A TSH blood test measures thyroid-stimulating hormone, the signal your pituitary gland sends to tell your thyroid how hard to work. [1] In the UK it is usually the first thyroid test, because a normal TSH makes a thyroid problem unlikely. [1] A high result often points to an underactive thyroid and a low result to an overactive one. [2]
Neither pattern is a diagnosis on its own. This article explains what the test measures, how clinicians read the patterns and what can skew a result. It also covers why the thyroid is rarely the whole weight story.
What does a TSH blood test measure?
A TSH test measures the hormone that controls your thyroid. TSH comes from the pituitary gland, a small gland at the base of the brain. It tells the thyroid how much hormone to release. [1]
The thyroid is a small butterfly-shaped gland in your neck, just in front of the windpipe. [3] It makes two hormones, thyroxine (T4) and triiodothyronine (T3). They control how fast your body uses energy and affect heart rate, body temperature, mood and weight. [4]
The British Thyroid Foundation compares the pituitary to a thermostat. When thyroid hormone drops a little below normal, it releases more TSH; when levels rise above normal, it stops. [1]
That is why TSH moves in the opposite direction to the thyroid itself. An underactive thyroid usually shows as a high TSH, and an overactive thyroid as a low one. [2]
Why is TSH the first thyroid test?
Because a normal TSH makes a thyroid problem very unlikely. The British Thyroid Foundation says that if TSH is normal, "it is very likely that your thyroid is functioning normally". [1]
NICE, the body that writes NHS clinical guidance, says to consider measuring TSH alone for adults when a pituitary problem is not suspected. If TSH is above the laboratory's reference range, free T4 is measured on the same sample. If it is below, free T4 and free T3 are measured. [5]
Laboratories call this add-on testing "cascading", and many UK laboratories run it automatically. [1] [5] NICE CKS, the NICE summaries written for GPs, calls TSH the single most sensitive and specific blood test for an overactive thyroid. [6]
"Free" T4 and T3 are the small unbound portions your body can use. Less than 1% of T4 is free, and that is the portion laboratories measure. [4]
What do high, low and normal TSH results mean?
A high TSH often means an underactive thyroid and a low TSH can indicate an overactive one. [2] On its own, an abnormal TSH shows an excess or shortage of thyroid hormone but not the reason. [2] Free T4, and sometimes free T3, follow. [2]
| Pattern | What it usually points to, and what happens next |
|---|---|
| High TSH, low free T4 | An underactive thyroid (hypothyroidism). A GP assessment follows. [1] [7] |
| Slightly high TSH, normal free T4 | Subclinical hypothyroidism, or mild thyroid failure, which can develop into the full condition over years. A repeat test 3 to 6 months later rules out a temporary rise. [1] [8] |
| Low TSH, high free T4 or free T3 | An overactive thyroid (hyperthyroidism). [1] [6] GP assessment, with possible referral for further tests. [3] |
| Low TSH, normal free T4 | Subclinical hyperthyroidism. A repeat test after 3 months. [5] [6] |
| Low TSH, low free T4 | A pituitary problem, or the effect of a significant illness elsewhere. Further tests through a GP or specialist. [1] |
Small rises in TSH can be temporary, caused by an illness unrelated to the thyroid. A clinician repeats one abnormal result before any conclusion. [2] [8] A clinician reads the numbers with your symptoms and history. [1]
What can an underactive or overactive thyroid feel like?
Underactive thyroid symptoms usually develop slowly and may be mild and easy to miss. [7] They have other causes too, so the NHS says to get them checked. [7]
An underactive thyroid. The NHS lists these common symptoms: [7]
- feeling extremely tired (fatigue)
- feeling cold more than usual
- putting on weight
- constipation
- difficulty concentrating or thinking clearly
- low mood or depression
- dry skin, dry hair or hair loss, and a croaky voice
The British Thyroid Foundation adds muscle weakness, cramps and aches, a slow heartbeat, slightly raised blood pressure and raised cholesterol. [9]
An overactive thyroid. The NHS lists these common symptoms and signs: [3] [10]
- nervousness, anxiety and irritability
- mood swings and difficulty sleeping
- persistent tiredness and weakness
- sensitivity to heat, and a swelling in the neck
- an irregular or unusually fast heart rate
- twitching or trembling
- weight loss, often despite an increased appetite
It is unlikely anyone has all of them, and they may develop gradually or suddenly. [10]
If tiredness and low mood persist with a normal thyroid, see low testosterone symptoms and their other causes. Loud snoring and feeling very tired in the day are symptoms of sleep apnoea, where breathing stops and starts during sleep. [11]
Is your thyroid the reason you are gaining weight?
