Thyroid Function Test Explained: What Your TSH, T4 and T3 Results Mean

A thyroid function test is one of the most frequently requested blood tests in UK general practice, and one of the most commonly misunderstood. Patients often arrive at an appointment holding a printout showing a TSH result and a free T4 result, unsure which number matters, whether a result close to the edge of the range is a problem, and whether they need treatment. This guide explains what each part of a thyroid blood test measures, how the results fit together, and what UK clinical guidance says about when an abnormal result actually requires action.

Quick Answer

A thyroid function test measures how well your thyroid gland is working. The main test is TSH (thyroid stimulating hormone), which is released by the pituitary gland in the brain to tell the thyroid how much hormone to produce. TSH is usually measured alongside free T4 (thyroxine) and sometimes free T3. A high TSH with a low free T4 points to an underactive thyroid (hypothyroidism). A low TSH with a high free T4 or free T3 points to an overactive thyroid (hyperthyroidism). Because TSH and thyroid hormones move in opposite directions, the pattern of results matters far more than any single number, and mildly abnormal results often need repeating before any conclusion is drawn.

Key Facts

What is a thyroid function test?

A thyroid function test, often abbreviated to TFT, is a blood test that assesses the activity of the thyroid gland. It is not a single measurement but a small group of related results that are interpreted together.

In the NHS, and in most private laboratories, the standard first-line test is TSH alone. Laboratories then use a system called cascade or reflex testing: if the TSH result falls outside the reference range, the laboratory automatically adds free T4 to the same blood sample, and adds free T3 if the picture suggests an overactive thyroid. This approach avoids unnecessary testing while making sure that abnormal results are properly characterised.

Some private thyroid profiles measure TSH, free T4 and free T3 on every sample regardless of the TSH result. This produces more numbers, but not always more useful information, and it is one reason private thyroid results can look more complicated than an NHS printout for the same person.

What does the thyroid gland actually do?

The thyroid is a butterfly-shaped gland at the front of the neck, sitting just below the Adam’s apple and wrapping around the windpipe. It produces two hormones: thyroxine (T4) and triiodothyronine (T3). These hormones set the pace of metabolism in almost every tissue in the body.

Thyroid hormones influence heart rate, body temperature, bowel activity, menstrual cycles, mood, concentration, cholesterol handling, skin and hair growth, and the rate at which the body uses energy. This is why thyroid disease produces such a wide and non-specific range of symptoms, and why thyroid testing is requested so often.

The feedback loop that explains your results

The thyroid does not work independently. It is controlled by the pituitary gland at the base of the brain, which releases TSH. The system behaves like a thermostat:

• If thyroid hormone levels fall, the pituitary senses this and increases TSH to stimulate the thyroid to work harder.
• If thyroid hormone levels rise, the pituitary reduces TSH to slow the thyroid down.

This inverse relationship is the single most important idea in reading thyroid results. It explains why a high TSH indicates an underactive thyroid, which is the opposite of what most people expect when they first see their results.

The relationship is also logarithmic rather than linear. A small fall in thyroid hormone output produces a proportionally much larger rise in TSH. That sensitivity is what makes TSH such a good screening test, and also why TSH can drift outside the range before a person has any symptoms at all.

What does each thyroid blood test measure?

Thyroid panels vary between laboratories and between NHS and private testing. These are the components you are most likely to see on a report.

TSH (thyroid stimulating hormone)

Despite the name, TSH is not made by the thyroid. It is a pituitary hormone that acts on the thyroid. It is the most sensitive single indicator of thyroid status in someone whose pituitary is working normally, which is why it is used as the first-line test.

TSH has a daily rhythm, peaking overnight and in the early hours and reaching its lowest point in the afternoon. The difference is modest but can be enough to move a borderline result from just inside the range to just outside it.

Free T4 (free thyroxine)

T4 is the main hormone the thyroid produces. Most of it circulates bound to proteins and is biologically inactive. Free T4 measures the small unbound fraction that is available to enter cells, which is why free T4 is measured rather than total T4 in almost all UK laboratories.

Free T4 tells you how much hormone the thyroid is actually delivering. Combined with TSH, it separates mild thyroid dysfunction from established disease.

Free T3 (free triiodothyronine)

T3 is the more biologically active hormone. Only a small proportion is made directly by the thyroid; most is produced when T4 is converted to T3 in the liver, kidneys and other tissues.

