Thyroid Symptoms: Which Blood Test Do You Need?

You've been tired for weeks, you feel the cold more than everyone around you, or your heart has started racing for no obvious reason. Someone mentions your thyroid, and it sounds plausible.

It might be. Tiredness, weight gain, feeling cold, low mood and a fast heartbeat can all have a thyroid cause, and they all have many other causes too, which is why a blood test rather than a symptom list is how you find out. The usual starting point measures TSH and free T4, and at our clinic the result is usually back in about a working day.

If you'd rather skip the reading, you can walk in for a thyroid function test at 117a Harley Street, 7 days a week.

Underactive or overactive: how do the symptoms differ?

An underactive thyroid tends to show as tiredness, weight gain, feeling cold, low mood, dry skin or constipation. An overactive thyroid tends to show as the opposite set: weight loss, feeling hot, anxiety, a fast or irregular heartbeat, tremor or trouble sleeping.

Every item on both lists overlaps with other conditions, and NICE reminds doctors that one symptom on its own rarely points to the thyroid. If tiredness is the main thing, our guide to feeling tired all the time explains the wider causes, and a fatigue assessment looks at the wider causes in one visit.

Which thyroid blood test should you book?

Most people start with Thyroid Profile 1. It measures TSH and free T4, costs £100 plus the £50 phlebotomy fee, and is usually back in 1 working day. Profile 3 adds free T3, and Profile 2 adds thyroid antibodies for a broader picture. You book all three the same way, walk-in or online, 7 days a week.

Every price is plus the £50 phlebotomy fee for the blood draw. The turnaround clock starts when your sample reaches the lab, which happens the same day, weekend samples included. You can compare all three profiles and book on the thyroid function test page.

How do you read your thyroid results?

TSH is the signal your pituitary gland sends to the thyroid. When the thyroid is slow, the pituitary shouts louder, so TSH rises. When the thyroid is overactive, TSH drops, often almost to zero. That see-saw is the one thing worth remembering: TSH moves in the opposite direction to how the thyroid is working.

The British Thyroid Foundation gives these as typical adult reference ranges. Your report will print its own laboratory's range, and that is the one to use.

Typical adult reference ranges. Source: British Thyroid Foundation. Ranges vary between laboratories and are different in pregnancy.

Doctors then read the two main numbers together. These are the common patterns:

Common thyroid result patterns. Sources: British Thyroid Foundation; UK Guidelines for the Use of Thyroid Function Tests (ACB, BTA, BTF).

Timing matters too. NICE advises against testing during an acute illness, because a bad cold or infection can nudge the numbers, and suggests waiting at least 6 weeks before repeating a test. If your result is outside the range, our guide to an abnormal result GP review explains what the follow-up involves.

What is subclinical hypothyroidism?

It means TSH is above the reference range while free T4 is still normal. The thyroid is working harder to keep up, without yet falling behind. It is common: the UK thyroid testing guidelines put it at around 10% of people over 60.

The first step is usually a repeat test about 3 months later, because TSH can rise for a while and settle on its own. A thyroid antibody test (TPO antibodies) also helps, since positive antibodies make a slow progression more likely. That is what Thyroid Profile 2 adds.

NICE guideline NG145 then gives doctors two clear lines. If TSH is 10 mU/L or higher on two tests 3 months apart, treatment with levothyroxine is considered. If you are under 65, with TSH above the range but below 10 on two tests and symptoms of an underactive thyroid, a 6-month trial of treatment may be offered. Untreated, the usual plan is a recheck once a year if you have antibodies, or every 2 to 3 years if you don't.

Already taking thyroid medication? Testing and timing

If you take levothyroxine, the test checks whether your dose is right, and the target is a TSH within the reference range. Free T4 can read a little higher in the hours after your daily tablet. The UK guidelines say this isn't clinically significant, but tell us what time you took it, and some people simply take that morning's tablet straight after the sample.

After a dose change, the level takes time to settle. NICE suggests checking TSH every 3 months until two results in a row are stable within range, then once a year. A test sooner than about 6 weeks after a change usually reflects the old dose rather than the new one.

Thyroid Profile 1 is the usual monitoring test, at £100 plus the £50 phlebotomy fee, with a GP follow-up to go through the result if you want one.

Thyroid tests and pregnancy

Pregnancy changes the thyroid picture. The British Thyroid Foundation notes that the TSH reference range in pregnancy differs from the usual adult range, so a result has to be read against pregnancy-specific ranges.

If you have an underactive thyroid and are pregnant or planning to be, the NHS advises telling your GP straight away, because further tests are needed and your levothyroxine dose is likely to change. The UK testing guidelines also recommend a thyroid check before conception for women with type 1 diabetes, and for anyone who has had postpartum thyroiditis before.

If you are planning a pregnancy and want your thyroid checked first, Thyroid Profile 1 or Profile 2 with antibodies is a sensible start, ideally alongside a private GP consultation so the result is read with your history.

When is it worth adding T3 or antibodies?

Free T3, in Profile 3, is most useful when TSH is low and an overactive thyroid is being considered. Thyroid antibodies, in Profile 2, come in when autoimmune thyroid disease is being considered, for example if thyroid problems run in your family or TSH is mildly raised. For a routine first check, TSH and free T4 usually come first.

Booking a broader profile straight away is fine. Plenty of people start with Profile 1 and widen the search only if the result or their symptoms point that way.

Could it be perimenopause rather than your thyroid?

The two overlap more than people expect, and NICE specifically reminds doctors that thyroid symptoms in menopausal women can be mistaken for menopause. Both can cause tiredness, low mood, weight change and disturbed sleep. If your periods are changing too, or your symptoms started around the usual age for perimenopause, a broader hormone picture often tells you more.

Our menopause blood test covers that wider hormone panel, and you can have it alongside or instead of a standalone thyroid profile.

Should you see a GP first, or just test?

A blood test gives you a result, and a doctor turns that result into a diagnosis and a plan. A GP visit first makes sense if you're unsure which profile fits, if other symptoms sit alongside the tiredness or weight change, or if you'd like the result explained the same week it arrives.

A private GP consultation is £150, and the GP can help you choose a profile before testing or go through the result with you once it is back. You don't need a GP referral to test, as our guide to a private blood test without a referral explains.

How do you prepare for a thyroid blood test?

There is no special preparation and no need to fast. Tell the clinic about any thyroid medicine you take and any biotin supplements, since NICE notes that high-dose biotin can give falsely high or low results. Walk in or book online at 117a Harley Street, 7 days a week, alongside any of our other private blood tests in London.

Already have a TSH result and want to know more about what the number means? Blood London's guide explains what your TSH result means.