Free Thyroxine (FT4)
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Evaluates Free T4 (Thyroxine), which helps assess thyroid gland function and metabolism.
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🧪 What is Free Thyroxine (Free T4 / FT4)?
Free T4 (FT4) refers to the unbound or biologically active form of Thyroxine (T4) — the major hormone produced by the thyroid gland.
- While most circulating T4 is bound to proteins (mainly thyroxine-binding globulin or TBG), free T4 represents the hormone available to enter cells and regulate metabolism.
- Free T4 is a more accurate marker of thyroid function than total T4, especially when protein levels are abnormal (e.g., pregnancy, illness, liver disease).
❓ Why is the Free T4 Test Done?
To:
- Diagnose and monitor hypothyroidism and hyperthyroidism
- Evaluate abnormal TSH levels
- Guide and monitor thyroid hormone replacement therapy
- Investigate infertility, fatigue, depression, weight gain, or palpitations
- Evaluate congenital hypothyroidism in newborns
📊 Normal Range
Test | Normal Range |
|---|---|
Free T4 (FT4) | 0.8 – 1.8 ng/dL (or 10 – 23 pmol/L) |
🔍 Values may vary slightly depending on laboratory method and reference range.
📈 Interpretation of Results
FT4 Level | Possible Interpretation |
|---|---|
🔺 High FT4 | - Hyperthyroidism (e.g., Graves’ disease, toxic adenoma) |
🔻 Low FT4 | - Hypothyroidism (e.g., Hashimoto’s thyroiditis) |
🧠 Associated Organs and Conditions
Organ/System | Role |
|---|---|
Thyroid gland | Produces T4 hormone (mostly as inactive precursor of T3) |
Pituitary gland | Regulates T4 via TSH secretion |
Liver & kidneys | Help convert T4 to T3 (active form) and eliminate hormones |
Pregnancy/Estrogen levels | Affect TBG, which can alter total but not free T4 |
🔄 Related / Follow-Up Tests
- TSH (Thyroid Stimulating Hormone) – Always interpret with FT4
- Free T3 – Active thyroid hormone
- Anti-TPO Antibodies – To evaluate autoimmune thyroiditis
- TRAb (TSH receptor antibodies) – For Graves’ disease
- Total T4 / T3 – Less useful but sometimes ordered
- Thyroid Ultrasound – For structural evaluation
- Radioactive Iodine Uptake (RAIU) – For hyperthyroidism subtype
- Lipid Profile / CBC / Liver function tests – To evaluate systemic effects
✅ Fasting Required?
Parameter | Fasting Required |
|---|---|
Free T4 | ❌ No fasting required |
(Though a morning sample is sometimes preferred for consistency, especially during monitoring) |
📝 Summary
Parameter | Summary |
|---|---|
What | Measures unbound, active thyroxine produced by the thyroid gland |
Why Test | To evaluate thyroid function, especially in suspected hypo- or hyperthyroidism |
Normal Range | 0.8 – 1.8 ng/dL (10–23 pmol/L) |
High FT4 | Suggests hyperthyroidism or excessive hormone therapy |
Low FT4 | Suggests hypothyroidism or pituitary dysfunction |
Follow-up Tests | TSH, Free T3, Anti-TPO, TRAb, ultrasound, RAIU |
Fasting Required | ❌ No, but morning preferred for consistency |
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