Usually only a small part of it, if at all. The British Thyroid Foundation says an underactive thyroid is generally associated with some weight gain, mostly from salt and water. [12] It adds that "massive weight gain is rarely associated with hypothyroidism". [12]
It puts the weight attributable to the thyroid at around 2 to 5 kg (5 to 10 pounds), depending on how underactive it is. [12] If weight gain is your only symptom, it is less likely that the thyroid is the sole cause. [12]
The condition is also less common in men. NICE CKS puts spontaneous hypothyroidism at 1 to 2% of people in the UK, up to 10 times more common in women than men. [8] In a European analysis, undiagnosed hypothyroidism affected 3.4% of men. [8]
Excess weight can also push TSH up on its own. NICE CKS notes that TSH can become raised in people with overweight or obesity, which may falsely suggest subclinical hypothyroidism. [8]
Checking TSH is still worth doing: the NHS says an underactive thyroid can be managed with treatment. [7] It is one of the medical causes worth excluding when weight loss has stalled.
What can skew a TSH result?
Several things can push TSH up or down without any thyroid disease. Tell the person arranging your test about all of them. [1]
- Biotin supplements. Biotin (vitamin B7) is in many multivitamins and hair, skin and nail supplements. The MHRA, the UK medicines regulator, says high intakes "may lead to falsely high or low test results". [13] NICE says clinicians should ask about it. [5]
- Some medicines. Some prescribed medicines affect thyroid function or the test itself, including some for heart rhythm and some for mental health. Tell your GP what you take; never stop a medicine without advice. [1] [2]
- Recent or acute illness. Serious illness and extreme stress can change thyroid results, a state called non-thyroidal illness. NICE says not to test during an acute illness unless the illness itself may be thyroid-related. [5] [2]
- Time of day. TSH follows a daily rhythm with a night-time peak, which NICE CKS says can be delayed by night shifts, irregular sleep, vigorous exercise and depression. [8]
- Iodine and kelp. Supplements containing iodine, including kelp, can affect thyroid results and thyroid function. [1] [9]
You do not need to fast or prepare for a TSH test. [2] [4]
Why do reference ranges differ between laboratories?
Because each laboratory sets its own range from its own method and population. The British Thyroid Foundation gives a typical adult TSH range of 0.4 to 4.0 mU/L, "only a guide" that "will vary according to laboratory". [1]
Age matters too. NICE CKS says the upper limit of the TSH range rises with age. [8] A mild TSH of 4.0 to 7.0 mU/L may be a normal adaptation to ageing. [8]
So read your result against the range printed on your own report, not a figure found online. The NHS says what counts as normal "varies depending on things such as your age and the exact testing technique used by the laboratory". [14]
How does TSH relate to cholesterol?
Closely enough that clinicians check one when the other is out of line. NICE lists an underactive thyroid among the common secondary causes of raised blood fats to rule out, with excess alcohol, uncontrolled diabetes and liver disease. [15] NICE CKS says hypothyroidism raises total and LDL cholesterol, and lists a cholesterol check among the tests to consider when it is suspected. [8]
The British Thyroid Foundation lists high cholesterol among the reasons to ask a GP for a thyroid test. [1] For what the cholesterol markers show, see how UK doctors read cholesterol results.
Where does TSH fit among the 10 markers?
HeMed starts with a blood test. You start with a needle-free test of 10 markers across your heart, liver, blood sugar and thyroid. TSH is the thyroid marker.
HeMed does not measure free T4 or free T3; if TSH is out of range, your GP can arrange them. [5] Fasting tests and hormone panels are not included; those are arranged through your GP.
A clinician reviews your blood test first. A retest every 6 months shows what's actually changing. For the other nine markers, see what a men's health blood test checks for and the ten numbers to know before a weight goal.
HeMed does not diagnose or treat the conditions described above.
When should you see a GP?
