Thyroid Blood Tests Explained 2026. What TSH, T4, T3 and Antibodies Actually Mean and Which Tests Are Worth Paying For
Most people who suspect a thyroid problem are given 1 test, told it is normal, and sent away. TSH alone is a reasonable screening test and a poor diagnostic one, because it tells you the pituitary's opinion of your thyroid without telling you the hormone levels, the cause, or whether an autoimmune process is under way. A complete picture takes 4 things. TSH, which rises when the thyroid underperforms and falls when it overperforms, with most UK laboratories reporting a reference range of roughly 0.4 to 4.0 mU/L. Free T4, the main circulating hormone, which distinguishes an overt problem from a borderline one.
Free T3, the active hormone that does the work in tissues, useful in hyperthyroidism and in assessing conversion. And antibodies, TPO and thyroglobulin for Hashimoto's and TSH receptor antibodies for Graves', which identify the cause rather than the level and which a standard panel routinely omits. Around 4% to 10% of adults have subclinical hypothyroidism, meaning a raised TSH with a normal free T4, and whether that needs treating depends on the number, your age, your symptoms and your antibody status rather than on the word abnormal. The mimics matter as much, because iron, B12 and vitamin D deficiency produce the same symptoms and are commoner. At The Wellness the panel is chosen after an assessment and arranged at an accredited laboratory, with a further appointment to go through the results properly. Targeted thyroid panel from £295, comprehensive panel from £495. Fees appear further down this page.
Reviewed by the medical team at The Wellness. Last updated August 2026.
Arrange a thyroid panel on WhatsApp or email team@thewellnesslondon.com or call 020 3951 3429.
What does each thyroid blood test measure
TSH is made by the pituitary and instructs the thyroid. When the thyroid underperforms the pituitary shouts louder and TSH rises, so a high TSH usually means an underactive gland. When the thyroid overproduces, the pituitary goes quiet and TSH falls, so a suppressed TSH usually means an overactive one. Most UK laboratories report a range of around 0.4 to 4.0 mU/L, though ranges vary between assays and the upper limit remains debated.
Free T4 is the main hormone the thyroid releases and the free fraction is the portion available to tissues. It separates overt disease, where TSH is abnormal and free T4 has moved with it, from subclinical disease, where TSH is abnormal and free T4 is still normal. Free T3 is the active hormone, converted from T4 mostly in the liver and kidneys. It matters most in hyperthyroidism, where T3 can rise before T4, and in assessing people who feel unwell on treatment despite a normal TSH.
Antibodies identify the cause. TPO antibodies are raised in the large majority of Hashimoto's thyroiditis and their presence in someone with a borderline TSH substantially raises the likelihood of progressing to overt hypothyroidism. Thyroglobulin antibodies add further information. TSH receptor antibodies confirm Graves' disease and are the test that distinguishes it from other causes of an overactive gland, which changes the treatment entirely.
What does subclinical hypothyroidism mean and does it need treating
Subclinical hypothyroidism means a raised TSH with a normal free T4, and it affects roughly 4% to 10% of adults, considerably more in older women. It is the single commonest thyroid finding and the one that causes most confusion, because the word abnormal appears on the report while the hormone levels are intact.
NICE guideline NG145 sets out the approach. Where TSH is above 10 mU/L on 2 occasions 3 months apart, levothyroxine is offered even without symptoms, because the risk of progression and of cardiovascular effects is meaningful. Where TSH sits between the top of the reference range and 10, the decision depends on you. In people under 65 with symptoms consistent with hypothyroidism, a 6-month trial of treatment is reasonable, continued only where symptoms measurably improve. In people over 80, watchful waiting is usually preferred, because mildly raised TSH in older age is common and treating it has not been shown to help.
Antibody status shifts that judgement. A raised TSH with positive TPO antibodies is far more likely to progress than the same number without them, which is why testing antibodies at the outset changes what happens next rather than adding a line to a report.
Ask what your results mean on WhatsApp or email team@thewellnesslondon.com.
