum concentrations of 
trijodothyronin, thyroxine and thyrotropin and also the T4-binding capacity of 
TBG were determined. The mean serum T3 concentration was found to be 
significantly lower in patients with hepatic coma when compared with euthyroid 
subjects. In the cases of renal insufficiency the serum T3 concentrations were 
in the normal range. Due to hormone loss through dialysis however, the mean 
value of the T3 concentrations was slightly lower than the average concentration 
of normal subjects. The obtained results agree with those of our earlier studies 
which showed that there are significant differences between liver artery and 
vein T3 concentrations in serum, whereas no such differences could be 
ascertained between serum concentrations in renal artery and vein. On the basis 
of these findings it is assumed that conversion of T4 into T3 occurs 
predominantly in the liver.