Abstract
Plasma creatine kinase (CK) is well known to be elevated in patients with hypothyroidism and decreased in those with hyperthyroidism; however, the mechanism of these changes remains obscure. In the present study, base-line plasma CK activity and the CK disappearance rate (kd) were determined in four conscious dogs before and after induction of hypo- and hyperthyroidism. Serum thyroxine (T4) concentrations during hyperthyroidism increased by an average of 762 ± 236% (SE) and weight decreased 13% versus a weight gain of 5% during hypothyroidism (P < 0.02) and a 73% reduction in serum T4. Dog MM CK was purified by diethylaminoethyl (DEAE) Sephadex A-50 chromatography and injected intravenously and the kd determined before and after induction of hypothyroidism with 131I and hyperthyroidism with L-thyroxine. Plasma CK activity was determined every 30 min to obtain the kd. During hypothyroidism kd decreased 31% and base-line CK activity increased 65%, in contrast to an increase in kd of 56% (P < 0.05) in hyperthyroidism that was associated with a decrease in base-line CK activity of 46% (P < 0.02). Neither L-thyroxine nor serum from hyperthyroid animals incubated in vitro with a known amount of CK inhibited the CK activity. Thus, changes in base-line CK activity associated with hypo- and hyperthyroidism are predominantly due to changes in the CK disappearance rate, which is increased in hyperthyroidism and decreased in hypothyroidism.