Free T3 is most useful when hyperthyroidism is suspected, because in some people T3 rises before T4 does, a pattern known as T3 toxicosis. It is far less useful in suspected hypothyroidism, and a free T3 in the lower part of the range in someone with a normal TSH is rarely meaningful on its own.

Thyroid peroxidase antibodies (TPO antibodies)

TPO antibodies indicate autoimmune thyroid disease. They are the marker most often used to identify Hashimoto’s thyroiditis, the leading cause of hypothyroidism in the UK.

NICE advises considering a TPO antibody test in adults whose TSH is above the reference range, and specifically advises against repeating it. Antibody levels do not track disease activity, so there is no clinical value in monitoring them over time.

TSH receptor antibodies (TRAb)

TRAb are the antibodies responsible for Graves’ disease. They stimulate the TSH receptor and drive the thyroid to overproduce hormone. This test is used mainly in secondary care to confirm the cause of hyperthyroidism, to guide the length of drug treatment, and to assess risk in pregnancy.

Thyroglobulin and thyroglobulin antibodies

Thyroglobulin is used almost exclusively in the follow-up of treated thyroid cancer, as a marker of residual or recurrent thyroid tissue. Thyroglobulin antibodies are measured alongside it because they interfere with the thyroglobulin assay. Neither test has a role in the routine assessment of an underactive or overactive thyroid, and neither belongs in a general health screen.

Why might your doctor request a thyroid function test?

Thyroid testing is requested for symptoms, for monitoring, and as part of the assessment of other conditions.

Symptoms that commonly prompt testing

• Persistent tiredness or low energy
• Unexplained weight gain or weight loss
• Feeling unusually cold or unusually hot
• Palpitations, tremor or anxiety
• Low mood, poor concentration or memory difficulties
• Hair thinning, dry skin or brittle nails
• Constipation or increased bowel frequency
• Irregular, heavy or absent periods
• Neck swelling, a visible goitre or a lump in the thyroid
• Muscle aches, cramps or weakness

These symptoms are common in the general population and most people who have them turn out to have a normal thyroid. That does not make the test unreasonable. Thyroid disease is treatable and easy to miss clinically, so it is sensible to exclude it early rather than late. Our article on chronic fatigue blood tests covers the wider set of tests usually considered alongside thyroid function.

Clinical situations where thyroid testing is routine

• Monitoring of established thyroid disease and levothyroxine treatment
• Investigation of subfertility, irregular cycles or recurrent miscarriage
• Confirmed or planned pregnancy in women with known thyroid disease
• New atrial fibrillation or unexplained tachycardia
• Unexplained raised cholesterol
• Osteoporosis or unexplained bone loss
• Assessment of possible polycystic ovary syndrome, where thyroid disease is one of the conditions that must be excluded first
• Before and during treatment with amiodarone or lithium
• New or worsening depression or cognitive change, particularly in older adults
• Presence of another autoimmune condition such as type 1 diabetes, coeliac disease or vitiligo

There is no national screening programme for thyroid disease in adults in the UK. Newborns are screened for congenital hypothyroidism through the blood spot test, but adult testing is done on the basis of symptoms or risk, not routinely by age.

What is the normal range for thyroid function tests?

There is no single national reference range for thyroid tests in the UK. Each laboratory sets its own range based on the analyser and assay it uses and the population it serves. This is why a free T4 of 11 pmol/L may be reported as normal by one laboratory and flagged as low by another, and why results from different laboratories should not be compared directly.

The figures below are typical of UK adult laboratories and are given for orientation only. Always read your result against the range printed on your own report.

A reference range is not a treatment threshold

This distinction causes a great deal of unnecessary worry. A reference range describes where about 95 per cent of results from a healthy reference population fall. By definition, around one in twenty healthy people will have a result slightly outside it and be perfectly well.

A treatment threshold is a separate clinical decision point supported by evidence that intervening improves outcomes. For thyroid disease these are not the same number. A TSH of 4.6 mU/L is outside a typical reference range but is nowhere near the point at which UK guidance recommends treatment, whereas a TSH of 12 mU/L confirmed on repeat testing is.

