The total T3 blood test measures the concentration of total triiodothyronine (T3) hormone in the bloodstream. T3 is one of the two primary thyroid hormones produced by the thyroid gland. It plays a crucial role in regulating metabolism, growth, and development. This test helps evaluate thyroid function and can aid in diagnosing conditions such as hyperthyroidism or hypothyroidism. It is often performed alongside other thyroid function tests, such as TSH and free T4, to obtain a comprehensive assessment of thyroid health.