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SUMMARY. The effects of the administration of 0.5 or 1 mg/kg of 1-thyroxine for 3 or 16 days were studied in 55 New Zealand white rabbits. Heart size was 29% above control after 16 days of 1-thyroxine, despite lower body weights. In an anesthetized open-chest animal, regional microspectrophotometric observations of small arteries and veins to determine oxygen extraction were combined with regional blood flow measurements using radioactive microspheres to determine regional oxygen consumption by the Fick principle. Vascular flow reserves were studied through measurement of blood flow after the administration of chromonar HC1, 10 mg/kg. Myocardial oxygen consumption was, respectively, 2.4 and 3.8 times control after 3 and 16 days of 1-thyroxine. This was accompanied by significant increases in both coronary blood flow and oxygen extraction. In control, oxygen extraction and consumption were higher in the subendocardial region compared to the subepicardial area. No significant regional differences were found in flow, oxygen extraction, or consumption after 1-thyroxine administration. Chromonar increased coronary blood flow 2.8 times in the control animals, but did not significantly increase flow or decrease vascular resistance in the rabbits given 1-thyroxine. Adenosine increased coronary blood flow 3.5 times in the control animals but only 2.2 times in animals given 1-thyroxine for 3 days. Animals given 1-thyroxine had hypertrophied hearts with increased oxygen consumption, markedly increased flow, and increased oxygen extraction but without regional left ventricular differences. (Circ Res 52: 272-279, 19S3)

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