Ranges that differ from the standard adult range

• Pregnancy. Thyroid physiology changes substantially. Trimester-specific ranges should be used, with TSH typically lower in the first trimester and free T4 falling as pregnancy progresses.
• Newborns and children. Ranges are considerably wider and change rapidly in the first weeks and months of life.
• Older adults. TSH tends to rise gradually with age. A TSH of 5 mU/L in an active 85 year old carries a different meaning from the same value in a 30 year old.
• People on levothyroxine. The aim is a TSH within the reference range, and monitoring is based on TSH rather than on symptoms alone.
• Treated thyroid cancer. TSH is often deliberately suppressed below the normal range as part of treatment.

How to read the pattern of your thyroid results

Interpretation depends on the combination of TSH and free T4, not on either value alone. The table below summarises the patterns seen in practice.

The last two rows are rare and almost always need specialist input. The first five cover the overwhelming majority of results seen in general practice.

What does a high TSH mean?

A raised TSH usually means the pituitary is having to push harder than normal to get the thyroid to produce enough hormone. In other words, it points towards an underactive thyroid.

How much it matters depends on the free T4:

• High TSH with low free T4 is overt primary hypothyroidism. The thyroid is failing to keep up and treatment with levothyroxine is normally indicated.
• High TSH with normal free T4 is subclinical hypothyroidism. The thyroid is under strain but still delivering enough hormone. This needs confirmation and thought rather than automatic treatment.

A single mildly raised TSH is not a diagnosis. TSH can rise temporarily after an illness, in the recovery phase after any acute condition, in poor sleep and shift work, and simply from having the blood taken early in the morning. Repeating the test after an interval, usually around three months, is standard practice unless the result is markedly abnormal or the person is clearly unwell.

Causes of an underactive thyroid

Autoimmune thyroiditis (Hashimoto’s disease)

This is by far the commonest cause in the UK and in other iodine-sufficient countries. The immune system gradually damages thyroid tissue, so hormone production declines slowly over years. TPO antibodies are positive in most cases. It is considerably more common in women and often runs in families alongside other autoimmune conditions.

Treatment for a previously overactive thyroid

Radioactive iodine treatment, thyroid surgery and, less often, antithyroid drug treatment can leave the thyroid underactive. Hypothyroidism after radioactive iodine is common and expected, and lifelong levothyroxine is usually needed after a total thyroidectomy.

Medication

• Amiodarone, used for heart rhythm problems, can cause either underactivity or overactivity
• Lithium, used in bipolar disorder
• Interferon and some immunotherapy or checkpoint inhibitor cancer treatments
• Certain tyrosine kinase inhibitors

Thyroiditis and postpartum thyroiditis

Inflammation of the thyroid can release stored hormone and cause a temporary overactive phase, followed by an underactive phase, before function often recovers. Postpartum thyroiditis follows this pattern in the year after childbirth and is frequently mistaken for postnatal depression or exhaustion. Because the picture changes over months, the timing of the blood test strongly influences what the result shows.

Less common causes

• Iodine deficiency, uncommon in the UK but seen in people avoiding all dairy, fish and iodised salt, and a relevant consideration in pregnancy
• Excess iodine, including from kelp and seaweed supplements and from iodine-containing contrast media
• Congenital hypothyroidism, detected through newborn screening
• Previous radiotherapy to the head or neck
• Pituitary disease causing central hypothyroidism, in which TSH is low or inappropriately normal despite a low free T4

Subclinical hypothyroidism: the grey zone

Subclinical hypothyroidism means a TSH above the reference range with a free T4 still within it. It is the most common abnormal thyroid result found in practice, and the one that generates the most uncertainty.

A significant proportion of people with a mildly raised TSH return to normal without any treatment, which is why confirmation matters before starting lifelong medication.

What UK guidance recommends

NICE guideline NG145 on thyroid disease sets out a clear approach for adults:

• Consider levothyroxine where TSH is 10 mU/L or higher on two separate occasions three months apart.
• Consider a six-month trial of levothyroxine in adults under 65 whose TSH is above the reference range but below 10 mU/L on two occasions three months apart, and who have symptoms of hypothyroidism.
• If symptoms do not improve once TSH is within the reference range, consider stopping levothyroxine rather than continuing indefinitely.
• Consider measuring TPO antibodies once in adults with a TSH above the reference range, and do not repeat the test.

NICE also advises taking into account features that make underlying thyroid disease more likely, such as symptoms, positive thyroid antibodies, or previous thyroid surgery or radioactive iodine treatment.

In adults over 65, and particularly over 80, the threshold for treating a mildly raised TSH is higher. Trial evidence has not shown symptomatic benefit in this group, and there are real risks from over-replacement, including atrial fibrillation and reduced bone density. Watchful monitoring is often the better course.

