Relationship between thyroid function and cardiotoxic properties of isoproterenol

The relationship between thyroid function and response to a synthetic adrenergic agent, isoproterenol, has been studied in the rat. Hypothyroidism after thyroidectomy or treatment with propylthiouracil for one or three weeks resulted in milder symptoms, reduced heart weights and less severe heart lesions after isoproterenol treatment than was seen in the controls. Thyroxine treatment on the other hand increased the mortality and heart weight after isoproterenol but had less marked effects on the myocardial necrosis. The myocardial infarct-like necrosis produced by isoproterenol is believed to be due to local ischemia resulting from a relative hypoxia in the myocardial fibres stimulated by this drug. The relationship seen between thyroid function and isoproterenol sensitivity supports this concept.