What condition is characterized by an underactive thyroid?

Hypothyroidism is a condition where the thyroid gland is underactive, leading to insufficient production of thyroid hormones such as thyroxine (T4) and triiodothyronine (T3). These hormones are crucial for regulating metabolism, energy levels, and overall bodily functions. As a result, individuals with hypothyroidism may experience symptoms such as fatigue, weight gain, cold intolerance, dry skin, and depression.

In contrast, hyperthyroidism, characterized by an overactive thyroid, leads to excessive hormone production and results in symptoms like increased heart rate, weight loss, and anxiety. Goiter refers to an enlargement of the thyroid gland, which can occur in both hypothyroidism and hyperthyroidism but is not a condition by itself. Cushing's syndrome is not related to thyroid function; rather, it is caused by excessive levels of cortisol in the body, often due to an adrenal gland issue or long-term use of corticosteroids.

Understanding these distinctions helps clarify why hypothyroidism directly correlates with underactivity of the thyroid gland, making it the correct response to the question.