Pregnancy and planned pregnancy are handled differently and are discussed below.

What does a low TSH mean?

A low or suppressed TSH usually means there is more thyroid hormone in circulation than the body needs, so the pituitary has reduced its signal. This points towards an overactive thyroid.

• Low TSH with high free T4 or free T3 is overt hyperthyroidism.
• Low TSH with normal free T4 and free T3 is subclinical hyperthyroidism.
• Low TSH with low free T4 suggests something other than a primary thyroid problem, such as pituitary disease or recent treatment for an overactive thyroid.

The most common non-thyroid explanation for a low TSH is taking slightly too much levothyroxine. In anyone already on treatment, the first question is the dose rather than a new diagnosis.

An overactive thyroid should be taken seriously. Untreated hyperthyroidism increases the risk of atrial fibrillation, osteoporosis and, rarely, a dangerous condition called thyroid storm. A clearly suppressed TSH with a high free T4 warrants prompt medical review rather than waiting for a routine appointment.

Causes of an overactive thyroid

Graves’ disease

The most common cause of hyperthyroidism in the UK. Antibodies stimulate the TSH receptor, driving continuous hormone overproduction. Graves’ disease can also affect the eyes, causing grittiness, bulging, double vision or lid retraction, and this eye involvement can appear before, during or after the thyroid problem itself.

Toxic nodules and toxic multinodular goitre

One or more thyroid nodules develop the ability to produce hormone independently of TSH control. This becomes more common with age and tends to produce a more gradual, milder picture than Graves’ disease.

Thyroiditis

Inflammation causes stored hormone to leak out of damaged thyroid cells. This produces a temporary overactive phase that is not caused by increased hormone production, so antithyroid drugs do not help. Subacute (de Quervain’s) thyroiditis typically follows a viral illness and causes a painful, tender thyroid. Postpartum and silent thyroiditis are painless.

Excess thyroid hormone from outside the body

Too high a dose of levothyroxine is the most frequent cause. Less commonly, thyroid hormone is taken deliberately for weight loss or energy, sometimes in unregulated supplements bought online, which is unsafe and can cause lasting harm.

Iodine and drug-induced causes

Amiodarone and iodine-containing contrast agents can trigger hyperthyroidism, particularly in people with pre-existing nodular thyroid disease. High-dose iodine or kelp supplements can do the same.

Subclinical hyperthyroidism

This means a low TSH with normal free T4 and free T3. As with subclinical hypothyroidism, the first step is to repeat the test, since a transient dip in TSH is common after illness and in early pregnancy.

Persistent subclinical hyperthyroidism matters more than its mild-sounding name suggests, particularly when TSH is fully suppressed below 0.1 mU/L. It is associated with an increased risk of atrial fibrillation and of bone loss, especially in people over 65 and in postmenopausal women. Specialist assessment is usually appropriate when it persists.

Confusing and discordant results

Some thyroid results do not fit any of the usual patterns. These are worth recognising, because they lead to unnecessary treatment if they are misread.

Non-thyroidal illness (sick euthyroid syndrome)

During serious illness, surgery, starvation or hospital admission, thyroid results can look abnormal without any thyroid disease being present. Free T3 typically falls first, TSH can be low during the illness and rebound above the range during recovery. This is why thyroid testing is generally avoided in acutely unwell people unless there is a specific reason, and why results taken during an admission are often repeated once the person has recovered.

Biotin interference

High-dose biotin, found in many hair, skin and nail supplements, interferes with the laboratory method used for many thyroid assays. It can produce results that mimic Graves’ disease, with a falsely low TSH and falsely high free T4 and free T3. Stopping biotin for at least two days before the test and repeating it resolves the picture. This is a genuinely common cause of alarming private thyroid results.

Macro-TSH and assay antibodies

Occasionally TSH binds to other proteins in the blood, forming a large complex that the assay still detects but which has no biological activity. The result is a persistently raised TSH in someone with a normal free T4 and no symptoms. Heterophile antibodies can distort results in a similar way. If results and clinical picture do not match, the laboratory can be asked to investigate interference before treatment is started.

Poor adherence to levothyroxine

Missing doses through the month and then taking tablets consistently in the days before a blood test produces a characteristic pattern of a raised TSH with a normal or even high free T4. Recognising it avoids repeated and unnecessary dose increases.

