Total Triiodothyronine (T3)
- Sample Type
- Serum
- Fasting
- No
- Unit
- ng/dL
Function
Triiodothyronine (T3) is one of the two major hormones produced by the thyroid gland. While the thyroid produces more T4 (thyroxine), T3 is the much more metabolically active form. Most T3 in the blood is bound to proteins (like thyroid-binding globulin). The 'Total T3' test measures both the bound and the free (unbound) portions of the hormone.
Why it is Ordered
It is primarily used to diagnose and monitor hyperthyroidism (overactive thyroid). In many cases of hyperthyroidism, T3 levels rise before T4 levels do. It is also used to diagnose 'T3 Toxicosis,' a condition where T4 is normal but T3 is elevated. It is less useful for diagnosing hypothyroidism, as it is often the last value to drop.
Associated Conditions
- Graves' Disease: An autoimmune cause of hyperthyroidism.
- Toxic Nodular Goiter: Small masses in the thyroid that overproduce hormone.
- Thyroiditis: Inflammation that causes a temporary leak of hormone into the blood.
- Hypothyroidism: Underactive thyroid (though TSH and Free T4 are preferred for this).
Check Your Result
Check Your Result
Enter your lab result to see where you stand compared to the standard reference range.
Why Context Matters
Total T3 levels are heavily influenced by the amount of thyroid-binding proteins in the blood. Conditions like pregnancy, liver disease, or the use of birth control pills can significantly alter the 'Total' count without affecting the 'Free' (active) hormone level. Additionally, high doses of Biotin (Vitamin B7) can interfere with the lab assay, causing falsely high results. Always cross-reference with Free T3 and TSH.
Lab ranges are statistical averages, not biological laws. "Normal" for a 20-year-old male isn't normal for a 60-year-old female.
Clinical References
Source-of-truth databases and clinical guidelines for Total Triiodothyronine (T3):
Official Sources
- ATA Guidelines & Statementsthyroid.org
Research & Guidelines
Related Indicators
Medically Reviewed by Dr. Binoy Babu, MBBS
Board Certified Doctor • 10+ Years Clinical Experience
Dr. Babu is a practicing physician dedicated to empowering patients with clear, actionable medical information. He founded 2opi to bridge the gap between complex lab reports and patient understanding, ensuring everyone has access to a reliable second opinion.
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