Which condition is characterized by an underactive thyroid?

Hypothyroidism is defined as a condition in which the thyroid gland does not produce enough thyroid hormones. This underactivity results in a slowing of the body's metabolism and can lead to a variety of symptoms, such as fatigue, weight gain, cold intolerance, depression, and slowed heart rate. The condition can arise from autoimmune diseases (such as Hashimoto's thyroiditis), certain medications, a lack of iodine in the diet, or damage to the thyroid gland itself.

In contrast, hyperthyroidism, characterized by an overactive thyroid, results in excess production of thyroid hormones and can lead to symptoms like weight loss, rapid heartbeat, and anxiety. Thyroid storm is an extreme form of hyperthyroidism that can be life-threatening and is characterized by a sudden and severe increase in these symptoms. A goiter refers to an enlargement of the thyroid gland, which can occur in both hypothyroidism and hyperthyroidism, but it does not specifically indicate whether the thyroid is overactive or underactive.

Thus, hypothyroidism is clearly linked to an underactive thyroid function, making it the correct answer.