A. Respiration rates were obtained by cumulative addition of glycerol-3P (G3P, 4mM), malate (4 mM), octanoyl-carnitine (0.4 mM), and pyruvate (1 mM). The increments in respiration rates are plotted to show the substrate preference of control and hypothyroid cardiac fibers (CT n=16, HYPO n=8). B. Enzymatic activity of total and mitochondrial MDH. C. mRNA expression of proteins of the β-oxidation pathway. PPAR: peroxisome proliferator activated receptor; MCAD: middle chain AcylCoA dehydrogenase; mCPTI: muscle carnitine palmitoyl transferase I. * p<0.05, ** p<0.01, *** p<0.001 versus CT.