Thyroid Function Blood Test

• TSH (measured): the pituitary's signal to the thyroid. High suggests the thyroid is underactive; low suggests excess thyroid hormone, or less commonly a pituitary problem.
• Free T4 (measured): the main hormone released by the thyroid, in its unbound form.
• Free T3 (measured): the more active hormone, in its unbound form. Most useful when TSH is low, to assess an overactive thyroid and its severity.

The laboratory counts this panel as 3 biomarkers, all measured by immunoassay. Thyroid antibodies (TPO and thyroglobulin antibodies) and TSH receptor antibodies are not included; thyroid antibodies are available as a separate test.

The thyroid is a butterfly-shaped gland at the front of the neck. It makes two hormones that set the pace of metabolism in almost every tissue: thyroxine (T4), which forms most of its output, and triiodothyronine (T3), the more active form. Much of the body's T3 is made by converting T4 in the tissues.

A feedback loop

The hypothalamus in the brain signals the pituitary gland, which releases TSH to stimulate the thyroid. When thyroid hormone levels rise, TSH falls; when they fall, TSH rises. Because TSH responds strongly to small changes in thyroid hormone, it is usually the most sensitive first test of thyroid function.

Why free hormones are measured

Most T4 and T3 in the blood is bound to carrier proteins and is inactive. The small free fraction is what reaches the tissues, so laboratories measure free T4 and free T3.

How UK guidance approaches testing

NICE guideline NG145 suggests that for adults without suspected pituitary disease, clinicians can start with TSH alone, adding free T4 if TSH is high and free T4 and free T3 if TSH is low. Measuring all three together gives a complete picture in one sample, which can be helpful when symptoms are unclear or when TSH may not reflect thyroid status reliably, such as in pituitary conditions. More tests do not by themselves lead to better decisions; their value is in how they are interpreted.

In the UK the most common cause of an underactive thyroid is Hashimoto's disease, an autoimmune condition, and the NHS reports that Graves' disease accounts for about 4 in 5 cases of an overactive thyroid.

Your report shows each result against the laboratory's reference range. The British Thyroid Foundation gives typical adult ranges of about 0.4 to 4.0 mU/L for TSH, 9 to 25 pmol/L for free T4 and 3.5 to 7.8 pmol/L for free T3, but stresses that ranges vary between laboratories and assay methods. Different ranges apply in pregnancy and childhood.

Common patterns

• High TSH, low free T4: suggests an underactive thyroid (hypothyroidism).
• Raised TSH, normal free T4: described as subclinical hypothyroidism. NICE suggests considering levothyroxine when TSH is 10 mU/L or higher on two occasions 3 months apart, and periodic monitoring otherwise.
• Low TSH, high free T4 and/or free T3: suggests an overactive thyroid (hyperthyroidism).
• Low TSH, normal free T4 and free T3: subclinical hyperthyroidism; NICE suggests repeat TSH monitoring.
• Low or normal TSH with low free T4: can reflect a pituitary problem or a response to serious illness elsewhere in the body.

Things that affect thyroid results

• Biotin: NICE advises asking about biotin intake, because biotin supplements can cause falsely high or low results on some thyroid assays.
• Acute illness: NICE advises against testing during acute illness unless thyroid disease is suspected as the cause, as illness can alter results temporarily.
• Medicines: amiodarone, lithium, steroids, some anticonvulsants and the contraceptive pill can affect results, as can supplements containing iodine.
• Pregnancy: TSH is often lower in the first trimester.
• Thyroid treatment: after a dose change, tests are not usually repeated sooner than 6 weeks, as levels take time to settle.

The British Thyroid Foundation notes that current guidelines do not recommend thyroid hormone treatment when levels are within the normal range. If your results need medical attention you will be advised to see your GP, and with your consent Juvenology can write to them. Please do not start, stop or change thyroid medication because of this result.

Thyroid conditions are common and can develop slowly, with symptoms that overlap with many other causes. NICE suggests considering thyroid tests for adults with symptoms that suggest thyroid disease, with depression or unexplained anxiety, and offering them to people with type 1 diabetes or other autoimmune conditions and new atrial fibrillation. The British Thyroid Foundation also lists neck swelling, a fast or irregular heartbeat and fertility problems as reasons to test.

A private test can be useful if you would like current results before seeing your GP, or to follow up a previous borderline result. If you already take thyroid medication, your GP will normally arrange monitoring.

This test suits adults with symptoms they would like to discuss with their GP, people following up a previous borderline TSH, and those with a personal or family history of thyroid or autoimmune disease.

May not be the right choice

• Healthy without symptoms: there is no UK recommendation for routine thyroid screening in adults without symptoms or risk factors.
• Acutely unwell: results during acute illness can be misleading. A very fast or irregular heartbeat, chest pain or severe breathlessness needs urgent medical help.
• Pregnancy: thyroid tests in pregnancy use different ranges and should be arranged by your GP or maternity team.
• On treatment: monitoring of levothyroxine or anti-thyroid medicine is normally arranged by your GP or specialist.

