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Triiodothyronine (T3)
88.00
The T3 blood test measures triiodothyronine, the active thyroid hormone that regulates metabolism. It is useful in diagnosing and monitoring thyroid conditions.
Turnaround time
1-2 days
Biomarkers count
1
Same-Day Appointments
UKAS Accredited Labs
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Under 18? Patients under 18 can only be seen at GB Medlabs and Clinilabs, Monday to Friday only.
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Details about Triiodothyronine (T3)
What is the Triiodothyronine (T3) test?
The Triiodothyronine (T3) test measures the level of T3 hormone circulating in the blood. T3 is the biologically active thyroid hormone responsible for regulating metabolism, heart rate, body temperature, and energy levels.
Most T3 is produced by conversion from thyroxine (T4) in peripheral tissues, while a smaller amount is produced directly by the thyroid gland. Measuring T3 helps assess how effectively thyroid hormones are being activated and utilised by the body.
This test is often used alongside TSH and T4 to evaluate thyroid function more precisely.
Why do I need a Triiodothyronine (T3) test?
T3 levels can become abnormal even when TSH and T4 are within reference ranges, particularly in certain thyroid conditions.
You may need this test if you experience:
- Unexplained weight loss or weight gain
- Palpitations or rapid heartbeat
- Heat intolerance or excessive sweating
- Anxiety or nervousness
- Fatigue or weakness
- Tremor
- Suspected hyperthyroidism
- Borderline or inconclusive thyroid results
- Monitoring known thyroid disease
This test helps identify active thyroid hormone imbalance and clarify unclear thyroid function results.
Benefits of the test
- Measures active thyroid hormone
Reflects hormone activity at tissue level. - Useful in hyperthyroidism assessment
T3 may be elevated before T4. - Complements TSH and T4 testing
Adds functional insight. - Helps explain persistent symptoms
Useful when other thyroid tests are inconclusive. - Supports treatment monitoring
Tracks response to therapy in some cases. - Simple blood-based testing
Requires only one blood sample. - Widely clinically accepted
Standard thyroid assessment marker. - Useful in complex thyroid cases
Helps identify T3-dominant conditions.
Step-by-step: How we offer testing
Clinic-based testing (UK-wide)
- Choose your nearest UK clinic
- Attend your appointment
- A clinician draws a blood sample
- Sample is sent to the laboratory
- Results are issued once analysis is complete
Home or hotel visit (+£60, UK-wide)
- Book a mobile phlebotomist
- Blood sample taken at your home or hotel
- No need to attend a clinic
- Sample securely transported to the laboratory
Your results explained
Low T3
- Reduced triiodothyronine levels
- UK standard (typical): <1.2 nmol/L
- May be associated with hypothyroidism or reduced hormone conversion
Normal T3
- T3 within reference range
- UK standard (typical): 1.2–3.1 nmol/L
- Suggests normal active thyroid hormone levels
High T3
- Elevated triiodothyronine levels
- UK standard (typical): >3.1 nmol/L
- May indicate hyperthyroidism or T3-dominant thyroid disease
Interpretation should consider TSH, T4, symptoms, and clinical context.
How It Works
Simple steps to get your results
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Book Online
Choose your test online and book in seconds. Select your preferred clinic location or home visit option.
2
Visit or Test at Home
Attend one of our UK or London clinics, arrange a home nurse visit, or use a finger-prick kit where available.
3
Get Your Results
Your sample is analysed by accredited UK laboratories, with secure results delivered directly to you.
Take control of your health with London Blood Tests
WhatsApp UsHow to Prepare for Your Test
Follow these guidelines for accurate results
Sample Timing
Where possible, attend your blood test between 7am and 12pm. Please wait until any short-term illness or infection has fully resolved before testing. Avoid intense exercise for 24–48 hours beforehand, as this can affect certain markers.
Fasting & Hydration
Fasting is not always required, but some tests may recommend it. If fasting is advised, avoid food for 8–12 hours before your appointment and drink water only. Stay well hydrated, as this helps with sample collection and accuracy.
Medications & Supplements
Continue prescribed medications unless advised otherwise by your clinician. Avoid vitamin, mineral, or biotin supplements for at least 24–48 hours before your test, as these can interfere with results. If you are unsure about any medication or supplement, please let us know before your appointment.
Hormones & Menstrual Cycle
For hormone-related tests, timing within your menstrual cycle may be important. If relevant, follow any specific guidance provided on your test page.
If you have any questions or special circumstances, our team is happy to advise before your appointment.
Frequently Asked Questions
T3 is the active hormone; T4 is a precursor.
Yes, particularly in certain thyroid conditions.
It may be helpful in selected cases.
No, this test measures total T3.
Yes, acute illness can lower T3 levels.
Fasting is not usually required.
Yes, some drugs influence thyroid hormone levels.
Sometimes, depending on clinical context.
Pregnancy can influence thyroid hormone levels.
It suggests increased thyroid hormone activity and supports further endocrine evaluation.
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