Key takeaways
- TSH, thyroid stimulating hormone, is made by the pituitary gland and tells the thyroid how much hormone to produce, so it is the standard first-line test for thyroid function.
- A high TSH generally points to an underactive thyroid, and a low TSH to an overactive one, but the pattern needs reading alongside free T4.
- Blood London's TSH test costs £71.25 plus the £50 blood draw, with results back in 1 day, and no GP referral is needed to book it.
- Your laboratory report shows the reference range next to your result, and a raised or lowered TSH is a starting point for a doctor's assessment, not a diagnosis on its own.
- If you take levothyroxine, many clinicians advise taking that day's dose after the blood sample so the result reflects your usual level.
TSH is the pituitary hormone that signals your thyroid gland to make more or less thyroid hormone, and a raised or lowered level is usually the first clue to whether your thyroid is underactive or overactive. You can book a TSH blood test at Blood London without a GP referral.
Book a TSH blood test at 117a Harley Street: £71.25 plus the £50 blood draw, results in 1 day.
What does TSH actually measure?
TSH is not made by the thyroid itself. It is released by the pituitary gland, at the base of the brain, which monitors how much thyroid hormone is circulating and adjusts its signal accordingly. When thyroid hormone levels fall, the pituitary raises TSH to push the thyroid to produce more. When thyroid hormone levels rise, the pituitary lowers TSH to slow the thyroid down. This feedback loop is why TSH, rather than the thyroid hormones themselves, is usually the first test doctors order: it tends to shift before symptoms become obvious, and it moves in the opposite direction to the underlying thyroid hormone level.
What does a high TSH result usually mean?
A high TSH generally means the thyroid gland is underactive and not producing enough hormone, so the pituitary has raised its signal to compensate. This pattern, known as hypothyroidism, can come with tiredness, weight gain, feeling cold, low mood, dry skin or constipation, though many people have few symptoms at all, especially early on. The most common cause in the UK is autoimmune thyroid disease, where the immune system gradually affects the gland. A single high result is usually followed by a repeat test and a free T4 measurement before any treatment decision is made.
What does a low TSH result usually mean?
A low TSH generally means the thyroid is producing more hormone than the body needs, so the pituitary has dialled its signal down. This pattern, hyperthyroidism, can come with a fast heart rate, weight loss, feeling hot, anxiety, tremor or sleep disturbance. As with a high result, some people have a low TSH with few or no symptoms. Common causes include Graves' disease, thyroid nodules, inflammation of the thyroid (thyroiditis) and taking more thyroid hormone medicine than you need. Rarely, a low TSH alongside a low free T4 reflects a pituitary problem rather than an overactive thyroid, which is one more reason a low TSH is followed up with further testing rather than acted on alone. A very fast or irregular heartbeat, chest pain or breathlessness needs urgent medical attention through NHS 111, or 999 if severe.
| TSH and free T4 pattern | What it commonly suggests |
|---|---|
| High TSH, low free T4 | An underactive thyroid (hypothyroidism) |
| High TSH, normal free T4 | An early or mild underactive pattern, sometimes called subclinical hypothyroidism |
| Low TSH, high free T4 | An overactive thyroid (hyperthyroidism) |
| Low TSH, normal free T4 | An early or mild overactive pattern, sometimes called subclinical hyperthyroidism |
Why is free T4 tested alongside TSH?
TSH tells you the pituitary's signal, but free T4 tells you what the thyroid gland is actually producing in response, so the two together give a fuller picture than either alone. This is particularly useful when a TSH result sits at the edge of its range, when someone has clear symptoms but a normal TSH, or when monitoring thyroid medication. Blood London's Thyroid Profile 3 measures TSH, free T4 and free T3 together for £142.50 plus the £50 blood draw, with results in 1 day. You can also add free T4 on its own for £71.25 plus the £50 blood draw, results in 1 day.
Can biotin supplements affect my TSH result?
Yes, high-dose biotin supplements can interfere with some thyroid immunoassays, including TSH, and can distort the result in either direction. If you take a biotin supplement, particularly one aimed at hair, skin or nails, mention this when you book so the phlebotomist and laboratory are aware. This is a recognised laboratory limitation rather than anything specific to Blood London's testing.
Do I need to stop my thyroid medication before the test?
Do not stop a prescribed thyroid medication without your clinician's advice, but timing the blood sample can matter. If you take levothyroxine, many clinicians advise having the blood drawn before that day's dose, or taking the dose after the sample, so the result reflects your usual level rather than a recent tablet. Ask the clinician who prescribes your medication for their preferred timing, since practice can vary.
What if my TSH is normal but I still have symptoms?
A normal TSH does not rule out every thyroid-related explanation for how you feel, since symptoms like fatigue, weight change or mood changes have many possible causes. If symptoms persist despite a normal TSH, a fuller thyroid profile including free T4 and free T3, or antibody testing such as thyroid peroxidase antibodies (£73.75 plus the £50 blood draw, results in 1 day) to check for autoimmune thyroid disease, can add detail. Anaemia can cause similar tiredness, so a full blood count is often checked alongside thyroid tests; our guide to full blood count results explains what it shows. A doctor can advise which further tests, if any, are worth pursuing given your particular symptoms.
How TSH testing works at Blood London
The TSH test costs £71.25 plus the £50 blood draw, which is charged once per visit even if you book more than one test. Results are usually back within 1 day and are sent to you by email. No fasting or special preparation is needed, and no GP referral is required to book. You can walk in or book online at 117a Harley Street, seven days a week, alongside any other private blood tests in London you want done in the same visit. You can add a doctor review of your report to your test for £75. Results are otherwise sent without interpretation, so if your TSH is outside range, arrange to discuss it with a doctor, for example through a GP appointment at Medical Express Clinic, which costs £150.
Book a TSH blood test at 117a Harley Street: £71.25 plus the £50 blood draw (£121.25 total), results in 1 day. Start your booking.
Frequently asked questions
Does a high TSH always mean an underactive thyroid?
Usually, but not always. A high TSH most often reflects an underactive thyroid, though a small number of cases have other explanations, which is why doctors repeat the test and add free T4 before concluding anything.
How long does it take to get TSH results?
Results from Blood London's TSH test are usually available within 1 day and are sent to you by email.
Can stress or illness change my TSH result?
Yes, acute illness and some medicines can temporarily shift TSH, which is one reason a single abnormal result is usually followed by a repeat test rather than acted on immediately.
Is fasting needed before a TSH blood test?
No fasting is needed for a TSH test at Blood London, and the blood draw can be booked at any time of day.
Should I test TSH alone or the full thyroid profile?
TSH is the standard first-line test. If it comes back abnormal, or you have symptoms despite a normal TSH, a fuller profile with free T4 and free T3, such as Blood London's Thyroid Profile 3, gives more detail.
Who should interpret my TSH result?
Your laboratory report will show the reference range next to your result, but a doctor should interpret it alongside your symptoms and history. You can add a doctor review to your test for £75 when you book.
References
- NHS: Underactive thyroid (hypothyroidism)
- NHS: Overactive thyroid (hyperthyroidism)
- NICE Clinical Knowledge Summaries: Hypothyroidism
- NICE Clinical Knowledge Summaries: Hyperthyroidism
- British Thyroid Foundation