See a GP if you think you might have an underactive or overactive thyroid. The NHS says the symptoms are common and can have other causes, but it is important to get them checked. [7] [10]
The British Thyroid Foundation lists other reasons to ask for a thyroid test. [1] They include swelling or thickening in the neck, an irregular or fast heart rate, and high cholesterol. [1]
Some symptoms need faster action. See a GP if palpitations keep coming back or last longer than a few minutes. [16] Call 999 or go to A&E if palpitations do not go away, or if they come with chest pain, breathlessness or feeling faint. [16]
Call 999 for sudden chest pain or discomfort that does not go away, or pain that spreads to your arm, neck, jaw, stomach or back. [17] Chest pain with sweating, sickness, light-headedness or breathlessness is also a 999 call. [17]
Frequently asked questions
What is a normal TSH level?
It depends on the laboratory. The British Thyroid Foundation gives a typical adult range of 0.4 to 4.0 mU/L and says ranges are only a guide and vary by laboratory. [1] The upper limit also rises with age. [8]
Read your result against the range printed on your own report, and let a clinician interpret it alongside your symptoms.
Can a TSH test tell me if my thyroid is causing weight gain?
Not by itself. It shows whether your thyroid is likely to be working normally. Even then, the British Thyroid Foundation puts the weight attributable to the thyroid at around 2 to 5 kg, much of it fluid. [12]
If weight gain is your only symptom, the thyroid is less likely to be the sole cause. [12]
Do I need to fast before a TSH blood test?
No. Lab Tests Online UK lists no preparation for a TSH test. [2]
Can biotin affect my thyroid results?
Yes. The MHRA says biotin, often in hair, skin and nail supplements, may interfere with the laboratory method used for thyroid tests (immunoassays). [13] A high intake may give falsely high or low results. [13]
NICE says clinicians should ask about biotin intake. [5] Tell your clinician about any supplements you take.
How often should TSH be checked?
There is no UK recommendation to screen adults without symptoms. [2] A mild TSH rise without symptoms may be monitored once a year or up to every 3 years. [2] TSH is not normally measured more often than every 6 weeks. [2] [5]
Does an abnormal TSH result mean I have a thyroid problem?
No. An abnormal TSH shows an excess or shortage of thyroid hormone but not the reason. [2] Free T4 and sometimes free T3 follow, and a borderline result is usually repeated. [8]
A clinician interprets the set with your symptoms and history.
Does HeMed measure free T4 or free T3?
No. HeMed measures TSH; if your TSH is out of range, your GP can arrange free T4 and free T3. [5] Fasting tests and hormone panels are not included; those are arranged through your GP.
HeMed provides clinician-led weight management. We do not diagnose or treat thyroid disease. This page is general health information, not medical advice. If you are worried about symptoms, speak to your GP, or use NHS 111.
Next step: see what the HeMed blood test measures.
Sources (17)
- British Thyroid Foundation, "Thyroid function tests", page updated 8 October 2026 (leaflet first issued 2008, revised to 2021): the pituitary "senses the level of thyroid hormones in your bloodstream, just as the thermostat in your living room senses the temperature"; "In most cases, your TSH level will be the first hormone to be measured, because if this is normal, it is very likely that your thyroid is functioning normally"; the "cascade" of T4 and T3 testing; typical adult reference range TSH 0.4 to 4.0 mU/L, "These ranges are only a guide and will vary according to laboratory"; a slightly raised TSH with a normal FT4 "is called subclinical hypothyroidism or mild thyroid failure. It may gradually develop into full-blown or clinical hypothyroidism over many years"; a low TSH with a low FT4 may reflect the pituitary or "any significant illness that doesn't involve your thyroid"; results can be influenced by medications, illnesses and supplements containing iodine or biotin; reasons to ask a GP for a thyroid blood test include neck swelling, an irregular or fast heart rate and high cholesterol. btf-thyroid.org/thyroid-function-tests. Accessed 10 November 2026.
- Association for Laboratory Medicine, Lab Tests Online UK, "Thyroid-stimulating hormone (TSH)", reviewed 30 September 2024: "A high TSH result often means an underactive thyroid gland caused by failure of the gland (hypothyroidism) although small elevations of TSH can be transitory due to non-thyroidal illness"; "A low TSH result can indicate an overactive thyroid gland (hyperthyroidism)"; "an abnormal TSH indicates an excess or deficiency in the amount of thyroid hormone available to the body, but it does not indicate the reason why"; some medicines may interfere with the results; "Extreme stress and acute illness may also affect TSH test results"; no preparation needed; "There is currently no recommendation in the UK for routine screening of adults"; mild increases without symptoms "may be monitored annually or up to every 3 years"; not routinely measured more often than every 6 weeks. labtestsonline.org.uk/tests/thyroid-stimulating-hormone-tsh. Accessed 10 November 2026.
