Serum TSH Level UK: Normal Range, High, Low & Weight

You open the NHS App and there it is, halfway down a list of numbers you didn't ask for. Serum TSH level: 5.2 mIU/L. Flagged. Possibly with a one-line comment from the practice that says something like "no action required" or, worse, nothing at all.

So you do what everyone does. You google it at eleven o'clock at night and find American sites quoting different numbers, forums full of people telling you your GP is wrong, and a lot of pages that never quite answer the question you're actually asking: what does my serum TSH level mean, and should I be worried?

What "serum TSH level" actually means on your blood test

TSH stands for thyroid-stimulating hormone, and the confusing part is that your thyroid doesn't make it. Your pituitary gland does, sitting underneath your brain, and its job is to nudge the thyroid in your neck into producing thyroxine (T4) and triiodothyronine (T3). Those two hormones set the pace of your metabolism.

"Serum" only tells you how it was measured. Blood is spun down, the cells are separated out, and the hormone is measured in the straw-coloured liquid left behind. It isn't a special or more advanced test than a plain "TSH". Your GP surgery's system prints it that way because that's how the lab labels it.

The bit that trips people up is the direction of travel. When your thyroid slows down, your pituitary shouts louder, so TSH rises. When your thyroid runs fast, the pituitary goes quiet and TSH drops. High number, sluggish thyroid. Low number, overactive one. It feels back to front, and almost everyone reads it the wrong way round the first time.

That sensitivity is also why TSH is the test the NHS reaches for first. NICE guidance recommends measuring TSH alone in adults when pituitary disease isn't suspected, then automatically adding free T4 if the TSH comes back above range, or free T4 and free T3 if it comes back below. Labs call this cascading. It's why you sometimes see extra results appear that nobody warned you about.

What counts as a normal TSH range in the UK

The British Thyroid Foundation publishes these typical adult reference ranges:

Now the caveat that matters more than the numbers themselves. These are a guide, not a national standard. Ranges vary by laboratory, by the analyser it uses, and by age. The Newcastle Hospitals laboratory directory, for example, quotes 0.3 to 4.5 mIU/L for adults, 0.5 to 4.3 for 11 to 18 year olds, and 0.7 to 8.4 for babies between three months and a year old.

So if your result reads 4.3 and the site you're reading says the top of normal is 4.0, that doesn't mean you've been missed. Check the range printed beside your own result. That's the one your GP is working from.

Pregnancy shifts things again, and the reference range used should ideally come from healthy pregnant women in the same population rather than the general adult range.

What a high serum TSH level means

A raised TSH means your pituitary is working harder to get a response out of your thyroid. What happens next depends on your free T4.

If TSH is high and FT4 is low, that's overt hypothyroidism, an underactive thyroid that needs treating. If TSH is high but FT4 is still inside the reference range, that's subclinical hypothyroidism, sometimes called mild thyroid failure. The second one is far more common, and it's where most of the confusion and most of the internet arguing happens.

Symptoms of an underactive thyroid, according to the NHS, tend to creep in slowly: fatigue, feeling cold, weight gain, constipation, difficulty concentrating, low mood, dry skin or hair loss, a hoarse voice, and heavier or irregular periods. Any one of them on its own means very little. Several of them together, alongside a raised TSH, starts to mean something.

For subclinical hypothyroidism, NICE sets out two fairly specific situations:

• If your TSH is 10 mIU/L or higher on two separate occasions three months apart, levothyroxine should be considered.
• If your TSH is above the reference range but below 10 mIU/L on two occasions three months apart, and you're under 65 and have symptoms, a six-month trial of levothyroxine can be considered. If your symptoms don't budge once TSH is back in range, NICE suggests considering stopping it again.

Which is the honest answer to "my TSH is 5.2, why won't anyone treat me?" A single reading of 5.2 with a normal FT4 sits well inside the territory where watching and repeating is the recommended approach, not because your GP is dismissing you, but because a fair proportion of those results drift back into range on their own.

What a low serum TSH level means

A TSH below the reference range points the other way. Your pituitary has gone quiet because there's already plenty of thyroid hormone circulating.

Low TSH with a raised FT4 or FT3 is consistent with hyperthyroidism, or thyrotoxicosis. Symptoms run roughly opposite to the underactive picture: weight loss despite a decent appetite, a racing or irregular heartbeat, tremor, sweating, heat intolerance, anxiety, loose stools, difficulty sleeping. Graves' disease, an autoimmune condition, is a common cause in the UK, which is why doctors measure TSH receptor antibodies to confirm it.

Low TSH with normal FT4 and FT3 is subclinical hyperthyroidism. NICE suggests seeking specialist advice when an adult has two TSH readings below 0.1 mIU/L at least three months apart together with evidence of thyroid disease, such as a goitre or positive antibodies, or symptoms of thyrotoxicosis.

One more pattern is worth knowing about, because it gets missed. A low or normal TSH alongside a low FT4 doesn't fit either box. That can point to a pituitary problem rather than a thyroid one, or simply reflect significant illness elsewhere in the body. It needs looking into rather than reassuring away.

Why a result can look abnormal when your thyroid is fine

Thyroid blood tests are sensitive, which is a strength and a nuisance in equal measure. A handful of things push the numbers around without any thyroid disease being present.

Being unwell is the big one. NICE advises against testing thyroid function during an acute illness unless the illness itself is suspected to be thyroid-related, because sickness distorts the result. If your blood was taken during a nasty chest infection, mention it.

