Shatney and coworkers, in 1984, reported that recent thyroidectomy in dogs improved survival following hemorrhagic shock induced by reservoir bleeding to a mean pressure (MAP) of 60 torr for 60 min. Since thyroidectomy reduces preshock MAP, their control dogs had a relatively greater fall in MAP during hemorrhage. To circumvent this inequity, this study was designed to elucidate the effects of recent thyroidectomy in 20 splenectomized dogs subjected to acute hemorrhagic shock induced by phlebotomy of a predetermined volume of blood independent of resultant MAP. During splenectomy, ten dogs had thyroidectomy with parathyroid preservation and ten dogs had sham neck operation; 12 days later, hemorrhagic shock was induced by the stepwise bleeding of 35 ml/kg body weight over 75 min. Parameters measured at preshock (baseline), postshock (PS), postresuscitation (PR), and day 2 included MAP, cardiac output (CO), wedge pressure (PCWP), T3, T4, TSH, and hematocrit (Hct). Hypothyroidism was confirmed by low T3 and T4. Baseline and PS hemodynamic parameters were similar for both groups; PR data showed a significant rise in CO (6.7 liter/min vs 3.9 liter/min) and a fall in PCWP (13.2 mmHg vs 18.6 mmHg) in the euthyroid dogs compared to the hypothyroid dogs. Increased CO persisted through day 2 in the euthyroid dogs (3.14 liter/min vs. 1.95 liter/min). Four thyroidectomized dogs died during shock compared to one control dog. Contrary to prior data, this study shows that recent thyroidectomy reduces the survival and compensatory response to hemorrhage. The mechanism leading to this impaired response needs to be defined.