Central hypothyroidism

If the pituitary or hypothalamus is not working properly, TSH cannot rise appropriately. Free T4 is low while TSH is low or unremarkable. This is uncommon but important, because relying on TSH alone will miss it entirely. It requires endocrine assessment and testing of the other pituitary hormones.

Symptoms associated with abnormal thyroid results

Thyroid symptoms are non-specific and overlap heavily with anaemia, iron deficiency, perimenopause, depression, sleep disorders and simple exhaustion. The table below shows the classic patterns, but many people have only a few of these features and some have none at all.

Two points are worth emphasising. Severity of symptoms correlates poorly with the size of the abnormality on the blood test, and older adults frequently present atypically, sometimes with nothing more than low mood, falls or new atrial fibrillation.

Thyroid function, fertility, pregnancy and the menopause

Fertility and menstrual cycles

Thyroid dysfunction disrupts ovulation and menstrual regularity in both directions. An underactive thyroid is associated with heavy or irregular bleeding, raised prolactin and reduced fertility. An overactive thyroid tends to cause light or absent periods. Thyroid function is therefore checked routinely in the assessment of subfertility, irregular cycles and recurrent miscarriage, and forms part of our Fertility MOT and pre-pregnancy health check.

Before and during pregnancy

Pregnancy places substantial extra demand on the thyroid. The baby depends entirely on maternal thyroid hormone for the first three months or so of development, and iodine requirements rise.

Key points for women who are pregnant or planning pregnancy:

• Trimester-specific reference ranges must be used. Standard adult ranges will mislead.
• Women already taking levothyroxine usually need their dose increased as soon as pregnancy is confirmed, and should contact their GP or midwife promptly rather than waiting for a routine appointment. Waiting until the booking appointment can mean weeks of under-replacement at the point it matters most.
• Thyroid function is monitored more frequently during pregnancy, typically every four to six weeks in the first half.
• Free T4 and free T3 are used rather than total T4 and T3, because binding proteins rise in pregnancy and inflate the total measurements.
• The levothyroxine dose usually needs reducing again after delivery.
• Untreated overt hypothyroidism in pregnancy carries real risks, so treatment thresholds are lower than outside pregnancy.

Carbimazole and propylthiouracil are both used in hyperthyroidism but their use in pregnancy is a specialist decision, and anyone taking antithyroid medication who is planning a pregnancy should seek advice before conceiving rather than afterwards.

After childbirth

Postpartum thyroiditis affects a meaningful minority of women in the year after delivery and is easily attributed to normal new-parent exhaustion. It typically causes a transient overactive phase followed by an underactive phase. Most women recover, but a proportion go on to develop permanent hypothyroidism, so follow-up testing is worthwhile.

Perimenopause and menopause

Thyroid disease and perimenopause share a striking number of symptoms: fatigue, low mood, poor sleep, brain fog, palpitations, hair thinning, weight change and altered periods. Both become more common in women in their forties and fifties, and both can be present at once. Checking thyroid function is a reasonable early step when these symptoms appear, precisely because the two conditions cannot be reliably separated on symptoms alone.

How thyroid results relate to other blood tests

Thyroid function is rarely tested in isolation, because the symptoms that prompt testing have several other common explanations.

Our overview of which blood test panel you actually need explains how these tests are usually grouped, and blood test results explained covers how to read a laboratory report in general.

How to prepare for a thyroid blood test

Thyroid function tests do not require fasting, and no special preparation is needed for most people. A few practical points make the result easier to interpret.

• Be consistent with timing. TSH is higher in the early morning and lower in the afternoon. If you are having repeat tests to track a borderline result or adjust a levothyroxine dose, aim for a similar time of day each time.
• If you take levothyroxine, take it after the blood test on the day of the test. Taking it beforehand can transiently raise free T4 and give a misleading impression of over-replacement. TSH itself is not affected by a single dose.
• Stop high-dose biotin supplements for at least 48 hours beforehand. This includes most hair, skin and nail supplements and some multivitamins.
• Tell whoever requests the test about relevant medication, including amiodarone, lithium, steroids, oestrogen-containing contraception or HRT, and any thyroid supplement bought online.
• Avoid testing during an acute illness unless there is a specific reason, since results are unreliable at that time.
• Mention if you are pregnant or could be, so that the correct reference ranges are applied.

The test itself is a single blood sample from a vein in the arm and takes a couple of minutes. Most UK laboratories report thyroid results within one to two working days.

