Test Summary
Analysis Type
Serum; Immunoassay
Collection method
In person at the lab; at-home where available
Why test
fT3
?
- Helps explain thyroid symptoms when TSH or Free T4 are unclear.
- Gauges severity when TSH is low and hyperthyroidism is suspected.
- Monitors response to treatment or dose changes.
- Assesses combination T4/T3 therapy or possible over-replacement.
What is
fT3
?
- Helps explain thyroid symptoms when TSH or Free T4 are unclear.
- Gauges severity when TSH is low and hyperthyroidism is suspected.
- Monitors response to treatment or dose changes.
- Assesses combination T4/T3 therapy or possible over-replacement.
What insights will i get from
fT3
?
- High: may suggest hyperthyroidism or too much thyroid medicine; confirm with TSH and Free T4.
- Low: may occur in hypothyroidism or serious illness without thyroid disease.
- Normal but symptoms: review TSH and Free T4; Free T3 alone rarely explains.
- Trends over time matter; compare with symptoms, TSH, and Free T4.
Sample type & collection
- Sample: Blood
- Fasting: No
- Best timing: Morning preferred for consistency
- Collection: In person at the lab; at-home where available
- Typical volume:
- Analysis: Serum immunoassay
- Prep tips: Avoid high-dose biotin 24-48 hours before testing
- Turnaround: Usually 1-3 business days
fT3
is best interpreted with:
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fT3
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Limits & interferences with
fT3
:
- Biotin supplements can skew results; stop high-dose 24-48 hours before testing.
- Recent illness, surgery, or fasting can lower T3 temporarily.
- Medications like amiodarone, steroids, and propranolol affect T4 to T3 conversion.
- Heparin and certain antibodies can cause assay interference.
- Special situations (when to confirm or adjust): pregnancy, critical illness, high biotin, or assay concerns.
Questions about
fT3
:
- Do I need to fast? No. You can test at any time.
- Should I stop my thyroid medication before testing? Keep taking it unless your clinician advises otherwise. If on liothyronine, they may request a pre-dose draw.
- What is the difference between Free T3 and Total T3? Free T3 measures unbound hormone; Total T3 includes hormone bound to proteins.
- Is Free T3 a first-line test? No. TSH is the main screening test. Free T3 supports the overall picture.
Sources:
- American Thyroid Association — Thyroid Function Tests. (2023). https://www.thyroid.org/thyroid-function-tests/
- MedlinePlus — T3 (Triiodothyronine) Test. (2023). https://medlineplus.gov/lab-tests/t3-triiodothyronine-test/
- NICE — Thyroid disease: assessment and management. (2019). https://www.nice.org.uk/guidance/ng145
- ARUP Consult — Thyroid Function Testing. (2024). https://arupconsult.com/content/thyroid-function-testing
- NIDDK — Thyroid Tests. (2021). https://www.niddk.nih.gov/health-information/diagnostic-tests/thyroid-tests
Medical disclaimer: This content is for informational purposes only and is not medical advice. Always discuss results with a qualified healthcare professional.
