Synopsis

Source of validity: Scientific
Rating (out of 5): 3

Desiccated thyroid extract (Thyroid) has a scientific basis for use in treating certain forms of "heart weakness," specifically when such cardiac symptoms are secondary to hypothyroidism (underactive thyroid). In the late 19th and early 20th centuries, physicians observed that patients with severe hypothyroidism (myxedema) often developed bradycardia, low cardiac output, and congestive heart failure. Administration of thyroid extract often resulted in marked improvement of these cardiac symptoms, as the underlying cause—insufficient thyroid hormone—was corrected. Modern research confirms that thyroid hormones exert direct effects on cardiac contractility, heart rate, and vascular resistance. In patients with hypothyroidism, supplementation with thyroid hormone can restore normal cardiac function. However, in euthyroid (normal thyroid function) individuals, there is no evidence that thyroid supplementation benefits the heart and may actually increase the risk of arrhythmia and other adverse cardiac events. Thus, the use of thyroid extract or hormone therapy for heart weakness is justified only in the context of hypothyroidism-induced cardiac dysfunction. Outside of this context, it is neither effective nor safe. The overall evidence rating is 3, reflecting clear benefit in a specific population (hypothyroid patients with cardiac symptoms), but lack of benefit or potential harm in the general population or in other types of heart disease.

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