Thyroid Function Blood Test
Fatigue, weight changes, hair thinning, or feeling the cold more than usual? These are classic signs your thyroid may not be performing as it should. This panel measures TSH, FT4 and FT3 — the three key markers.
Doctor-led
UKAS labs
Results in 2-3 days
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How it works
Book a home visit
A health professional takes your sample at home or work
From £65
Book a clinic
Professional blood draw at a nearby partner clinic.
From £40
Free tracked return
Post your kit in any Royal Mail postbox with the pre-paid, tracked label.
Doctor-written report
Every Brooksby Medical result is written and reviewed by a GP. You’ll get plain-English explanations for each marker and clear next steps.
Dr James Coleman, MBChB, MRCGP
Founder & GP, Brooksby Medical
Every report is personally reviewed with clear, actionable advice.
Data secure • UKAS-accredited labs • Doctor-led care
What’s measured in this test
Each biomarker helps reveal insights into your health.
Free Thyroxine (FT4)
What is Free Thyroxine (FT4)?
Free Thyroxine (FT4) is one of the main hormones produced by your thyroid gland. FT4 is the active hormone that is available to travel throughout your body, regulating your metabolism, energy levels, and body temperature.
What does a High Result mean?
A high FT4 level usually means your thyroid gland is overactive (hyperthyroidism). This is most often caused by Graves' disease or thyroid nodules. A temporary rise can occur due to thyroiditis. Some supplements, particularly biotin, can interfere with the lab test.
What does a Low Result mean?
A low FT4 level usually means your thyroid gland is underactive (hypothyroidism). If your FT4 is low and your TSH is raised, this strongly points to a problem with the thyroid gland itself.
Free Triiodothyronine (FT3)
What is Free Triiodothyronine (FT3)?
Free Triiodothyronine (FT3) is one of the active hormones produced by your thyroid gland. It is the most potent of the thyroid hormones, meaning it has the strongest effect on your body's metabolism and energy levels.
What does a High Result mean?
A high FT3 level often means your thyroid gland is overactive (hyperthyroidism). This is most commonly caused by Graves' disease, thyroid nodules, or thyroiditis. Sometimes only FT3 is high while FT4 remains normal — known as T3 toxicosis.
What does a Low Result mean?
A low FT3 result is usually not a sign of a true underactive thyroid unless FT4 is also low and TSH is high. Low FT3 on its own is often linked to other acute or chronic illnesses rather than a thyroid gland problem.
Thyroid Stimulating Hormone
What is TSH?
TSH stands for Thyroid-Stimulating Hormone. It is a hormone produced by the pituitary gland (a master gland located in the brain). The pituitary gland acts like a thermostat, constantly measuring the amount of thyroid hormone in your blood.
The Signal: TSH is the signal the pituitary sends to the thyroid gland telling it how hard to work and how much hormone (FT4 and FT3) to produce.
The Best Test: Because of this feedback loop, TSH is the best initial test to check your overall thyroid function.
What does a High Result mean?
A higher TSH level usually means your pituitary gland is trying to push the thyroid to work harder.
Underactive Thyroid (Hypothyroidism): This is the most common reason. The thyroid gland is not producing enough hormone, so the pituitary increases the TSH signal to compensate.
What does a Low Result mean?
A low TSH level means your pituitary gland has slowed or shut down the signal because it senses there is too much thyroid hormone in your blood.
Overactive Thyroid (Hyperthyroidism): This is the most common reason.
Frequently asked questions
What does a thyroid function blood test check?
This test measures three essential thyroid hormones:
TSH — the pituitary signal telling your thyroid how hard to work
Free T4 (FT4) — the main hormone your thyroid produces
Free T3 (FT3) — the active hormone converted from T4
Together they reveal whether your thyroid is overactive, underactive, or functioning normally.
What symptoms suggest a thyroid problem?
Common symptoms of thyroid dysfunction include:
Underactive (hypothyroid): fatigue, weight gain, feeling cold, low mood, constipation, dry skin
Overactive (hyperthyroid): weight loss, anxiety, tremor, rapid heartbeat, heat intolerance, diarrhoea
When should I take a thyroid blood test?
For the most accurate results, collect your sample between 6–10 AM when thyroid hormones are most stable. If you take thyroid medication (levothyroxine), take it as normal but delay liothyronine until after the sample.
Is fasting required for a thyroid test?
No fasting is required, but timing matters — morning samples (6–10 AM) give the most reliable results. Avoid biotin supplements for 48 hours before testing as they can interfere with the assay.
How will I receive my thyroid results?
Secure online results with a GP-written explanation from Brooksby Medical in 2–3 working days.
Related reading
Articles from our GP that relate to this test - explaining your results, what they mean, and when to take action.
THE BRIEFING
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