A. Thyrotoxicosis
Hyperthyroidism (thyrotoxicosis) is caused by an excessive secretion of thyroid hormone, L-thyroxine. This hormone has direct cardiac effects manifesting in a marked increase in heart rate and myocardial contractility.
1. Clinical Features
Symptoms of thyrotoxicosis include heat intolerance, excessive sweating, nervousness, irritability, anxiety, marked weight loss, increase in appetite, and palpitations with fast heart rates. Palpitations may be caused by a marked increase in heart rate, but the heart rhythm may become irregular because of the occurrence of atrial fibrillation. Physical signs include tachycardia, agitation, enlargement of the thyroid gland, and typicaleye signs. Atrial fibrillation with a rapid heart rate of more than 150 beats may force patients to seek attention. Increas¬ing shortness of breath is usually caused by excessive cardiac work resulting in myocardial dysfunction and heart failure.
2. Management
The cardiovascular manifestations of thyrotoxicosis are often resistant to therapy. Atrial fibrillation does not respond to treatment with digoxin, and the beta-blocking agents are most useful in controlling the fast heart rate evidenced in sinus tachycardia or caused by atrial fibrillation until specific treatment of the thyroid distur¬bance becomes effective. Irradiation with radioactive iodine or surgical removal is usually successful.

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