Free T4
Free T4 measures the unbound, active form of thyroxine, the main hormone produced by the thyroid gland. It is one of the most important tests for evaluating thyroid function.
Category: Thyroid Panel
Also known as: free t4, ft4, thyroxine free, free thyroxine, T4 Free, slobodni t4
Reference range: 0.8–1.8 ng/dL (Source: ABIM)
Standard unit: ng/dL
High values may indicate
High free T4 may indicate hyperthyroidism, Graves' disease, thyroiditis, or excessive thyroid hormone medication.
Low values may indicate
Low free T4 may indicate hypothyroidism, pituitary disorder, severe illness, or iodine deficiency.
Frequently asked questions
- What is a normal Free T4 level?
- A typical reference range for Free T4 is 0.8–1.8 ng/dL (source: ABIM). Reference ranges vary by laboratory, age, and sex, so always compare against the range on your own lab report.
- What units is Free T4 measured in?
- Free T4 is most commonly reported in ng/dL. Labs in different countries may use other units — you can convert between them with BloodId's blood test unit converter.
- Where do Free T4 reference ranges come from?
- BloodId uses ABIM standard reference ranges and educational information from MedlinePlus (see our methodology). Reference ranges still vary by lab, age, and sex, so always compare against your own report.
- Can I track Free T4 over time?
- Yes. Upload results from any lab and BloodId charts your Free T4 over time against its reference range, so you can see whether the trend is rising, falling, or stable.