- NHS, "Overactive thyroid (hyperthyroidism)", page last reviewed 11 April 2023 (its stated next review date, 11 April 2026, had passed on the access date): "The thyroid is a small butterfly-shaped gland in the neck, just in front of the windpipe"; "about 10 times more common in women than men"; the symptom list; see a GP, who can arrange a blood test and refer for further tests. nhs.uk/conditions/overactive-thyroid-hyperthyroidism. Accessed 10 November 2026.
- Association for Laboratory Medicine, Lab Tests Online UK, "FT4", reviewed 29 July 2026: T4 "makes up nearly all of what we call thyroid hormone", T3 "less than 10%"; "Less than 1% is unattached: this is the free thyroxine (FT4). It is this free portion that the body can actually use"; thyroid hormones "control your metabolism" and "affect your heart rate, body temperature, mood, weight, and many other functions"; no special preparation; high-dose biotin "can interfere with some FT4 laboratory tests". labtestsonline.org.uk/tests/ft4. Accessed 10 November 2026.
- NICE, "Thyroid disease: assessment and management", NG145, published 20 November 2019, last updated 12 October 2023: recommendation 1.2.8, consider measuring TSH alone when pituitary disease is not suspected, then free T4 if TSH is above the reference range, and free T4 and free T3 if below; 1.2.6, do not test during an acute illness unless the illness may be due to thyroid dysfunction; 1.2.10, repeat no sooner than 6 weeks if symptoms worsen; 1.2.11, ask about biotin intake "because a high consumption of biotin from dietary supplements may lead to falsely high or low test results"; the guideline defines subclinical hyperthyroidism as TSH below the reference range with FT3 and FT4 within it. nice.org.uk/guidance/ng145. Accessed 10 November 2026.
- NICE Clinical Knowledge Summaries, "Hyperthyroidism", last revised January 2025: serum TSH "has the highest sensitivity and specificity of any single blood test used in the evaluation of suspected thyrotoxicosis"; suspected subclinical hyperthyroidism, "Repeat TFTs 3 months after the initial result to exclude other causes of a transiently suppressed TSH"; high-dose biotin can cause spuriously low TSH results. cks.nice.org.uk/topics/hyperthyroidism. Accessed 10 November 2026.
- NHS, "Underactive thyroid (hypothyroidism)", page last reviewed 28 April 2025: "This can cause symptoms like tiredness and weight gain. It can be managed with treatment"; "Symptoms of an underactive thyroid may be mild and may not be easy to notice", with the common-symptom list quoted in the article; "Some of the symptoms are common and can be caused by other conditions. But it's important to get them checked by a GP"; diagnosis by blood tests for TSH and thyroxine. nhs.uk/conditions/underactive-thyroid-hypothyroidism. Accessed 10 November 2026.
- NICE Clinical Knowledge Summaries, "Hypothyroidism", last revised November 2024: prevalence of spontaneous hypothyroidism "1 to 2%", "up to 10 times more common in women than in men"; a meta-analysis of 17 European studies found undiagnosed hypothyroidism in 4.9% of people, "6.4% in females and 3.4% in males"; suspected subclinical hypothyroidism, "repeat TFTs 3 to 6 months after the initial result to exclude other causes of a transiently raised TSH"; "one abnormal TSH level should be followed by a repeat blood test"; "Obesity: may affect the HPT axis and serum TSH can become raised in overweight or obese people, which may falsely suggest subclinical hypothyroidism"; "Mild TSH elevation (4.0 to 7.0 mU/L) may be a normal physiological adaption to ageing"; the nocturnal TSH peak may be delayed in night shift workers, irregular sleepers, after vigorous exercise and in depression; "hypothyroidism increases total cholesterol and low-density lipoprotein concentrations", so serum lipids are checked. cks.nice.org.uk/topics/hypothyroidism. Accessed 10 November 2026.
