Key takeaways
- TSH is made by the pituitary gland and tells your thyroid how much hormone to produce, which makes it the standard first test for thyroid function.
- A high TSH generally points to an underactive thyroid and a low TSH to an overactive one, with free T4 confirming the pattern.
- You can book a TSH test without a GP referral or fasting, and results come back in 1 day.
- If you take levothyroxine, many clinicians advise taking that day's dose after the blood sample, so the result reflects your usual level.
- Your report shows the reference range next to your result, and a doctor reads it alongside your symptoms.
You've been tired for months, the cold gets to you more than it used to, or your heart races for no obvious reason. Someone suggests checking your thyroid, and the test they usually mean is TSH.
TSH, thyroid stimulating hormone, is the pituitary hormone that signals your thyroid to make more or less thyroid hormone. 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 (£121.25 total), results in 1 day.
What does TSH actually measure?
Here's the part that surprises most people: TSH isn't made by the thyroid at all. It's released by the pituitary gland at the base of the brain, which keeps track of how much thyroid hormone is circulating and adjusts its signal to match.
When thyroid hormone levels fall, the pituitary raises TSH to push the thyroid to make more. When they rise, it lowers TSH to slow the thyroid down. It works rather like a thermostat turning the heating up when the room gets cold.
That 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 is underactive and not making enough hormone, so the pituitary has raised its signal to compensate. This is hypothyroidism. It can bring 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 making more hormone than the body needs, so the pituitary has dialled its signal down. This is hyperthyroidism. It can bring 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. That's one more reason a low TSH is followed up with further testing before anything is done.
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. Free T4 tells you what the thyroid is actually producing in response, so together they give a fuller picture than either alone. That's most useful when a TSH result sits at the edge of its range, when someone has clear symptoms but a normal TSH, or when thyroid medication is being monitored.
Our Thyroid Profile 3 measures TSH, free T4 and free T3 together for £142.50, with results in 1 day. You can also add free T4 on its own for £71.25, also in 1 day. Both are plus the £50 blood draw, which is charged once per visit however many tests you book.
Can biotin supplements affect my TSH result?
Yes. High-dose biotin supplements can interfere with some thyroid immunoassays, including TSH, and distort the result in either direction. If you take biotin, particularly a supplement aimed at hair, skin or nails, mention it when you book so the phlebotomist and laboratory are aware. It's a recognised laboratory limitation that applies wherever you're tested.
Do I need to stop my thyroid medication before the test?
Keep taking any prescribed thyroid medication unless your clinician advises otherwise, but the timing of your blood sample can matter. If you take levothyroxine, many clinicians advise having blood drawn before that day's dose, or taking the dose after the sample. That way the result reflects your usual level rather than a recent tablet. Practice varies, so ask the clinician who prescribes your medication for their preferred timing.
What if my TSH is normal but I still have symptoms?
Symptoms like fatigue, weight change or mood changes have many possible causes, and a normal TSH can't rule out every thyroid-related one. If symptoms persist, a fuller thyroid profile with free T4 and free T3 can add detail. So can antibody testing such as thyroid peroxidase antibodies, which checks for autoimmune thyroid disease (£73.75, results in 1 day).
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 doing for your particular symptoms.
How TSH testing works at Blood London
No fasting or special preparation is needed, and no GP referral is required. Results are usually back within 1 day and sent to you by email. 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.
Results are sent without interpretation unless you add a doctor review of your report, which costs £75. If your TSH is outside range, it's worth discussing with a doctor, for example at a GP appointment at Medical Express Clinic, which costs £150. 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. That's 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 repeated before anything is acted on.
Is fasting needed before a TSH blood test?
No. You don't need to fast 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 our Thyroid Profile 3, gives more detail.
Who should interpret my TSH result?
Your report shows 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