Thyroid Blood Tests
Understand your thyroid. Compare your testing options. Know what your results may mean.
Marylebone Diagnostic Centre provides private thyroid blood testing from Baker Street, London, including TSH, FT4, FT3, T3, T4 and thyroid antibody testing.
Whether you are investigating symptoms, monitoring known thyroid disease or trying to understand an abnormal result, this guide explains the main thyroid markers and helps you compare MDC's thyroid profiles.
What is a thyroid blood test?
A thyroid blood test measures hormones and, where appropriate, antibodies that provide information about how the thyroid gland is functioning.
TSH is commonly used as the initial marker. FT4 is used alongside TSH in particular circumstances or when TSH is abnormal, while FT3 can provide additional information, particularly when an overactive thyroid is suspected. Thyroid antibodies can help investigate an autoimmune cause.
NICE recommends TSH alone as an initial test for many adults when secondary thyroid dysfunction is not suspected, with FT4 measured when TSH is high and FT4 plus FT3 when TSH is low. (NICE)
Important: More testing is not automatically better. The appropriate thyroid profile depends on the clinical question.
Thyroid Test Results Chart: TSH, FT4 & FT3
Thyroid results are best interpreted as a pattern rather than as individual numbers.
Use the reference ranges printed on your own laboratory report. A result marked “high” or “low” below means that it is outside that laboratory's reference interval. Open an explanation to learn more.
| TSH | FT4 / FT3 | Pattern that may be considered |
|---|---|---|
| Within range | FT4 within range | Usually consistent with normal thyroid functionThis is generally reassuring. Persistent symptoms still deserve review because other conditions can cause similar symptoms. |
| High | FT4 low | Possible primary hypothyroidismThis pattern suggests primary hypothyroidism. A clinician should assess your symptoms, medicines and results before advising treatment. |
| High | FT4 within range | Possible subclinical hypothyroidismA raised TSH with normal FT4 may indicate subclinical hypothyroidism. Repeat testing and clinical context help determine whether monitoring or treatment is appropriate. |
| Low | FT4 and/or FT3 high | Possible hyperthyroidismThis can indicate hyperthyroidism and needs clinical review. FT3 may be high even when FT4 is within range. |
| Low | FT4 and FT3 within range | Possible subclinical hyperthyroidismThis pattern may indicate subclinical hyperthyroidism. A clinician may repeat testing and check for medication effects or other causes. |
| Low or inappropriately within range | FT4 low | Requires further clinical assessmentA normal TSH does not always exclude a problem. This pattern can occur with pituitary conditions, significant illness or medication effects and needs clinical interpretation. |
This chart is an educational guide, not a diagnosis.
Thyroid results must be considered alongside symptoms, medical history, medications, pregnancy status where relevant, illness and the laboratory's own reference ranges.
High-dose biotin supplements can also interfere with some thyroid blood tests and produce falsely high or low results, so patients should tell the clinical team about supplements they take. NICE specifically recommends asking about biotin intake when thyroid dysfunction is being investigated. (NICE)
Do not start, stop or alter thyroid medication based solely on this chart.
This broadly follows the logic used in NICE thyroid assessment: raised TSH leads to FT4 assessment, while low TSH leads to FT4 and FT3 assessment. (NICE)
TSH, FT4, FT3, T4, T3 & Thyroid Antibodies Explained
TSH — Thyroid-Stimulating Hormone
TSH is produced by the pituitary gland and signals the thyroid to produce thyroid hormones. It is one of the principal markers used to assess thyroid function.
Simple explanation: TSH is the signal sent to the thyroid.
FT4 — Free Thyroxine
FT4 measures the free, biologically available portion of thyroxine circulating in the blood. It is particularly useful when interpreted alongside TSH.
Simple explanation: FT4 helps show how much available thyroxine is circulating.
FT3 — Free Triiodothyronine
FT3 measures free triiodothyronine, an active thyroid hormone. It can be particularly useful when investigating suspected hyperthyroidism; NICE recommends measuring FT3 alongside FT4 when TSH is below the reference range. (NICE)
Simple explanation: FT3 measures an active circulating thyroid hormone.
Total T4 & Total T3
Total T4 and T3 measure both bound and unbound hormone in the bloodstream. They are included in some of MDC's more extensive thyroid profiles.
A-TPO — Thyroid Peroxidase Antibodies
TPO antibodies can help investigate whether an autoimmune process is affecting the thyroid. NICE recommends considering TPO antibody measurement in adults with TSH above the reference range. (NICE)
A-TG — Thyroglobulin Antibodies
Thyroglobulin antibodies are another marker associated with thyroid autoimmunity. Antibody results should be interpreted alongside thyroid function and the clinical picture rather than used alone to decide whether the thyroid is underactive or overactive.
