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🧪 What is Free Triiodothyronine (FT3)?

Free T3 (FT3) is the biologically active form of Triiodothyronine (T3) — one of the two primary hormones produced by the thyroid gland.

Only about 20% of T3 is secreted directly by the thyroid.
The remaining 80% is converted from T4 (Thyroxine) in the liver and other tissues.
FT3 refers to the unbound (free) fraction of T3 circulating in the blood, available to act on cells.

❓ Why is the Free T3 Test Done?

To:

Diagnose hyperthyroidism or T3 toxicosis
Evaluate thyroid hormone activity when TSH or Free T4 is borderline
Assess thyroid function in patients with goiter, palpitations, weight loss, or fatigue
Monitor thyroid hormone replacement therapy
Evaluate suspected pituitary or hypothalamic dysfunction

📊 Normal Range

Test	Normal Range
Free T3 (FT3)	2.3 – 4.2 pg/mL (or 3.5 – 6.5 pmol/L)

🔍 Lab-specific ranges may vary slightly depending on assay used.

📈 Interpretation of Results

FT3 Level	Clinical Interpretation
🔺 High FT3	- Primary hyperthyroidism (Graves’ disease) - T3 toxicosis (early or mild hyperthyroidism with elevated T3 but normal T4) - Thyroid nodules (toxic adenoma) - Over-supplementation with thyroid hormone
🔻 Low FT3	- Hypothyroidism (especially late stages) - Non-thyroidal illness syndrome (euthyroid sick syndrome) - Severe malnutrition or fasting - Chronic illness (liver, kidney, heart disease)

⚠️ FT3 is often normal in early hypothyroidism and drops in severe systemic illness despite normal thyroid function.

🧠 Associated Organs and Conditions

Organ/System	Relevance
Thyroid gland	Produces some T3 and most T4 (precursor)
Liver & Kidneys	Convert T4 to T3 (via deiodination)
Pituitary gland	Regulates T3 via TSH
Hypothalamus	Controls TSH secretion through TRH

🔄 Related / Follow-Up Tests

TSH (Thyroid Stimulating Hormone) – Most sensitive initial test
Free T4 – Often interpreted with FT3 to evaluate full thyroid function
Total T3 – Less useful than FT3 in most cases
Anti-TPO Antibodies – To assess autoimmune thyroid disease (e.g., Hashimoto’s)
TRAb (TSH receptor antibodies) – For Graves’ disease
Thyroid Ultrasound – For nodules or goiter
RAIU Scan – To assess functional activity of the thyroid in hyperthyroidism

✅ Fasting Required?

Parameter	Requirement
Free T3	❌ No fasting required, but some clinicians prefer morning sampling for consistency, especially during therapy monitoring

📝 Summary

Parameter	Summary
What	Measures free (unbound), biologically active T3, the key active thyroid hormone
Why Test	To evaluate hyperthyroidism, borderline thyroid dysfunction, or treatment monitoring
Normal Range	2.3 – 4.2 pg/mL (3.5 – 6.5 pmol/L)
High FT3	Indicates hyperthyroidism, especially T3 toxicosis
Low FT3	May reflect hypothyroidism or non-thyroidal illness
Follow-up Tests	TSH, Free T4, Anti-TPO, TRAb, thyroid ultrasound, RAIU
Fasting Required	❌ No, but morning preferred for consistency

Free Triiodothyronine (FT3)

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