Established clinical use: thyroid function tests are the standard way to diagnose and monitor overactive and underactive thyroid conditions.

Population patterns: autoimmune thyroid disease is more common in women, and the NHS notes Hashimoto's disease most often begins between ages 30 and 50.

Areas of ongoing research and caution: mildly raised TSH is common in older adults, and researchers continue to study whether TSH naturally drifts upwards with age and when mild changes are worth treating. Because of this uncertainty, NICE sets clear thresholds and repeat-testing intervals before treatment is considered, rather than treating a single result.

What this test cannot tell you: it does not identify the cause of a thyroid problem, measure metabolism or energy directly, predict lifespan or give a "biological age". There is no evidence-based "optimal" thyroid range for longevity.

• Biotin: biotin (vitamin B7) is found in many hair, skin and nail supplements. Tell us if you take it; the laboratory or your clinician can advise how long to pause it before your sample.
• Thyroid medication: keep to your usual routine unless your clinician advises otherwise, and note when you last took your dose.
• Timing: fasting is not needed. If you are comparing with earlier results, a similar time of day makes comparison easier.
• After a dose change: wait at least 6 weeks before retesting.
• Illness: if you are acutely unwell, wait until you have recovered unless a clinician advises otherwise.
• Other medicines and supplements: list everything you take, including iodine or kelp supplements.

Choose how your sample is taken when you book. We confirm the collection option, any collection fee and the expected turnaround time before you book.

• Juvenology Clinic, Maidstone (82 King Street, Maidstone ME14 1BH): Available.
• Partner clinic near you, anywhere in the UK: Available on request. We confirm availability for your postcode before you book.
• Postal home finger-prick kit: Ask before ordering. We offer a home kit only where the laboratory confirms this test is suitable for a finger-prick sample.

Sample: Venous blood sample (serum); sample tube type confirmed by the laboratory

Collection notes: No fasting needed. Tell us about biotin supplements before booking; the laboratory or your clinician can advise how long to pause them. Keep thyroid medication to your usual routine and note the time of your last dose. A postal finger-prick kit is offered only if the laboratory confirms suitability.

The price of this test covers laboratory analysis and the laboratory's results report. The report lists each result beside the laboratory's reference range.

Beyond the blood test: optional support, priced separately

• Personalised written interpretation report: £50
• General consultation: £50
• Results consultation (results discussion and follow-up advice): £100
• Personalised programme: from £150, after we assess whether a programme suits you

Consultations are with Marina, an NMC-registered nurse. She explains what your results show and how they relate to lifestyle. No prescription service is offered with these tests.

If a result suggests you need medical follow-up, we will recommend that you see your GP and, with your consent, can provide a letter to take with you. A blood test cannot rule out every condition. If you have symptoms, speak to your GP or NHS 111, and call 999 in an emergency.

Why measure TSH, free T4 and free T3 together?

TSH alone is often the first test, but it can be misleading in pituitary conditions, during illness or with some medicines. Having all three in one sample shows the thyroid's output and the pituitary's response together, which can avoid a second sample.

I feel tired all the time, but my results are normal. What next?

Tiredness has many possible causes, including low iron, low vitamin B12, poor sleep, low mood and other conditions. Normal thyroid results make an untreated thyroid problem less likely; your GP can help look for other causes.

Does this test show whether I have Hashimoto's disease?

No. It shows whether thyroid hormone levels are normal. Thyroid antibodies, available as a separate test, show whether there is an autoimmune process.

How is this different from a TSH-only test?

A TSH-only test is a sensible first check for many people. This panel adds free T4 and free T3, which help to classify an abnormal TSH and can reveal patterns that TSH alone would miss.

I take levothyroxine. Should I change my dose based on this result?

No. Share the results with your GP or specialist, who will interpret them alongside your symptoms and timing of your dose.

Is interpretation included in the price?

The price covers laboratory analysis and the laboratory's report with reference ranges. You can add an optional personalised written interpretation report (£50) or a results discussion consultation (£100).

1. Thyroid disease: assessment and management (NG145). National Institute for Health and Care Excellence, 2019, updated 2023. https://www.nice.org.uk/guidance/ng145
2. Thyroid function tests. British Thyroid Foundation, revised 2026. https://www.btf-thyroid.org/thyroid-function-tests
3. Thyroid-stimulating hormone (TSH). Lab Tests Online UK. https://labtestsonline.org.uk/tests/thyroid-stimulating-hormone-tsh
4. Underactive thyroid (hypothyroidism). NHS. https://www.nhs.uk/conditions/underactive-thyroid-hypothyroidism/
5. Overactive thyroid (hyperthyroidism). NHS. https://www.nhs.uk/conditions/overactive-thyroid-hyperthyroidism/