When to repeat the test

• After a change in levothyroxine dose, wait around six to eight weeks before rechecking. Testing sooner produces a result that has not yet stabilised.
• For a mildly abnormal TSH with no clear symptoms, repeating after about three months is standard.
• Once stable on levothyroxine, annual testing is usual for most people.

Understanding your results

When reviewing a thyroid result, a clinician is working through a short sequence of questions rather than reacting to a single number.

1. Is the TSH inside or outside the laboratory’s own reference range?
2. If it is outside, what is the free T4 doing? That determines whether this is overt or subclinical disease.
3. How far outside the range is it? A TSH of 4.5 mU/L and a TSH of 45 mU/L are entirely different situations.
4. Do the symptoms fit the biochemistry, or are they better explained by something else?
5. Is there anything that could distort the result: recent illness, biotin, pregnancy, missed or extra levothyroxine doses, amiodarone or lithium?
6. Has this been confirmed on a second sample, or is it a single reading?
7. What did previous results show? A TSH that has climbed steadily over three years means something different from one that has bounced around.

The last question is often the most useful and the most neglected. A trend over time carries far more information than a single snapshot, which is one reason it is worth keeping copies of your own results.

When should you see a GP?

Arrange a routine appointment if you have:

• A thyroid result outside the reference range that has not been explained to you
• Persistent fatigue, weight change, hair loss or low mood that is not improving
• Symptoms that continue despite a thyroid result reported as normal, since another cause needs looking for
• A family history of thyroid or autoimmune disease with new symptoms
• Planned pregnancy while taking levothyroxine, ideally before conceiving

Seek prompt medical advice, within days rather than weeks, if you have:

• A markedly abnormal result, for example a clearly suppressed TSH with a high free T4
• Palpitations, a persistently fast or irregular pulse, or noticeable tremor
• Significant unintentional weight loss
• A newly confirmed pregnancy while taking levothyroxine
• A new lump in the neck, hoarseness lasting more than three weeks, or difficulty swallowing
• New eye symptoms such as bulging, double vision or reduced vision alongside thyroid symptoms

Seek urgent care the same day for severe agitation or confusion with fever and a racing heart, or for extreme drowsiness with a very low body temperature in someone with known thyroid disease. Both are rare but are medical emergencies.

Treatment and management

Treating an underactive thyroid

Levothyroxine taken once daily is the standard treatment. It is a synthetic form of T4, identical to the hormone the thyroid produces, and it is generally very effective.

• Take it on an empty stomach, usually 30 to 60 minutes before breakfast, or at bedtime well after eating. Consistency matters more than the specific time.
• Separate it by at least four hours from iron, calcium, indigestion remedies containing calcium or magnesium, and multivitamins, because these substantially reduce absorption.
• Doses are usually started lower and increased more cautiously in older people and in anyone with heart disease.
• Blood tests are repeated around six to eight weeks after any dose change, then annually once stable.
• The aim is a TSH within the reference range. Deliberately suppressing TSH to relieve symptoms is not recommended, as it increases the risk of atrial fibrillation and bone loss.

NICE recommends levothyroxine alone as first-line treatment for primary hypothyroidism and advises against routine use of liothyronine (T3), either alone or in combination. Some people continue to feel unwell despite a normal TSH, and this is a genuine and frustrating situation. Current UK practice is to look carefully for other explanations, including iron deficiency, vitamin D deficiency, coeliac disease, sleep disorders, perimenopause and depression, before attributing symptoms to inadequate thyroid replacement.

Treating an overactive thyroid

Management depends on the cause and is usually directed by an endocrinologist.

• Antithyroid drugs. Carbimazole is the usual first choice in the UK, with propylthiouracil as an alternative in specific circumstances. Anyone taking these medicines should seek urgent medical advice if they develop a fever, sore throat or mouth ulcers, as these can signal a rare but serious drop in white cells.
• Beta blockers. Used short term to control palpitations and tremor while other treatment takes effect.
• Radioactive iodine. A definitive treatment that commonly leads to hypothyroidism afterwards, which is then treated with levothyroxine.
• Surgery. Considered for large goitres, suspected cancer, or when other treatments are unsuitable.
• Observation. Thyroiditis is often self-limiting and needs symptom control rather than antithyroid drugs.

Monitoring is more frequent during treatment, typically every six weeks until TSH returns to the reference range.

