Hashimoto's thyroiditis and subclinical hypothyroidism
Hashimoto’s thyroiditis is an autoimmune attack on the thyroid gland, and it is the commonest cause of an underactive thyroid in the UK. It often begins silently: thyroid peroxidase (TPO) antibodies appear in the blood years before any hormone shifts, then the gland slowly fails, passing through subclinical hypothyroidism (a raised TSH with normal thyroid hormone) before reaching overt disease. NICE only routinely treats the subclinical stage once TSH reaches 10 mIU/L, or earlier with a time-limited trial if you are under 65 and symptomatic. Selenium and gluten get a lot of attention, but the honest evidence for both is thinner than the internet suggests.
Key facts
- Autoimmune (Hashimoto’s) thyroiditis is the most common cause of hypothyroidism in iodine-sufficient countries including the UK, and underactive thyroid affects roughly 15 in 1,000 women and 1 in 1,000 men.23
- TPO antibodies are positive in about 90 percent of Hashimoto’s cases. They mark cause and future risk, not present illness: you can be antibody-positive with a normal TSH and feel completely well.4
- With a raised TSH and positive antibodies, the risk of progressing to overt hypothyroidism is roughly 4 percent per year; with a raised TSH alone it is about 2.6 percent.5
- NICE NG145 advises levothyroxine for subclinical hypothyroidism when TSH is 10 mIU/L or higher on two tests three months apart, and a 6-month trial under 65 if TSH is above range but under 10 with symptoms.1
- A 2024 meta-analysis of 35 randomised trials found selenium lowers TPO antibodies but does not change free T4 or thyroid volume, and the clinical meaning is uncertain.6
The autoimmune driver behind most UK hypothyroidism
Hashimoto’s thyroiditis (chronic autoimmune thyroiditis) is a condition in which the immune system mistakes thyroid tissue for a threat. Over years, immune cells infiltrate the gland, hormone-producing follicles are lost, and thyroxine output slowly falls. It is the single biggest reason UK adults end up on thyroid hormone replacement, and it is around ten times more common in women than men, rising with age.23 This is a distinct topic from how to read a thyroid blood test; our health library covers the thyroid panel marker by marker. Here the focus is the disease: what the antibodies mean, how it progresses, when treatment is justified, and what actually helps.
What TPO antibodies actually mean
Thyroid peroxidase is the enzyme the gland uses to build thyroid hormone. TPO antibodies are immune proteins aimed at that enzyme, and they are the fingerprint of autoimmune thyroid disease: positive in roughly 90 percent of people with Hashimoto’s.4 A second antibody, thyroglobulin antibody (TgAb), is positive in about half of cases and adds little once TPO status is known, so the NHS does not test it first-line.4
The crucial nuance is that antibodies predict risk rather than describe current function. Plenty of people are TPO-positive with a normal TSH and no symptoms; what the antibodies do is raise the odds of the gland failing later. So NICE recommends measuring TPO antibodies once when TSH is above the reference range, and explicitly advises not repeating the test, because tracking the titre does not usefully guide management.1
The natural history: euthyroid to subclinical to overt
Hashimoto’s rarely arrives all at once. It tends to move through recognisable stages, though not everyone travels the whole way and the pace varies enormously.
| Stage | TSH | Free T4 | TPO antibodies | How it usually feels |
|---|---|---|---|---|
| Euthyroid autoimmunity | Normal | Normal | Often positive | Usually no symptoms |
| Subclinical hypothyroidism | Raised | Normal | Often positive | None, or mild and non-specific |
| Overt hypothyroidism | Raised (often high) | Low | Often positive | Fatigue, cold intolerance, weight gain, low mood |
The classic long-term data come from the Whickham survey, a UK community cohort followed for twenty years. A raised TSH alone carried about a 2.6 percent annual risk of developing overt hypothyroidism, but a raised TSH and positive antibodies together pushed that to roughly 4 percent per year; positive antibodies with a normal TSH carried about 2 percent.5 The combination of a borderline TSH and positive TPO antibodies is the strongest everyday signal that the gland is likely to keep failing.
Subclinical hypothyroidism is also common and frequently self-corrects. Up to half of people with a TSH between 5 and 10 will drift back into the normal range without any treatment, which is exactly why NICE asks for two tests three months apart before acting.1
When NICE recommends treating subclinical hypothyroidism
This is where the antibody and symptom picture turns into a decision. NICE guideline NG145 sets out clear thresholds for adults.1
- TSH 10 mIU/L or higher, confirmed on two occasions three months apart: consider levothyroxine.1
- TSH above range but under 10, in an adult under 65 with symptoms of hypothyroidism: consider a 6-month trial of levothyroxine, and stop it if symptoms do not improve once TSH is back in range.1
- Otherwise monitor, not treat: yearly if there are features of thyroid disease such as positive antibodies, or every two to three years if not.1
NICE explicitly says that raised thyroid autoantibodies are one of the features that should tip the balance towards treating or monitoring more closely.1 Age matters too. The TRUST trial randomised 737 adults over 65 with subclinical hypothyroidism (TSH about 4.6 to 19.9) to levothyroxine or placebo and found no improvement in symptoms or tiredness from treatment, which is part of why the over-65 threshold is more conservative.7 Levothyroxine is started at roughly 1.6 micrograms per kilogram per day in under-65s without heart disease, and at a cautious 25 to 50 micrograms daily with titration in older adults or those with cardiovascular disease.1
Pregnancy is the major exception to the wait-and-see approach. Because adequate thyroid hormone matters for fetal development, guidelines treat far more readily: women who are TPO-positive are generally treated with levothyroxine once TSH rises above the pregnancy reference range (around 4 mIU/L), and treatment may be considered when TSH is between 2.5 and 4.0. If you are pregnant, trying to conceive, or having fertility treatment, thyroid status should be checked and managed promptly rather than monitored.8
The selenium and gluten questions, honestly
Selenium. Selenium is genuinely part of thyroid biology and is the most studied supplement here. A 2024 systematic review and meta-analysis by Huwiler and colleagues pooled 35 randomised trials and found selenium significantly lowered TPO antibodies (and TSH in people not yet on hormone replacement), but produced no significant change in free T4, free T3, thyroglobulin antibodies or thyroid volume. The authors cautioned that, because of assay differences, only the effect size and not its clinical meaning can be interpreted, and about half the trials studied selenium-deficient people, so benefits in a selenium-replete UK diet may be smaller.6 In short, selenium can move an antibody number with no proven effect on how you feel or on the disease course.
