Triiodothyronine (T3) is the most biologically active thyroid hormone that regulates metabolism, growth, and development throughout the body. This test measures T3 levels in the blood and is particularly useful for diagnosing T3 thyrotoxicosis, a condition where only T3 is elevated while T4 remains normal. T3 is often elevated in hyperthyroidism and can help confirm the diagnosis when other thyroid tests are borderline or normal. This test is especially valuable for patients with symptoms of hyperthyroidism such as rapid heartbeat, weight loss, fatigue, or muscle weakness when other thyroid tests don't clearly indicate the problem. T3 levels can also be affected by various factors including nutrition, certain medications, and non-thyroidal illnesses. This test is commonly ordered as part of comprehensive thyroid evaluation or when T3-specific thyroid disease is suspected.