Free Thyroxine (Free T4)
Free thyroxine (Free T4) is a thyroid function test that measures the level of unbound or free thyroxine hormone in the bloodstream. This test helps assess thyroid function and diagnose various thyroid disorders. Here’s how Free T4 is used to evaluate thyroid function:
- Thyroid function assessment: Free T4 levels provide important information about thyroid function, specifically regarding the production and activity of thyroid hormones. Alongside other thyroid function tests like Free T3 and TSH, Free T4 levels help diagnose and differentiate various thyroid disorders.
- Hyperthyroidism: Elevated levels of Free T4 are typically observed in hyperthyroidism, a condition characterized by an overactive thyroid gland. Hyperthyroidism can result in symptoms such as weight loss, rapid heart rate, anxiety, heat intolerance, and tremors.
- Hypothyroidism: Conversely, decreased levels of Free T4 may indicate hypothyroidism, which occurs when the thyroid gland does not produce enough thyroid hormones. Hypothyroidism can lead to symptoms such as fatigue, weight gain, cold intolerance, constipation, and depression.
- Monitoring thyroid treatment: Free T4 levels are monitored in individuals undergoing treatment for thyroid disorders, such as hypothyroidism or hyperthyroidism. Adjustments to thyroid hormone replacement therapy (e.g., levothyroxine) or antithyroid medications (e.g., methimazole) may be made based on Free T4 levels to achieve optimal thyroid function.
- Thyroid hormone binding disorders: Free T4 levels are not affected by proteins that bind thyroid hormones, such as thyroxine-binding globulin (TBG). Therefore, Free T4 measurements are particularly useful in evaluating thyroid function in conditions where protein binding may be altered, such as pregnancy, liver disease, or certain medications.
- Thyroid hormone conversion: Free T4 is converted to triiodothyronine (T3), the active form of thyroid hormone, primarily in peripheral tissues. Measuring Free T4 levels helps assess the availability of thyroxine for conversion to T3, which is crucial for regulating metabolism and other physiological functions.