Diet, supplements and common thyroid myths

Thyroid health attracts a large amount of misleading advice online. A few points based on current evidence:

• Iodine supplements are not a general remedy. Iodine deficiency is uncommon in the UK, and excess iodine can trigger both underactive and overactive thyroid disease. Kelp and seaweed supplements can contain very high and variable doses.
• Selenium is not routinely recommended. Evidence for benefit in autoimmune thyroid disease is inconsistent, and high intakes are harmful.
• A gluten-free diet does not treat thyroid disease unless you also have coeliac disease, which is worth testing for given the association.
• Cruciferous vegetables are not a practical concern. Broccoli, cabbage and kale would need to be eaten in extreme quantities to affect thyroid function in someone with adequate iodine intake.
• Thyroid glandular and desiccated thyroid products bought online are not licensed in the UK, contain unpredictable amounts of hormone, and can cause harm.
• Thyroid disease is not usually the explanation for substantial weight gain. Untreated hypothyroidism typically accounts for a few kilograms, much of it fluid, and treatment does not generally produce large weight loss.

What does help is treating the diagnosed problem properly, taking medication consistently, and looking for the other common causes of the same symptoms.

Private thyroid testing in London

Private thyroid testing is straightforward: a blood sample is taken at the clinic and sent to an accredited laboratory, with results usually available within one to two working days. At AristoGP in Central London we use The Doctors Laboratory, a UKAS-accredited laboratory, and can arrange TSH, free T4, free T3 and thyroid antibodies alongside related tests such as full blood count, ferritin, B12, folate, vitamin D and a lipid profile.

The part that matters most is not the sample but the interpretation. A number without context is of limited value, and direct-to-consumer thyroid panels frequently generate anxiety about results that are clinically unimportant, or reassurance about results that are not. If you have received thyroid results you do not understand, a GP can review them alongside your symptoms, medical history, medication and previous results, and advise whether repeat testing, further investigation, treatment or simple monitoring is appropriate.

You can read more about our blood tests and laboratory services or book a private GP consultation at our clinic near Chancery Lane and Farringdon.

Frequently asked questions

Do I need to fast before a thyroid blood test?

No. Thyroid function tests do not require fasting. You may be asked to fast if other tests such as glucose or a lipid profile are being taken from the same sample.

Why is a high TSH a sign of an underactive thyroid?

Because TSH is the signal from the pituitary gland telling the thyroid to work, not a measure of thyroid hormone itself. When the thyroid produces too little hormone, the pituitary increases TSH to try to compensate. A high TSH therefore reflects a thyroid that is struggling.

Is a TSH of 5 something to worry about?

Usually not by itself. A TSH slightly above the reference range with a normal free T4 is subclinical hypothyroidism, and a significant proportion of these results return to normal without treatment. Standard practice is to repeat the test after about three months and to consider symptoms, thyroid antibodies and age before deciding anything.

Should I take my levothyroxine before a blood test?

It is better to have the blood taken first and then take your tablet. Taking levothyroxine shortly beforehand can raise the free T4 result temporarily and suggest you are on too high a dose when you are not. Do not skip doses in the days before the test, as this distorts the result in the opposite direction.

Does the time of day matter?

TSH is naturally higher in the early morning and lower in the afternoon. For a one-off test this rarely changes management, but for borderline results or dose adjustments it is helpful to be tested at a similar time each time.

Can supplements affect my thyroid results?

Yes. High-dose biotin, found in many hair, skin and nail supplements, interferes with the laboratory assay and can produce results that falsely look like an overactive thyroid. Stopping biotin for at least 48 hours before testing avoids this. Iodine and kelp supplements can genuinely alter thyroid function rather than just the test result.

Do I need free T3 tested?

Not usually. Free T3 is helpful when an overactive thyroid is suspected, because T3 can rise before T4. In suspected underactivity it adds little, and a free T3 towards the lower end of the range alongside a normal TSH is not on its own evidence of a thyroid problem.

What does it mean if my thyroid antibodies are positive?

Positive TPO antibodies indicate autoimmune thyroid disease. If your thyroid function is currently normal, it does not mean you have hypothyroidism, but it does increase the likelihood of developing it over time, so periodic monitoring is sensible. Antibody levels do not need repeating, since they do not guide treatment.

Can stress cause thyroid problems?

