1. The plasma adenosine 3′:5′-cyclic monophosphate (cyclic AMP) response to intravenously administered glucagon has been studied in nine hyperthyroid, five hypothyroid and ten euthyroid patients.
2. Concentrations of the nucleotide rose to a peak at 15 min and declined to near basal values by 120 min, the response being greatest in hyperthyroid and least in hypothyroid subjects. The mean peak concentrations were 1028 nmol/l in the hyperthyroid group, 252 nmol/l in the hypothyroid group and 534 nmol/l in the euthyroid group.
3. These results could not be accounted for by differences in serum insulin response.
4. It is suggested that the response of plasma cyclic AMP to glucagon may serve as an index of tissue thyroid status.