Which thyroid tests are not worth paying for
Being honest about this saves people money and confusion. Reverse T3 is measured by several private panels and marketed as a measure of impaired conversion. It rises predictably in acute illness, starvation and stress, it has no established reference range for outpatient use, and no guideline body recommends it for diagnosing or managing thyroid disease. A raised result almost never changes treatment.
Thyroid function testing during an acute illness is similarly unhelpful. Non-thyroidal illness syndrome, sometimes called sick euthyroid, produces a low T3, a variably low TSH and a confusing picture in people whose thyroids are entirely normal, so testing during or shortly after a significant illness produces results that need repeating anyway.
Iodine and selenium testing is rarely useful in the UK, where iodine deficiency is uncommon, and supplementing iodine can worsen autoimmune thyroid disease rather than help it. Salivary and hair mineral testing has no validated role. And repeating a TSH weeks after a dose change tells you little, because the axis takes 6 to 8 weeks to settle, which is the interval to use.
Biotin deserves a specific mention because it causes real errors. High-dose biotin, common in hair, skin and nail supplements, interferes with many thyroid immunoassays and can produce results that mimic Graves' disease. Stop it for at least 48 hours before testing and tell whoever is interpreting the result that you take it.
How should thyroid tests be timed and prepared for
Timing matters more than most people are told. TSH follows a daily rhythm, peaking overnight and falling through the morning, so a sample taken at 8am can read meaningfully higher than one taken at 3pm from the same person. For consistency, test in the morning and use the same time of day for repeat testing, which matters when small differences are driving a treatment decision.
If you already take levothyroxine, take your dose after the blood test rather than before, since taking it beforehand transiently raises free T4 and can make a dose look higher than it is. Biotin supplements should be stopped for at least 48 hours. Fasting is not required for thyroid tests, although if the panel includes glucose or lipids you will be asked to fast for those.
After any dose change, wait 6 to 8 weeks before retesting. The pituitary responds slowly and testing at 3 weeks produces a number that will have moved again by 8. In pregnancy the picture changes entirely, with different trimester-specific ranges and a need for prompt assessment, so anyone pregnant or planning pregnancy with known or suspected thyroid disease should be assessed rather than waiting for a routine interval.
What do thyroid blood tests cost in London
Consultant endocrinology appointments in the Harley Street district run £250 to £450 before any test is ordered, with the panel and any follow-up charged separately. Advanced biomarker panels of 80 to 100 markers are sold at £600 to £1,000. Comprehensive imaging programmes at the top of the London market exceed £32,000. Below all of it sits a tier defined by its conditions rather than its price, a TSH sold as a standalone finger-prick test with no antibodies, no mimics screened and nobody to read the number against your symptoms.
At The Wellness the panel is chosen after an assessment and the interpretation appointment is included. All figures are from prices.
Executive Health Programme, the most comprehensive assessment with full biomarker profiling and coordinated imaging, from £11,995.
Executive Body Scan, consultant-reported imaging including the thyroid with full bloods and consolidated review, from £5,995.
Complete Thyroid Assessment, thyroid and neck ultrasound, full thyroid and metabolic panel including ferritin, B12 and vitamin D, extended review and coordination of specialist referral where indicated, from £1,795.
Combined thyroid assessment, ultrasound with the full thyroid panel and review appointment, from £1,295.
Comprehensive Blood Panel covering TSH, free T4, free T3, TPO and thyroglobulin antibodies alongside ferritin and iron studies, B12, folate, vitamin D, full blood count, HbA1c, kidney and liver function, with consultation and interpretation, from £495.
Targeted thyroid panel covering TSH, free T4, free T3 and antibodies with interpretation, from £295.
Interpretation of thyroid results taken elsewhere, by video or in person, from £150.
TSH receptor antibodies, thyroid ultrasound and endocrinology referral arranged where the picture indicates.
Assessments within the week in Marylebone, 3 minutes from Baker Street, with blood collection arranged at an accredited laboratory.
