Which of the following is a symptom of hyperthyroidism?

Hyperthyroidism is a condition characterized by an overproduction of thyroid hormones, leading to a variety of symptoms caused by an accelerated metabolism. Exophthalmos, or bulging eyes, is particularly associated with hyperthyroidism, especially in cases of Graves' disease. This symptom occurs due to the accumulation of immune cells and inflammatory substances behind the eyes, which can lead to swelling and protrusion.

The other symptoms listed, while they may occur in different conditions, do not align with the typical manifestation of hyperthyroidism. Fatigue is often associated with hypothyroidism, weight gain is more commonly related to reduced metabolic activity seen in hypothyroidism as well, and a deepened voice can occur in conditions where there is an abnormality in hormone levels, but is not characteristic of hyperthyroidism. Thus, the presence of exophthalmos serves as a distinctive indicator of hyperthyroidism, marking it as the correct choice in this context.