Selenium has a narrow safety margin. The UK tolerable upper intake is around 300 micrograms a day from all sources, and chronic excess can cause selenosis (hair and nail changes, gastrointestinal upset, neuropathy). Routine selenium is not recommended by NICE for thyroid disease, and high-dose supplements should not be taken indefinitely without good reason. This is an optional, unproven add-on, not a treatment.6
Gluten. The gluten question has two separate parts. First, there is a genuine, well-established link with coeliac disease: the two share genetic risk, and coeliac disease is several times more common in people with autoimmune thyroid disease, which is why NICE recommends offering coeliac testing at diagnosis of autoimmune thyroid disease.19 If you have coeliac disease, a strict gluten-free diet is essential. Second, and separately, is whether cutting gluten helps Hashimoto’s in people without coeliac disease. Here the evidence is weak: a 2025 meta-analysis could pool only three small randomised trials (about 110 participants) and found no consistent effect on thyroid hormones, TSH or antibodies, concluding the data do not support a gluten-free diet for everyone with Hashimoto’s.10
Living with it
For most people, Hashimoto’s is a slow, manageable condition. Once you genuinely need treatment, levothyroxine (T4) is the NICE first-line therapy and the goal is to keep TSH within the reference range; NICE does not routinely recommend liothyronine (T3) or natural thyroid extract, citing insufficient evidence of benefit and uncertain long-term safety, and the latter has no UK marketing authorisation.1 Take levothyroxine on an empty stomach and separate it from calcium and iron, which reduce its absorption.1
If you are not yet being treated, monitoring beats supplements: know your TSH, free T4 and antibody status, and recognise that mild symptoms can come from many causes, not just the thyroid. A balanced diet, adequate but not excessive iodine, and not chasing your antibody titre down are sensible defaults. Our stack builder helps track what has been tested and when, and our start page explains how to use these guides without overtreating a number.
What to ask your GP
- Have my TPO antibodies been checked, and combined with my TSH does that change how closely we should monitor or whether to treat?
- My TSH is raised but under 10: do I meet the NICE criteria for a 6-month levothyroxine trial, given my symptoms and age?
- Should we repeat the test in three months before deciding, since a single mildly raised TSH often settles on its own?
- I have autoimmune thyroid disease: should I be tested for coeliac disease, as NICE suggests?
- Is selenium worth trying in my case, or is the evidence too weak to justify it?
References
- National Institute for Health and Care Excellence. Thyroid disease: assessment and management (NG145). Published 2019, updated 2023. nice.org.uk/guidance/ng145.
- Chaker L, Razvi S, Bensenor IM, et al. Hypothyroidism. Nat Rev Dis Primers / Lancet. 2024. Autoimmune thyroiditis as the commonest cause in iodine-sufficient regions. thelancet.com.
- NHS. Underactive thyroid (hypothyroidism): causes and overview. nhs.uk, accessed 2026.
- Fröhlich E, Wahl R. Thyroid autoimmunity: role of anti-thyroid antibodies in thyroid and extra-thyroidal diseases. Front Immunol. 2017;8:521. TPOAb positive in ~90% and TgAb in ~50% of Hashimoto’s. PMC5418333.
- Vanderpump MPJ, Tunbridge WMG, et al. The incidence of thyroid disorders in the community: a twenty-year follow-up of the Whickham Survey. Clin Endocrinol (Oxf). 1995;43(1):55-68. onlinelibrary.wiley.com.
- Huwiler VV, Maissen-Abgottspon S, Stanga Z, et al. Selenium supplementation in patients with Hashimoto thyroiditis: a systematic review and meta-analysis of randomized clinical trials. Thyroid. 2024;34(3):295-313. PMC10951571.
- Stott DJ, Rodondi N, Kearney PM, et al. Thyroid hormone therapy for older adults with subclinical hypothyroidism (TRUST). N Engl J Med. 2017;376(26):2534-2544. nejm.org.
- Alexander EK, Pearce EN, Brent GA, et al. 2017 ATA guidelines for the diagnosis and management of thyroid disease during pregnancy and the postpartum. Thyroid. 2017;27(3):315-389. thyroid.org.
- Coeliac UK. Autoimmune thyroid disease and coeliac disease (related conditions). coeliac.org.uk, accessed 2026.
- Systematic review and meta-analysis. Effects of a gluten-free diet in non-coeliac Hashimoto’s thyroiditis. Nutrients. 2025;17(21):3437. Three RCTs, ~110 participants; no consistent effect. PMC12609533.
This article is educational and does not constitute medical advice, diagnosis, or a treatment recommendation. Medication uses described as “off-label” are not licensed for that purpose in the UK and should only be considered under qualified clinical supervision. Always speak to your GP, pharmacist, or a registered specialist before starting, stopping, or changing any treatment. If you have severe or alarm symptoms - unintentional weight loss, blood in your stool, difficulty swallowing, persistent vomiting, a fever, or severe pain - seek urgent medical care.