What condition is characterized by an overactive thyroid?

Thyrotoxicosis is the condition characterized by an overactive thyroid, which results in excessive production of thyroid hormones. This hyperactivity can lead to various symptoms, including increased metabolism, weight loss, rapid heartbeat, anxiety, and tremors. Thyrotoxicosis can arise from several underlying causes, such as Graves' disease, toxic multinodular goiter, or excessive intake of thyroid hormone.

Understanding the other options clarifies why thyrotoxicosis is the correct answer. Hypothyroidism is the opposite condition, marked by an underactive thyroid resulting in insufficient hormone production. Adrenal insufficiency pertains to inadequate production of adrenal hormones, not thyroid hormones. A goiter refers to an enlarged thyroid gland, which can occur in both hyperthyroidism and hypothyroidism but does not specifically indicate an overactive thyroid.