Then there's biotin. High-dose biotin, popular in hair, skin and nail supplements, interferes with the assay and can push results falsely high or falsely low. NICE now advises clinicians to ask about biotin intake in anyone with suspected thyroid dysfunction. So if you take a hair growth supplement, say so before the blood test rather than after the panic.

Plenty of ordinary medicines shift the numbers too: the contraceptive pill, steroids, anticonvulsants, anti-inflammatories, lithium and amiodarone, along with any supplement containing iodine.

And levels simply fluctuate. That's the whole reason NICE builds repeat testing into its thresholds rather than acting on one number.

One more deserves its own paragraph, because it affects so many women. Symptoms of thyroid dysfunction and symptoms of the menopause overlap almost completely, and NICE flags this directly. Tiredness, weight change, low mood, disturbed sleep, brain fog. If you're in your late forties and both are on the table, both deserve considering rather than one being assumed.

Thyroid function and weight: what the numbers can and can't explain

This is usually the reason people are reading the result in the first place. The weight has gone up, nothing obvious has changed, and a slightly raised TSH looks like it might be the answer.

Sometimes it's part of the answer. It's rarely the whole of it, and the size of the effect is smaller than most people hope.

The American Thyroid Association's summary, reproduced by the British Thyroid Foundation, puts it plainly: in general, around 5 to 10 pounds of body weight may be attributable to the thyroid, depending on how severe the hypothyroidism is. Most of that extra weight is accumulated salt and water rather than fat. Large weight gain is rarely down to an underactive thyroid alone. And once treatment starts, the weight loss that follows is usually under 10% of body weight, sometimes less than that, because hypothyroidism develops so slowly that other things have often changed alongside it.

The same source is blunt about the other direction too. Thyroid hormone has been used as a weight loss tool in the past, and pushing the dose above what your body needs produces weight loss that comes straight back when you stop, along with muscle protein loss and other metabolic problems. It isn't a treatment for obesity, and no reputable UK prescriber will use it as one.

None of which means the tiredness isn't real or the weight gain isn't happening. It means that if your TSH is 5.5 and you've gained three stone, the thyroid is one thread and there are others worth pulling. We've written separately about why the scale can stall even in a calorie deficit, and about what your HbA1c result means, which is the other blood test that turns up alongside this one and causes just as much midnight googling.

Thyroid results and weight loss medication

If you've been reading about GLP-1 treatments like Mounjaro or Wegovy, a thyroid result changes some of the calculus, though probably less than you'd assume.

Having a benign thyroid condition doesn't rule you out. The British Thyroid Foundation's guidance on GLP-1 medicines and thyroid conditions, updated in May 2026, says there's no specific reason people with non-cancerous thyroid conditions shouldn't use these medicines, though it may be sensible to wait until your thyroid hormone levels are normal before starting.

There is one hard stop. GLP-1 medicines should not be used by anyone with a personal or family history of medullary thyroid carcinoma (MTC), or of Multiple Endocrine Neoplasia type 2 (MEN 2). That comes from rodent studies showing a signal for this rare thyroid cancer. Human studies over the short to medium term haven't shown an increased thyroid cancer risk, but the contraindication stands, and any honest prescriber will ask you about it. If you've had a different type of thyroid cancer, that's a conversation with your specialist rather than an automatic no.

If you take levothyroxine, expect your thyroid function to be rechecked roughly two to three months after substantial weight loss, since your dose may need adjusting. Anyone on carbimazole or propylthiouracil should tell their endocrinologist and GP before starting.

Two practical points sit underneath all of this. GLP-1 medicines are prescription-only in the UK, and the MHRA is clear that a consultation with a healthcare professional has to happen before any prescription is issued, precisely so that questions like your thyroid history get asked. If you want the mechanism rather than the rules, our guide to how GLP-1s work covers it, and NHS eligibility for weight loss injections explains the criteria.

What happens after an abnormal result

Knowing the likely sequence takes some of the sting out of waiting.

First come more tests on the same sample. If TSH came back high, the lab adds FT4. If it came back low, FT4 and FT3. You may also be offered thyroid peroxidase antibodies (TPOAbs), which help work out whether an autoimmune process is behind a raised TSH. NICE says not to repeat that antibody test once it's been done, so if you've had it, you've had it.

Then a repeat, usually in three months, particularly for borderline results. Two readings three months apart is the threshold NICE uses for most subclinical decisions.

Levothyroxine follows if it's warranted. It's the first-line treatment for primary hypothyroidism. For adults under 65 without heart disease, NICE suggests starting at around 1.6 micrograms per kilogram of body weight per day, rounded to the nearest 25 micrograms. For anyone 65 or over, or with a history of cardiovascular disease, it's a slower start of 25 to 50 micrograms daily with titration upwards. NICE does not routinely recommend liothyronine or natural thyroid extract, on the grounds that the evidence for benefit over levothyroxine is thin and the long-term effects uncertain.

After that, monitoring. TSH every three months until it's stable, meaning two similar in-range measurements three months apart, then annually. One thing that catches people out: if your TSH was very high to begin with, or you were untreated for a long stretch, it can take up to six months to come back into range. Feeling better often lags behind the blood test by weeks.

When to speak to a doctor sooner

Book an appointment rather than waiting for a routine repeat if you have a swelling or thickening in your neck, a fast or irregular heartbeat, new eye symptoms such as bulging, double vision or persistent grittiness, or you're pregnant or planning a pregnancy and have any thyroid result outside the normal range. Untreated thyroid dysfunction in pregnancy carries real risks, and doses usually need changing early.

This article is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional before starting any treatment.