What does it measure?
T3 is an active thyroid hormone made partly by the thyroid and partly by conversion from T4 in body tissues. Free T3 measures the small unbound portion. T3 testing is most often useful when an overactive thyroid is suspected, especially if TSH is low and free T4 is not raised. [1]
Before the test
- Sample
- A blood sample from a vein.
- Preparation
- Usually no special preparation is needed. Mention thyroid treatment and supplements that may affect the assay.
What can the result mean?
A high result may support excess thyroid hormone when the rest of the pattern fits. A low result can occur during significant illness or low energy intake without a primary thyroid disorder. Normal T3 does not rule out an underactive thyroid. [1]
When is it useful, and how is it read?
T3 testing is more useful in selected evaluations for thyroid excess, for example when TSH is suppressed but free T4 is not raised. For suspected thyroid deficiency, TSH and free T4 usually provide the more useful starting pattern; a free T3 assay has additional reliability limits. [2] [4]
What can affect it?
Free T3 assays have limitations, and some clinicians prefer total T3 for particular questions. Medicines, biotin interference, and the timing of T3-containing treatment can affect results. Comparing values between different laboratories may be misleading. [1] [2]
Low T3 does not automatically mean an underactive thyroid
T3 can fall during serious illness or a major reduction in food intake even when the thyroid is not the primary problem. Treating that number alone as proof of hypothyroidism can miss the actual cause. [4] [5]
Questions you might have
Does normal T3 rule out an underactive thyroid?
No. T3 may remain normal even when thyroid hormone supply is low. TSH and free T4 are usually more informative for that question. [2]
Why is T3 sometimes useful when T4 is normal?
Some overactive-thyroid patterns raise T3 before T4. Symptoms and TSH help interpret that combination. [1]
What comes next?
Start with TSH and free T4, then ask whether T3 adds information for your situation. Your clinician can decide whether confirmation or a different test is useful. Avoid starting thyroid hormone or supplements simply to raise free T3 when the reason for the change is unclear. [1] [2]
What specific thyroid question does free T3 help answer that TSH and free T4 do not?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
- MedlinePlus — Triiodothyronine T3 Tests
- American Thyroid Association — Thyroid Function Tests
- MedlinePlus. TSH (Thyroid-stimulating hormone) Test.
Books we consulted
References that informed our explanations of the test and its context.
- Pagana KD, Pagana TJ, Pagana TN. Mosby’s Diagnostic and Laboratory Test Reference. 14th ed. Elsevier; 2019.
- Moisio MA, Moisio EW. Understanding Laboratory and Diagnostic Tests. Delmar Publishers; 1998.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