Severe physical stress and acute illness can temporarily alter thyroid results without thyroid disease being present. Psychological stress is not an established cause of autoimmune thyroid disease, although it can produce many of the same symptoms, which is a common source of confusion. Our article on thyroid problems and burnout looks at this overlap in more detail.

Will treating my thyroid help me lose weight?

Only modestly. Untreated hypothyroidism typically causes a few kilograms of gain, much of it fluid retention, and treatment reverses that rather than producing substantial weight loss. Levothyroxine should never be used as a weight loss treatment, as excess thyroid hormone damages the heart and bones.

Is hypothyroidism lifelong?

Autoimmune hypothyroidism and hypothyroidism after thyroid surgery or radioactive iodine are usually permanent and need lifelong treatment. Hypothyroidism caused by thyroiditis, including postpartum thyroiditis, is often temporary, and treatment can sometimes be withdrawn under supervision with repeat testing.

My results are normal but I still feel exhausted. What now?

A normal thyroid result is useful information, not a dead end. Common alternative explanations include iron deficiency, vitamin B12 or vitamin D deficiency, coeliac disease, anaemia, sleep apnoea, depression, perimenopause and the after-effects of viral illness. A structured review of symptoms with appropriate further tests is the logical next step.

How often should thyroid function be checked?

Once stable on levothyroxine, annually is usual. After a dose change, around six to eight weeks. In pregnancy, considerably more often. For someone with positive antibodies but normal function, periodic testing is reasonable, with the interval agreed with your doctor.

Can I get a thyroid test privately without seeing a GP first?

Yes, private thyroid testing can be arranged directly. The more useful question is what happens next, since an isolated number is of limited value without clinical context. Arranging the test alongside a consultation means abnormal or borderline results can be interpreted properly rather than left to guesswork.

Key takeaways

• TSH comes from the pituitary gland, not the thyroid, and moves in the opposite direction to thyroid hormone levels.
• A high TSH suggests an underactive thyroid; a low TSH suggests an overactive one.
• The combination of TSH and free T4 determines whether a problem is overt or subclinical.
• Reference ranges vary between laboratories and are not the same as treatment thresholds.
• Mildly abnormal results are common, frequently temporary, and should be confirmed on a repeat sample before treatment is started.
• UK guidance supports treating subclinical hypothyroidism when TSH is 10 mU/L or above on two occasions three months apart, and considering a six-month trial in symptomatic adults under 65 with a lower but persistently raised TSH.
• Biotin supplements, acute illness, pregnancy and inconsistent levothyroxine use are frequent causes of misleading results.
• Pregnancy requires trimester-specific ranges, and women on levothyroxine usually need a dose increase as soon as pregnancy is confirmed.
• Thyroid symptoms overlap heavily with iron deficiency, B12 deficiency, coeliac disease and perimenopause, so a normal result should prompt further assessment rather than end it.
• Results should always be interpreted alongside symptoms, medication, previous results and clinical context.

Sources and further reading

• NICE NG145: Thyroid disease: assessment and management
• NICE CKS: Hypothyroidism
• NICE CKS: Hyperthyroidism
• NHS: Underactive thyroid
• NHS: Overactive thyroid
• BNF: Levothyroxine sodium
• British Thyroid Association

This article reflects UK clinical practice and recognised professional guidance available at the time of writing. Guidance changes as new evidence emerges, and clinicians should always work from the most recent national and local recommendations.

Related articles

• Ferritin blood test explained
• Full blood count (FBC) explained
• Chronic fatigue blood tests: what to ask for and why
• Can thyroid problems cause burnout?
• Private blood tests in London: which panel do you actually need?
• Blood test results explained

These articles form part of the AristoGP Medical Knowledge Base, a growing collection of evidence-based patient information explaining common investigations, conditions and treatments in clear language.

Written and medically reviewed

Written and medically reviewed by Dr Georgios Schoinas, General Practitioner, AristoGP, Central London.

Last medically reviewed: 4 September 2026.

Medical disclaimer

This article is intended for general educational purposes and does not replace individual medical assessment, diagnosis or treatment. Blood test results cannot be interpreted in isolation, and the information here should not be used to diagnose yourself or to change any prescribed medication.

If you have symptoms that concern you or results you do not understand, speak to your GP or another appropriately qualified healthcare professional. If you develop severe symptoms such as chest pain, a very rapid or irregular heartbeat, significant breathlessness, collapse, or extreme drowsiness, seek urgent medical attention.