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Free T3 (Triiodothyronine) Test
⚠️ Update approaching🔬 Evidence: StrongEndocrine & Metabolism
Diğer adları: Free triiodothyronine, FT3, T3 test
⚠️
Discuss your test results with your doctor. This page is for informational purposes only and does not provide a diagnosis.
⚡ Key Facts
• Measures the unbound, biologically active fraction of triiodothyronine
• Normal adult range: approximately 2.0–4.4 pg/mL (3.1–6.8 pmol/L)
• Particularly important in T3 thyrotoxicosis evaluation
• Highly sensitive to non-thyroidal illness
🧪 What Does This Test Measure?
The free T3 test measures the unbound fraction of triiodothyronine, the most metabolically active thyroid hormone, particularly useful in evaluating T3 thyrotoxicosis.
📋 Why Is It Ordered?
Ordered for hyperthyroidism evaluation, T3 thyrotoxicosis investigation, and monitoring of antithyroid treatment.
🔧 Preparation
Prolonged fasting is generally not required. For individuals on thyroid medication, timing relative to medication dose may be relevant.
📊 Reference Ranges
General adult range: approximately 2.0–4.4 pg/mL (3.1–6.8 pmol/L)
Pregnancy-specific ranges apply.
⚠️ Reference ranges vary by laboratory and assay method.
⬆️ High Values
Elevated free T3 may be associated with hyperthyroidism, T3 thyrotoxicosis, or excessive thyroid medication.
⬇️ Low Values
Low free T3 is commonly seen during severe systemic illness (non-thyroidal illness syndrome). It may also be associated with advanced hypothyroidism or malnutrition.
⚙️ What Can Affect Results?
Acute and chronic illness, medications, nutritional status, assay method, pregnancy, and age may affect free T3 results.
🔬 Evidence Summary
Strong evidence: 3 guidelines, 1 systematic review, 3 reviews, 1 observational study.
📚 Most Important Studies on This Topic
1
Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association
Guideline· Endocrine Practice · 2012 · PMID: 22954017
2
Guidelines for the Treatment of Hypothyroidism: Prepared by the ATA Task Force on Thyroid Hormone Replacement
Guideline· Thyroid · 2014 · PMID: 25266247
3
Mechanisms behind the non-thyroidal illness syndrome: an update
Review· Journal of Endocrinology · 2010 · PMID: 20016054
✅ Key Takeaways
💡
What you learned: Free T3 is the most metabolically active thyroid hormone. It is especially useful for evaluating T3 thyrotoxicosis but is sensitive to non-thyroidal illness.
⛔
A free T3 result alone cannot diagnose thyroid disease. TSH, free T4, and clinical context are needed.
🔬 Sources Used on This Page
9 sources · Most recent publication: 2023📋
Guideline
Expert society and guideline recommendations
3
sources
📊
Systematic review / meta-analysis
Combined analysis of multiple studies
1
source
📖
Review
Comprehensive topic evaluation
4
sources
👁
Observational
Observational and cohort studies
1
source
Overall assessment: Evidence level for this topic is strong. This page is supported by 3 guidelines, 1 systematic review/meta-analysis, 4 reviews, 1 observational study.
📝 Questions to Ask Your Doctor
Be prepared for your appointment. Add questions to your list.
📋 References
[PMID: 37655789] Thyroid Stimulating Hormone and Thyroid Hormones (Triiodothyronine and Thyroxine): An American Thyroid Association-Commissioned Review of Current Clinical and Laboratory Status. Thyroid, 2023.
[PMID: 28056690] 2017 Guidelines of the American Thyroid Association for the Diagnosis and Management of Thyroid Disease During Pregnancy and the Postpartum. Thyroid, 2017.
[PMID: 25266247] Guidelines for the Treatment of Hypothyroidism: Prepared by the ATA Task Force on Thyroid Hormone Replacement. Thyroid, 2014.
[PMID: 22954017] Clinical Practice Guidelines for Hypothyroidism in Adults: Cosponsored by the American Association of Clinical Endocrinologists and the American Thyroid Association. Endocrine Practice, 2012.
[PMID: 20016054] Mechanisms behind the non-thyroidal illness syndrome: an update. Journal of Endocrinology, 2010.
[PMID: 20395623] Report of the IFCC Working Group for Standardization of Thyroid Function Tests; Part 2: Free Thyroxine and Free Triiodothyronine. Clinical Chemistry, 2010.
[PMID: 17991805] The clinical significance of subclinical thyroid dysfunction. Endocrine Reviews, 2008.
[PMID: 14722150] Subclinical Thyroid Disease: Scientific Review and Guidelines for Diagnosis and Management. JAMA, 2004.
[PMID: 11836274] Serum TSH, T(4), and Thyroid Antibodies in the United States Population (1988 to 1994): NHANES III. Journal of Clinical Endocrinology & Metabolism, 2002.
Last reviewed: 3/30/2026
Next review: 3/30/2027
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⚖️ This page does not replace medical advice. Make treatment decisions with your doctor.
Content is based on scientific studies indexed in PubMed and current clinical guidelines.
Content is based on scientific studies indexed in PubMed and current clinical guidelines.