Compare Thyroid Blood Tests at MDC
You do not necessarily need the largest profile.
Choose testing according to the reason for investigation. If you already have medical advice about which markers are required, select the corresponding profile.
A focused thyroid function profile measuring the main regulatory signal and free thyroxine.
Sample: Blood
Price includes phlebotomy
Adds FT3 to provide a broader assessment of circulating thyroid hormones.
Sample: Blood
Price includes phlebotomy
Turnaround: 1–3 working days
An extended hormone profile containing both free thyroxine and total T3/T4 measurements.
Sample: Blood
View Thyroid Profile 3Combines thyroid function with thyroid antibodies, providing information about hormone status and possible thyroid autoimmunity.
Sample: Blood
Price includes phlebotomy
Turnaround: 1–3 working days
MDC's most extensive currently listed thyroid profile, combining free and total thyroid hormones with thyroid antibodies.
Sample: Blood
Price includes phlebotomy
Turnaround: 1–3 working days
Which Thyroid Test Might I Need?
My TSH is high / hypothyroidism is being investigated
FT4 is particularly important. Thyroid antibodies may also be considered depending on the clinical context.
→ Profile 1 or a profile including antibodies where clinically appropriateMy TSH is low / hyperthyroidism is being investigated
FT4 and FT3 become particularly relevant.
→ Profile 2 or another appropriate profileAutoimmune thyroid disease is being investigated
Thyroid function plus appropriate antibody testing may be useful.
→ Thyroid Profile 4I have been advised to obtain broader thyroid testing
MDC offers broader hormone and antibody combinations.
→ Compare Profiles 3–5When Might Thyroid Testing Be Considered?
Thyroid dysfunction can produce a wide range of symptoms, and individual symptoms are not specific enough to diagnose a thyroid condition.
Possible symptoms can include:
Key message: Symptoms can suggest that thyroid testing may be appropriate, but symptoms alone cannot tell whether your thyroid is underactive, overactive or functioning normally.
Do I Need to Fast for a Thyroid Blood Test?
For thyroid testing alone, fasting is not usually required.
However, preparation may differ if thyroid markers are being collected alongside other blood tests.
Patients should tell the clinical team about medication and supplements they take.
Particularly important: biotin. High biotin intake can interfere with some thyroid assays and lead to misleading results. (NICE)
Patients taking prescribed thyroid medication should follow their clinician's instructions and should not alter treatment simply to prepare for a test unless advised to do so.
Thyroid Testing: Quick Answers
Is a thyroid test a blood test?
Yes. Routine thyroid function testing is performed using a blood sample.
What does TSH measure?
TSH is the pituitary signal that regulates thyroid hormone production.
What does a high TSH mean?
A high TSH can occur when the thyroid is underactive, but it needs to be interpreted alongside FT4 and the clinical context.
What does a low TSH mean?
A low TSH can occur with an overactive thyroid, but FT4 and FT3 help determine the overall pattern.
Can my TSH be high while FT4 is normal?
Yes. This pattern can occur in subclinical hypothyroidism and requires interpretation in context.
What are thyroid antibodies?
Thyroid antibodies can provide evidence of thyroid autoimmunity but do not alone show whether the thyroid is underactive or overactive.
Do I need to fast?
Usually not for thyroid testing alone.
Can biotin affect thyroid results?
Yes. High biotin intake can interfere with some thyroid laboratory tests.
Learn More About Your Thyroid
Does a Thyroid Test Require Blood?
Explains how thyroid testing is performed and which markers can be measured.
TSH Blood Test Explained
What TSH measures, why it changes and how it is interpreted with other thyroid markers.
Thyroid Testing in London
An overview of private thyroid investigation and common reasons people seek testing.
Thyroid Blood Tests Explained: TSH, FT4, FT3, Antibodies & Results
Frequently Asked Questions
What is a normal TSH level?
Does a high TSH mean hypothyroidism?
Does low TSH mean hyperthyroidism?
Can I have thyroid symptoms with normal results?
What is the difference between FT4 and FT3?
What do positive thyroid antibodies mean?
Do thyroid tests require blood?
Do I need a GP referral for private thyroid testing?
Clinical References
- NICE — Thyroid disease: assessment and management (NG145), including recommendations on TSH, FT4, FT3, TPO antibodies and biotin interference.
- British Thyroid Foundation — Thyroid Function Tests, a patient-facing explanation of why TSH is often tested first and when FT4, FT3 and antibodies are added.
