Cardiac function in acute hypothyroidism.

It has been established that chronic hypothyroidism may affect cardiac function by several mechanisms. It is not known how long the patient has to be hypothyroid for cardiac involvement to develop. This study was undertaken to assess the effect of a short period of hypothyroidism (10 days) on cardiac function. Nine patients who had had total thyroidectomy, had received ablative radioiodine for thyroid cancer and were euthyroid on replacement therapy were studied while both euthyroid and hypothyroid. Cardiac assessment was performed by X-ray, ECG, echocardiography and gated blood-pool scans. After 10 days of hypothyroidism, the left-ventricular ejection fraction failed to rise after exercise in 4 of the 9 patients studied, which was significant (P less than 0.002). No significant changes in cardiac size or function at rest were detected. This functional abnormality in the absence of any demonstrable change in cardiac size and the absence of pericardial effusion with normal basal function suggest that short periods of hypothyroidism may reduce cardiac reserve, mostly because of alterations in metabolic function.