Total Triiodothyronine (T3 total)
Total triiodothyronine (T3 total) is a thyroid function test that measures the total amount of triiodothyronine hormone in the bloodstream, including both bound and unbound forms. While Free T3 (unbound or free triiodothyronine) is considered the biologically active form of T3, Total T3 provides additional information about thyroid function and can be used to evaluate various conditions:
- Assessment of thyroid function: Total T3 levels are used in conjunction with other thyroid function tests, such as TSH, Free T4, and Free T3, to evaluate overall thyroid function. Abnormal Total T3 levels may indicate hyperthyroidism (elevated levels) or hypothyroidism (decreased levels).
- Hyperthyroidism (overactive thyroid): Elevated levels of Total T3 are often observed in hyperthyroidism, a condition characterized by excessive production of thyroid hormones by the thyroid gland. Hyperthyroidism can lead to symptoms such as weight loss, rapid heart rate, anxiety, heat intolerance, and tremors.
- Thyroid hormone binding disorders: Total T3 levels are influenced by proteins that bind thyroid hormones, such as thyroxine-binding globulin (TBG). Changes in TBG levels or alterations in thyroid hormone binding capacity can affect Total T3 levels, even if Free T3 levels remain normal. Therefore, Total T3 measurements may be useful in evaluating thyroid function in conditions where protein binding is altered, such as pregnancy, liver disease, or certain medications.
- Monitoring thyroid replacement therapy: Total T3 levels may be monitored in individuals undergoing treatment for thyroid disorders, such as hypothyroidism, with thyroid hormone replacement therapy (e.g., levothyroxine). However, Free T3 levels are considered more accurate for monitoring thyroid replacement therapy because they reflect the active form of T3 available to tissues.
- Secondary evaluation of thyroid disorders: Total T3 levels may provide additional information in cases where Free T3 measurements are unavailable or inconclusive. However, it's important to interpret Total T3 levels in the context of clinical symptoms, medical history, and other thyroid function tests.