The Thermogenic Effect of Thyroid Hormone and Its Clinical Implications

The Thermogenic Effect of Thyroid Hormone and Its Clinical Implications

J. Enrique Silva, MD

From Jewish General Hospital, Montreal, Quebec, Canada.

PHYSIOLOGY IN MEDICINE: A SERIES OF ARTICLES LINKING MEDICINE WITH SCIENCE

Co-sponsored by the American College of Physicians and the American Physiological Society

Physiology in Medicine: Dennis A. Ausiello, MD, Editor; Dale J. Benos, PhD, Deputy Editor; Francois Abboud, MD, Associate Editor; William J. Koopman, MD, Associate Editor

Annals of Internal Medicine: Paul Epstein, MD, Series Editor

Clinical Principles

There is a popular belief, even among physicians, that weight changes are frequent manifestations of thyroid dysfunction.

Thyrotoxicosis is usually associated with weight loss and heat intolerance. Cold intolerance, but not weight gain, is a prominent feature of hypothyroidism.

Pathophysiologic Principles

Thyroid hormones increase heat production only in homeothermic species.

Thermogenesis is energy dissipation as heat.

The thermogenic effect of thyroid hormones is closely linked to increased appetite and lipogenesis to ensure fuel availability and avoid wasting.

Major thermogenic mechanisms include increased turnover of adenosine triphosphate (ATP) and programmed reduction in the efficiency of ATP synthesis.

Reduced efficiency in ATP synthesis is relatively more important than increased ATP turnover in the transition from hypothyroidism to euthyroidism.

Clinicians are very familiar with cold or heat intolerance in patients with altered thyroid function. While nonspecific, these symptoms have some predictive value in the diagnosis of thyroid dysfunction. Involuntary changes in weight are also perceived as indicators of abnormal thyroid function, often prompting laboratory testing of thyroid gland function. Most of the clinical manifestations of thyroid dysfunction in adults are directly or indirectly derived from the stimulating effect of thyroid hormone on thermogenesis and energy turnover. Even so, the relationship between these effects and energy balance and body weight is not simple. This review examines the thermogenic effect of thyroid hormone and how it affects some of the manifestations of hypo- and hyperthyroidism, energy balance, and body weight.