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Free T3 (Triiodothyronine) Test

✅ Up to date🔬 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.

✅ 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

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Last reviewed: 3/30/2026
Next review: 6/30/2026

🔗 Related Topics

🩺 Hipotiroidizm🩺 Hipertiroidizm🧪 TSH (Tiroid uyarıcı hormon)🧪 Serbest T4 (sT4)🧪 Anti-TPO (tiroid peroksidaz antikoru)
⚖️ 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.