T4 (Thyroxine), Free

The Free T4 (Thyroxine) test measures the level of unbound T4 hormone circulating in the blood. Free T4 is the biologically active form of thyroxine, which plays a vital role in regulating metabolism, energy production, and overall cellular function. This test is frequently used alongside TSH testing to evaluate thyroid function.

1. What Is Free T4?

• Free T4 is the portion of total T4 that is not bound to proteins and is available to enter tissues and exert metabolic effects.

• It is the active form of the primary thyroid hormone produced by the thyroid gland.

• T4 is converted into T3 (triiodothyronine) in the body, which is even more metabolically active.

2. Why Is the Free T4 Test Performed?

This test is used to:

• Diagnose and monitor thyroid disorders, such as hypothyroidism and hyperthyroidism.

• Evaluate abnormal TSH levels or investigate symptoms of thyroid dysfunction.

• Monitor the effectiveness of thyroid replacement therapy.

• Assess thyroid function in people with pituitary or hypothalamic disease.

3. Free T4 Reference Range

• Normal range: 0.8 – 1.8 ng/dL (may vary slightly by lab)

• Interpretation depends on context and should be assessed alongside TSH levels.

4. Interpreting Free T4 Results

🔹 Elevated Free T4:

• Suggests hyperthyroidism or excessive thyroid hormone replacement.

• Common in Graves’ disease, thyroiditis, or overmedication with levothyroxine.

🔹 Low Free T4:

• Indicates hypothyroidism, especially when accompanied by elevated TSH.

• May also result from pituitary dysfunction (secondary hypothyroidism).

5. What Can Affect Free T4 Levels?

• Thyroid hormone medications or iodine supplements

• Liver and kidney function (affect hormone binding and clearance)

• Severe illness or stress (can temporarily affect hormone levels)

• Changes in thyroid-binding globulin levels due to estrogen or steroids

6. How Is the Test Performed?

• A blood sample is drawn from a vein, usually in the morning.

• Fasting is typically not required unless ordered with other tests.

7. When to See a Doctor

🚨 Contact your healthcare provider if you:

• Experience fatigue, weight changes, depression, dry skin, or temperature sensitivity.

• Are taking thyroid hormone therapy and require regular monitoring.

• Have abnormal TSH results or a family history of thyroid disorders.

🔹 Key Takeaways

✅ Free T4 is the active form of thyroxine and is crucial for assessing thyroid function.

✅ It provides valuable insight when used with TSH and other thyroid tests.

✅ Monitoring Free T4 helps guide diagnosis and treatment of thyroid conditions.