Triiodothyronine (T3) Test

The Triiodothyronine (T3) test measures the level of T3 hormone in the blood. T3 is one of the two main hormones produced by the thyroid gland (the other being thyroxine or T4) and is essential for metabolism, energy regulation, and organ function.

Types of T3 Tests

1. Total T3 – Measures both bound and free T3 (T3 attached to proteins + active T3).
2. Free T3 (FT3) – Measures only the unbound, active form of T3 that is available for use by the body.

Why is the T3 Test Done?

🔹 To diagnose hyperthyroidism (overactive thyroid)
🔹 To evaluate thyroid function when TSH & T4 levels are abnormal
🔹 To monitor thyroid disorders, like Graves’ disease
🔹 To assess T3 thyrotoxicosis, a rare condition where only T3 is elevated

Normal T3 Levels (May vary by lab)

• Total T3: 80 – 200 ng/dL
• Free T3: 2.0 – 4.4 pg/mL

What Do Abnormal T3 Levels Mean?

✅ High T3 (Hyperthyroidism)

• Graves’ disease (most common)
• Toxic multinodular goiter
• Thyroiditis (early stage)
• Too much thyroid medication
• High iodine intake

❌ Low T3 (Hypothyroidism or Non-Thyroidal Illness)

• Hypothyroidism (e.g., Hashimoto’s thyroiditis)
• Severe illness (“Low T3 Syndrome”)
• Liver disease (since T3 is converted from T4 in the liver)
• Malnutrition or prolonged fasting

How is the Test Performed?

✔ A blood sample is drawn from a vein
✔ No fasting is required
✔ Results usually available in 24-48 hours