1. Serum creatine kinase and oral temperature were measured in 20 patients with primary hypothyroidism before and after 48 h bed rest. Fifteen of these patients were heated during the 48 h period. The remaining five acted as control subjects. In addition, creatine kinase and oral temperature were measured in five control subjects after a 30 min period of exercise and again after a 30 min period of resting.
2. The oral temperature rose and the serum creatine kinase levels fell only in those patients who were actively warmed. In the control subjects the period of exercise followed by resting caused no significant change in creatine kinase levels.
3. A subnormal body temperature appears to be an important determinant of the raised serum creatine kinase levels seen in patients with primary hypothyroidism.