Which symptom might suggest the presence of hypothyroidism?

The symptom that suggests the presence of hypothyroidism is constant tiredness despite ample sleep. In hypothyroidism, the thyroid gland doesn't produce enough thyroid hormones, which are crucial for regulating metabolism and energy levels in the body. As a result, individuals may experience fatigue or a persistent feeling of low energy, even if they are getting enough sleep. This symptom is particularly characteristic of hypothyroidism because it reflects the body's overall sluggishness and reduced metabolic activity.

In contrast, increased energy levels and weight loss despite increased appetite are inconsistent with hypothyroidism. Individuals with hypothyroidism typically experience weight gain or difficulty losing weight due to a lower metabolic rate. An irregular heartbeat can occur with thyroid dysfunction but is more commonly associated with hyperthyroidism, where excess thyroid hormones lead to an increased heart rate. Therefore, constant tiredness despite ample sleep aligns well with the features of hypothyroidism, making it the correct answer.