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Free T4 (Thyroxine) Test

✅ Up to date🔬 Evidence: StrongEndocrine & Metabolism
Diğer adları: Free thyroxine, FT4, T4 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 thyroxine in the blood • Normal adult range: approximately 0.8–1.8 ng/dL (10–23 pmol/L) • Used alongside TSH to evaluate thyroid function • Pregnancy-specific reference ranges apply

🧪 What Does This Test Measure?

The free T4 test measures the unbound, biologically active fraction of thyroxine in the blood, used alongside TSH to evaluate thyroid function.

📋 Why Is It Ordered?

Ordered for thyroid function evaluation, interpretation of abnormal TSH results, and monitoring of thyroid 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: 0.8–1.8 ng/dL (approximately 10–23 pmol/L) Pregnancy-specific ranges are lower, especially in the first trimester. ⚠️ Reference ranges vary by laboratory and assay method.

⬆️ High Values

Elevated free T4 may be associated with hyperthyroidism, excessive thyroid medication dose, or physiological rise in early pregnancy.

⬇️ Low Values

Low free T4 may be associated with hypothyroidism, insufficient thyroid medication dose, or a pituitary-related disorder.

⚙️ What Can Affect Results?

Assay method, medications, pregnancy, binding protein changes, acute illness, and biotin supplements may affect free T4 results.

🔬 Evidence Summary

Strong evidence: 3 guidelines, 1 systematic review, 2 reviews, 1 observational study.

✅ Key Takeaways

💡

What you learned: Free T4 reflects the biologically active thyroid hormone level. It is most informative when interpreted alongside TSH.

⛔

A free T4 result alone cannot diagnose thyroid disease. TSH and clinical context are needed.

🔬 Sources Used on This Page

8 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
3
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, 3 reviews, 1 observational study.

📝 Questions to Ask Your Doctor

Be prepared for your appointment. Add questions to your list.

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

🔗 Related Topics

🩺 Hipotiroidizm🩺 Hipertiroidizm🧪 TSH (Tiroid uyarıcı hormon)🧪 Serbest T3 (sT3)🧪 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.