The energy relationships between cytosolic and mitochondrial metabolism were 
studied in the hearts from euthyroid, hypothyroid, and hyperthyroid rats. 
Isolated mitochondria showed high respiratory control ratios and impermeability 
to exogenous NADH. Hypo- and hyperthyroidism, respectively, resulted in lower 
and higher contents of both cytochromes per mitochondrion and mitochondrial 
protein per gram of wet weight of heart without changes in the ratio of 
cytochrome c to cytochrome aa3. In isolated perfused heart, the hyperthyroid 
state led to an increase in work rate and thereby an elevation of Vo2, which 
resulted in an increase oxidation-reduction turnover number for the cytochromes. 
An agreement was found between [ATP]/[ADP][Pi] of cytosolic free adenine 
nucleotides and the value calculated from a mathematical model of mitochondrial 
respiration. This implies that mitochondrial respiration is controlled at the 
cytochrome oxidase reaction and that oxidative phosphorylation in intact tissue 
is tightly coupled irrespective of thyroid state. It is concluded that thyroid 
hormone causes an increase in the mitochondrial mass, mitochondrial cytochrome 
content, and respiratory rate, and consequently expands the capacity of 
oxidative metabolism without an uncoupling effect on oxidative phosphorylation.