Total Thyroxine (T4 total)
Total thyroxine (T4 total) is a thyroid function test that measures the total amount of thyroxine hormone in the bloodstream, including both bound and unbound forms. While Free T4 (unbound or free thyroxine) is considered the biologically active form of T4, Total T4 provides additional information about thyroid function and can be used to evaluate various conditions:
- Assessment of thyroid function: Total T4 levels are used alongside other thyroid function tests, such as TSH, Free T4, and Total T3, to evaluate overall thyroid function. Abnormal Total T4 levels may indicate hyperthyroidism (elevated levels) or hypothyroidism (decreased levels).
- Hypothyroidism (underactive thyroid): Decreased levels of Total T4 are typically observed in primary hypothyroidism, a condition characterized by insufficient production of thyroid hormones by the thyroid gland. Hypothyroidism can lead to symptoms such as fatigue, weight gain, cold intolerance, constipation, and depression.
- Hyperthyroidism (overactive thyroid): Conversely, elevated levels of Total T4 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 T4 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 T4 levels, even if Free T4 levels remain normal. Therefore, Total T4 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 T4 levels may be monitored in individuals undergoing treatment for thyroid disorders, such as hypothyroidism, with thyroid hormone replacement therapy (e.g., levothyroxine). However, Free T4 levels are considered more accurate for monitoring thyroid replacement therapy because they reflect the active form of T4 available to tissues.
- Secondary evaluation of thyroid disorders: Total T4 levels may provide additional information in cases where Free T4 measurements are unavailable or inconclusive. However, it's important to interpret Total T4 levels in the context of clinical symptoms, medical history, and other thyroid function tests.