Free Triiodothyronine (FT3)
- Sample Type
- Serum
- Fasting
- No
- Unit
- pg/mL
Function
Triiodothyronine (T3) is one of the two major hormones produced by the thyroid gland. While the gland produces more T4, T3 is significantly more potent. 'Free' T3 refers to the portion of the hormone that is not bound to carrier proteins, making it biologically active and available to body tissues. It plays a central role in regulating basal metabolic rate, body temperature, and heart rate.
Why it is Ordered
FT3 is primarily used to diagnose hyperthyroidism and to determine the severity of the condition. It is particularly useful in cases of 'T3 Thyrotoxicosis,' where TSH is low and T4 is normal, but T3 is elevated. It is less useful for diagnosing hypothyroidism, as it is often the last value to drop in failing thyroid function.
Associated Conditions
- Graves' Disease: An autoimmune condition causing hyperthyroidism.
- Toxic Nodular Goiter: Small lumps in the thyroid that overproduce hormones.
- Thyroiditis: Inflammation that can cause a temporary release of excess T3.
- Non-Thyroidal Illness Syndrome: A state where FT3 levels drop during severe systemic illness despite a healthy thyroid.
Check Your Result
Check Your Result
Enter your lab result to see where you stand compared to the standard reference range.
Why Context Matters
FT3 levels are highly sensitive to acute illness and starvation; in a state known as 'Euthyroid Sick Syndrome,' FT3 levels will drop significantly as the body attempts to conserve energy, which can be misidentified as thyroid disease. Biotin (Vitamin B7) supplements are a major confounding factor, often causing falsely high FT3 results in many common lab assays. Stress and certain medications like amiodarone can also skew results.
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 Free Triiodothyronine (FT3):
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.
Get Notified When Dr. Babu Is Available