Why do patients with anorexia nervosa exhibit low T3 and T4 levels?

Patients with anorexia nervosa often experience low levels of T3 (triiodothyronine) and T4 (thyroxine) primarily due to malnutrition affecting hormone levels. In the state of severe caloric restriction and malnutrition, the body prioritizes energy conservation, which affects various metabolic processes, including the production of thyroid hormones.

When the body is deprived of essential nutrients, it reduces the synthesis of T3 and T4 as part of a broader adaptation mechanism to conserve energy and protect against further weight loss. This downregulation is a physiological response to the lack of caloric intake and is often referred to as "low T3 syndrome" or "euthyroid sick syndrome," where T3 levels decrease while TSH (thyroid-stimulating hormone) levels may remain normal in many cases.

The other suggestions, while potentially contributing factors in different contexts, do not directly cause the low thyroid hormone levels in patients specifically suffering from anorexia nervosa. Thyroid dysfunction may arise secondary to the nutritional deficiency, physical activity might influence thyroid hormone levels, and insufficient dietary iodine can lead to broader thyroid issues, but in the scenario of anorexia nervosa, malnutrition is the predominant factor influencing the reduction in thyroid