Home → Tests → Free T4
Free T4 (Thyroxine) Test
⚠️ Update approaching🔬 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.
📚 Most Important Studies on This Topic
1
Guidelines for the Treatment of Hypothyroidism: Prepared by the ATA Task Force on Thyroid Hormone Replacement
Guideline· Thyroid · 2014 · PMID: 25266247
2
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
3
Subclinical Thyroid Disease: Scientific Review and Guidelines for Diagnosis and Management
Systematic review· JAMA · 2004 · PMID: 14722150
✅ 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.
📋 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: 20395623] Report of the IFCC Working Group for Standardization of Thyroid Function Tests; Part 2: Free Thyroxine and Free Triiodothyronine. Clinical Chemistry, 2010.
[PMID: 15989900] Amiodarone and the Thyroid. American Journal of Medicine, 2005.
[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
🔗 Related Topics
⚖️ 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.