Which of the following can be a cause of weight loss?

Hyperthyroidism can lead to weight loss due to an increase in the metabolic rate. This condition involves the overproduction of thyroid hormones, particularly thyroxine (T4), which accelerates the body's metabolism. As a result, individuals with hyperthyroidism may experience symptoms such as unintended weight loss, increased appetite, and rapid heart rate. The elevated metabolic rate causes the body to burn more calories than usual, contributing to weight loss despite normal or increased food intake.

In contrast, increased calorie intake typically results in weight gain, and excessive hydration does not directly correlate with weight loss, even if it could lead to temporary weight fluctuations. Regular exercise alone, while beneficial for overall health and potentially contributing to weight loss, requires a calorie deficit to result in a net loss of weight. Thus, hyperthyroidism is the only choice that directly causes weight loss through metabolic changes.