This prospective case-controlled study investigated the role of the pituitaryethyroidal axis in the prediction of mortality in dogs. Serum thyrotropin, thyroxine and free thyroxine were measured at admission and daily until death or discharge in dogs ill with parvoviral diarrhoea. Serum thyroxine and free thyroxine in ill dogs were significantly lower than in controls and also significantly lower in nonsurvivors than in survivors on days 1-4. Serum thyrotropin at admission in ill dogs was similar to controls, but was significantly higher in nonsurvivors than in survivors. Thyrotropin concentrations became significantly lower in nonsurvivors vs survivors by day 4.