An FT4 blood test measures the level of free thyroxine (T4) in your blood. Thyroxine is a hormone produced by your thyroid gland, and the "free" portion refers to the T4 not bound to proteins in your blood. This unbound T4 is the biologically active form, meaning it's the part that actually affects your body's tissues and functions. The test helps doctors assess your thyroid gland's function.
What Does the FT4 Test Show?
The FT4 test results help determine if your thyroid is producing the correct amount of hormones.
- Low FT4 levels: Could indicate hypothyroidism (underactive thyroid), where your thyroid doesn't produce enough hormones.
- High FT4 levels: Could suggest hyperthyroidism (overactive thyroid), where your thyroid produces too much hormone.
Doctors often use the FT4 test in conjunction with other thyroid tests, such as TSH (thyroid-stimulating hormone), to get a complete picture of thyroid function. A high TSH with a low FT4 strongly suggests hypothyroidism, while a low TSH with a high FT4 points towards hyperthyroidism.
Why is the FT4 Test Important?
The FT4 test is crucial because:
- It directly measures the active form of the thyroid hormone, providing a more accurate assessment of thyroid function compared to total T4 tests.
- It helps diagnose and monitor thyroid disorders.
- It helps guide treatment decisions and monitor treatment effectiveness.
Examples of situations where an FT4 test may be ordered:
- Symptoms suggestive of thyroid problems (fatigue, weight changes, etc.).
- Monitoring thyroid hormone replacement therapy.
- Investigating abnormal TSH levels.
- Evaluating potential thyroid dysfunction during pregnancy.
