Which of the following is a symptom of hyperthyroidism?

In hyperthyroidism, the overproduction of thyroid hormones accelerates metabolism, leading to a variety of symptoms. One of the hallmark signs is an increased heart rate, also known as tachycardia. This occurs because the elevated levels of thyroid hormones stimulate the heart's activity, causing it to beat faster in order to meet the increased metabolic demands of the body.

In contrast, weight gain, fatigue, and cold intolerance are more typically associated with hypothyroidism, which is a deficiency of thyroid hormones. In that condition, metabolism slows down, leading to weight gain, a feeling of fatigue, and a tendency to be sensitive to cold. Thus, the correct answer, which highlights increased heart rate, aligns with the typical presentation of hyperthyroidism and reflects the physiological effects of excess thyroid hormone on cardiovascular function.