- British Thyroid Foundation, "Hypothyroidism" leaflet, revised 2021, page updated 17 September 2026: most common symptoms include fatigue, increased awareness of the cold, weight gain, low mood, memory problems, muscle weakness, cramps and aches, slow heartbeat, slightly raised blood pressure and raised cholesterol; "A thyroid function blood test is a simple and accurate way to check whether your thyroid gland is working properly"; causes include "Some health foods taken in excess, e.g. kelp (seaweed)". btf-thyroid.org/hypothyroidism-leaflet. Accessed 10 November 2026.
- NHS, "Overactive thyroid (hyperthyroidism): symptoms", page last reviewed 11 April 2023 (its stated next review date had passed on the access date): "it's unlikely you'll experience all of them. The symptoms may develop gradually or suddenly"; the lists of common symptoms and signs quoted in the article; "These symptoms and signs can have a number of causes. But a blood test can often help to determine whether they're caused by a thyroid problem." nhs.uk. Accessed 10 November 2026.
- NHS, "Sleep apnoea", page last reviewed 11 May 2026: "Sleep apnoea is when your breathing stops and starts while you sleep"; night-time symptoms "your breathing stopping and starting; making gasping, snorting or choking noises; waking up a lot; loud snoring"; by day "feel very tired; find it hard to concentrate; have mood swings; have a headache when you wake up"; "It may help to ask someone to stay with you while you sleep so they can check for the symptoms"; see a GP for the main symptoms; untreated, it can lead to "depression or mood changes". nhs.uk/conditions/sleep-apnoea. Accessed 10 November 2026.
- British Thyroid Foundation, "Thyroid and weight: the science" (reproducing American Thyroid Association text), page updated 10 October 2025: "Most of the extra weight gained in hypothyroid individuals is due to excess accumulation of salt and water. Massive weight gain is rarely associated with hypothyroidism. In general, 5-10 pounds of body weight may be attributable to the thyroid, depending on the severity of the hypothyroidism"; "if weight gain is the only symptom of hypothyroidism that is present, it is less likely that the weight gain is solely due to the thyroid". btf-thyroid.org/thyroid-and-weight-the-science. Accessed 10 November 2026.
- Medicines and Healthcare products Regulatory Agency, "MHRA Safety Roundup: May 2025", published 13 June 2025: "Biotin, also known as vitamin B7, is a water-soluble vitamin often found in multi-vitamins and dietary supplements marketed for hair, skin, and nail growth. Biotin may interfere with thyroid immunoassays and the risk of interference increases with higher doses of biotin. Healthcare professionals should ask adults, children and young people with suspected thyroid dysfunction about their biotin intake because a high consumption of biotin may lead to falsely high or low test results." gov.uk/drug-device-alerts/mhra-safety-roundup-may-2025. Accessed 10 November 2026.
- NHS, "Overactive thyroid (hyperthyroidism): diagnosis", page last reviewed 11 April 2023 (its stated next review date had passed on the access date): "Your thyroid hormone levels will be compared to what's normal for a healthy person of your age"; "What's considered normal varies depending on things such as your age and the exact testing technique used by the laboratory." nhs.uk. Accessed 10 November 2026.
- NICE, "Cardiovascular disease: risk assessment and reduction, including lipid modification", NG238, published 14 December 2023, recommendation 1.4.3: "Exclude possible common secondary causes of dyslipidaemia (such as excess alcohol intake, uncontrolled diabetes, hypothyroidism, liver disease and nephrotic syndrome) before referring for specialist review." nice.org.uk/guidance/ng238. Accessed 10 November 2026.
- NHS, "Heart palpitations", page last reviewed 17 March 2026: causes include "an overactive thyroid (hyperthyroidism)"; see a GP if palpitations "keep coming back or they are happening more often" or "last longer than a few minutes"; call 999 or go to A&E if they "do not go away" or come with "chest pain; shortness of breath; feeling faint or fainting". nhs.uk/symptoms/heart-palpitations. Accessed 10 November 2026.
- NHS, "Chest pain", page last reviewed 8 August 2023 (its stated next review date, 8 August 2026, had passed on the access date): call 999 for "sudden pain or discomfort in your chest that does not go away", "pain that spreads to your left or right arm, or your neck, jaw, stomach or back", or "chest pain and you feel sweaty, sick, light headed or short of breath". nhs.uk/symptoms/chest-pain. Accessed 10 November 2026.
Numbered markers in the page text link to these entries. Statistics describe populations, not any one person’s results.
Published 10 November 2026.