Why The Wellness is the best place in London for thyroid blood tests
Because the panel is chosen for you and the result is read against you. A TSH interpreted without antibodies tells you a level without a cause. A borderline TSH interpreted without ferritin, B12 and vitamin D produces a treatment decision made in the dark, since those deficiencies produce the same symptoms and are commoner than thyroid disease. And a number read against a laboratory range rather than against your age, your symptoms, your antibody status and your previous results is a data point rather than a diagnosis.
The practical details are handled properly too. Morning sampling for consistency, dose taken after the test rather than before, biotin stopped 48 hours beforehand, and repeat testing at 6 to 8 weeks rather than 3. Those 4 things alone account for a large share of the confusing thyroid results people carry around for years.
And the tests that do not help are not sold. No reverse T3, no hair mineral analysis, no iodine panel in a country where deficiency is rare and supplementation can make autoimmune disease worse. What you pay for is the panel that changes what happens next, and an appointment where somebody explains it.
Arrange your panel on WhatsApp or call 020 3951 3429.
Frequently asked questions
What thyroid blood tests should I have
TSH, free T4, free T3 and thyroid antibodies as a minimum, with TSH receptor antibodies added where an overactive thyroid is suspected. Alongside them, ferritin and iron studies, B12, folate, vitamin D, full blood count and HbA1c, because those deficiencies produce the same symptoms and are commoner than thyroid disease.
What is a normal TSH level
Most UK laboratories report a reference range of roughly 0.4 to 4.0 mU/L, though ranges vary between assays. A raised TSH with a normal free T4 is subclinical hypothyroidism, affecting around 4% to 10% of adults, and whether it needs treating depends on the number, your age, your symptoms and your antibody status rather than on the word abnormal.
Should I be treated for subclinical hypothyroidism
NICE guideline NG145 advises offering levothyroxine where TSH is above 10 mU/L on 2 occasions 3 months apart. Between the top of the range and 10, a 6-month trial is reasonable in people under 65 with symptoms, continued only if symptoms improve, while watchful waiting is usually preferred over 80. Positive TPO antibodies make progression considerably more likely.
Is reverse T3 worth testing
No. It rises predictably in acute illness, starvation and stress, has no established outpatient reference range, and is not recommended by any guideline body for diagnosing or managing thyroid disease. A raised result almost never changes treatment, which is why we do not sell it.
When should I have my thyroid blood test
In the morning, because TSH peaks overnight and falls through the day, and at the same time of day for repeat tests. Take levothyroxine after the test rather than before, stop biotin supplements at least 48 hours beforehand because they interfere with many thyroid assays, and wait 6 to 8 weeks after any dose change before retesting.
Can I have thyroid symptoms with a normal TSH
Yes, and it is common. The symptoms overlap almost entirely with iron deficiency, vitamin D and B12 deficiency, perimenopause, sleep apnoea and depression, all of which are commoner than thyroid disease. A normal TSH is a reason to test those rather than a reason to stop looking.
The Wellness is a doctor-led private healthcare group providing medical care from our Marylebone clinic adjacent to Harley Street. All doctors are GMC-registered. Blood analysis is performed by accredited laboratories and imaging is arranged at specialist centres and reported by consultant radiologists. This article is general information and not a substitute for medical advice about your own health.
Enquire now on WhatsApp or email team@thewellnesslondon.com or call 020 3951 3429.
References. NICE guideline NG145, thyroid disease assessment and management, including thresholds for treating subclinical hypothyroidism. NICE Clinical Knowledge Summaries on hypothyroidism and hyperthyroidism. British Thyroid Association and Association for Clinical Biochemistry guidance on thyroid function testing. Published evidence on diurnal variation in TSH, on biotin interference with immunoassays, and on non-thyroidal illness syndrome. Published prevalence data on subclinical hypothyroidism in adult populations. Published guidance on thyroid function in pregnancy and trimester-specific reference ranges.
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