Hyperthyroidism and the heart: study of left ventricular function in preclinical hyperthyroidism.

To study the response of the target organs to minor increments of thyroid hormone levels, left ventricular ejection fraction (LVEF) the primary indicator of left ventricular function, was measured in patients with preclinical hyperthyroidism and the results were compared with those obtained in euthyroid subjects and in patients with toxic adenoma. In euthyroid subjects and in patients with preclinical hyperthyroidism LVEF in response to exercise involving an isometric static work load revealed a minor increase, whereas in manifest hyperthyroidism (toxic adenoma) an opposite response with an impairment of left ventricular functional capacity was found. The results showed that the pituitary responds more sensitively to minor changes in the circulating thyroid hormone levels than does the heart and indicated that the typical hyperthyroid heart changes associated with the evolution of toxic adenoma develop gradually.