Interpret Your Free Thyroxine Results
Enter your Free T4 value below and get an instant, free AI interpretation — a plain-English explanation checked against WHO/IFCC reference ranges.
Medical Disclaimer
This AI-generated interpretation is for educational purposes only and does not constitute medical advice, diagnosis, or treatment. Results may vary based on individual health factors not captured here.
Always consult a qualified healthcare professional before making any health decisions. Read full disclaimer →
Free Thyroxine Normal Ranges
Free T4 is the active, unbound form of thyroxine (T4) and represents only ~0.
| Age Group | Reference Range | Unit | Notes |
|---|---|---|---|
| Adults (18–64) | 0.8 – 1.8 | ng/dL | — |
Also reported in: pmol/L.Conversion factor: 1 pmol/L = 12.870 ng/dL.
Why Doctors Order the Free Thyroxine Test
Free T4 is the essential complement to TSH for confirming thyroid dysfunction and grading its severity. In primary hypothyroidism: TSH is high + FT4 is low (overt) or TSH is high + FT4 is normal (subclinical).
How to Enter Your Free T4 Result
Free Thyroxine can be reported in ng/dL or pmol/L. Choose the unit printed next to your result on the lab report so it is compared with the matching reference range.
What a Low or High Free T4 Can Be Associated With
Below the reference range
- Hashimoto's thyroiditis (most common cause of primary hypothyroidism)
- Post-radioiodine therapy
- Post-thyroidectomy
- Severe iodine deficiency
- Central hypothyroidism (pituitary or hypothalamic disease)
Above the reference range
- Graves' disease
- Toxic multinodular goiter
- Toxic thyroid adenoma
- Thyroiditis (transient: Hashimoto thyroiditis "storm", subacute, postpartum)
- Excess levothyroxine therapy
These are common associations, not a diagnosis. A single result is read alongside your symptoms, history and other tests — discuss it with your doctor.
What Can Change Your Free T4 Result
Before you read too much into a result, consider whether any of these applied:
- Biotin supplements (interferes with FT4 immunoassays)
- Pregnancy (FT4 assays may be unreliable; direct dialysis methods preferred)
- Severe non-thyroidal illness (sick euthyroid — FT4 may be low despite normal thyroid)
- Heparin (displaces T4 from binding proteins in vitro)
Preparing for the test: No fasting required. Avoid biotin supplements 24 hours before.
Symptoms That Often Prompt Free T4 Testing
- Low FT4: fatigue, weight gain, constipation, cold intolerance, dry skin, depression
- High FT4: weight loss, heat intolerance, anxiety, palpitations, tremor
Want the full clinical guide to Free Thyroxine?
Reference ranges, causes, symptoms, preparation, and FAQs.