The relationship between serum creatine kinase and thyroid hormones: in vivo and in vitro studies.

Creatine kinase activity was measured in sera submitted for thyroid investigations. Although, as groups, hypothyroid and hyperthyroid subjects had higher and lower creatine kinase results than normal, the assay was not satisfactory as a screening test for predicting thyroid status in the individual. Attempts to inhibit the serum enzyme in vitro using high concentrations of thyroxine and tri-iodothyronine were unsuccessful. The inverse relation between serum creatine kinase and thyroid status is unlikely, therefore, to be due to a direct inhibitory effect of the hormones on the enzyme.