T4 (Thyroxine), Total

The Total T4 (Thyroxine) test measures the total amount of the thyroid hormone T4 in the blood, including both the free (active) and protein-bound (inactive) forms. T4 is produced by the thyroid gland and plays a key role in regulating metabolism, growth, and energy levels.

1. What Is T4?

• T4 (thyroxine) is the main hormone produced by the thyroid gland.

• It exists in two forms in the blood: bound to proteins (mostly thyroxine-binding globulin) and free (unbound and active).

• Total T4 reflects the combined amount of both forms, giving an overall measure of thyroid hormone production.

2. Why Is the Total T4 Test Performed?

This test is used to:

• Help diagnose thyroid disorders such as hypothyroidism and hyperthyroidism.

• Monitor thyroid function in people taking thyroid hormone replacement therapy.

• Evaluate pituitary function when TSH levels are abnormal.

• Screen for thyroid dysfunction, especially in combination with TSH testing.

3. Total T4 Reference Range

• Normal range: 4.5 – 12.5 µg/dL (may vary slightly by lab)

• Should be interpreted alongside TSH and Free T4 levels.

4. Interpreting Total T4 Results

🔹 Elevated Total T4:

• May indicate hyperthyroidism (overactive thyroid), often accompanied by low TSH.

• Can also be elevated in pregnancy or with high levels of binding proteins.

🔹 Low Total T4:

• Suggests hypothyroidism (underactive thyroid), especially when TSH is elevated.

• May also be caused by protein deficiencies or pituitary disorders.

5. What Can Affect T4 Levels?

• Levels of binding proteins (influenced by estrogen, liver function, and medications)

• Pregnancy (can increase binding proteins and total T4)

• Medications such as steroids, anticonvulsants, and birth control pills

• Acute or chronic illnesses

6. How Is the Test Performed?

• A blood sample is drawn from a vein.

• No special preparation is usually required unless specified by your doctor.

7. When to See a Doctor

🚨 Contact your healthcare provider if you:

• Experience symptoms such as weight changes, fatigue, depression, or temperature sensitivity.

• Are being treated for a thyroid condition and require ongoing monitoring.

• Have abnormal TSH results or signs of thyroid hormone imbalance.

🔹 Key Takeaways

✅ Total T4 provides an overview of total thyroid hormone levels, including both active and inactive forms.

✅ It is most useful when interpreted with TSH and Free T4 levels.

✅ Abnormal results may indicate hypo- or hyperthyroidism, or changes in protein-binding capacity.