Health condition · Clinically reviewed
Hypothyroidism, the guide that answers what to do next.
An underactive thyroid is one of the most treatable causes of persistent tiredness - once it is spotted. Here is how it is diagnosed, what the numbers mean, and how to get your dose right.
Why trust this guide
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Clinically reviewed
Written by our editorial team and reviewed by a registered UK clinician before publication.
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Sourced from guidance
Every claim is checked against NICE, BTA or peer-reviewed sources you can see at the end.
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Current for 2026
Reflects UK guidance on diagnosis thresholds, treatment targets, and the role of T3.
Key facts
Hypothyroidism at a glance.
The essentials, in plain English - what it is, how common it is, what the numbers mean, and how it is treated in the UK today.
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What it is
An underactive thyroid gland that makes too little thyroid hormone - slowing metabolism, mood, energy and heart rate.
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How common
Around 1 in 20 UK adults; more common in women and rising with age.
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Main cause
Autoimmune (Hashimoto’s thyroiditis) - the immune system attacks the thyroid.
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Diagnosis
TSH and free T4 blood tests. TPO antibodies confirm the autoimmune cause where diagnosis is uncertain.
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Treatment
Once-daily levothyroxine, taken on an empty stomach - dose adjusted to normalise TSH.
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Prognosis
Excellent on adequate replacement - most people feel completely well.
Why this guide matters
A simple test, a lifelong tablet, a normal life.
Hypothyroidism is common, easy to diagnose and highly treatable - once the right questions are asked. The three points below shape the rest of this guide.
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A simple blood test settles it
TSH and free T4 are decisive for most people.
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The dose matters more than the drug
Right target TSH, taken correctly, transforms symptoms.
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Life stages need adjustments
Pregnancy, illness and new medicines all change requirement.
How the diagnosis is made
From first symptom to a settled dose.
The steps a UK GP will normally follow, in order - so you know what to expect and why.
Phase 1 · Recognising
Symptoms and the first blood tests
Phase 2 · Confirming
Antibodies and ruling out other causes
Phase 3 · Managing
Levothyroxine and recheck at 6-8 weeks
- 01
Recognising
Symptoms and risk factors
Tiredness, weight gain, cold intolerance, constipation, low mood - alongside family history or postnatal onset.
- 02
Recognising
TSH plus free T4
Raised TSH with low free T4 confirms overt hypothyroidism. Mildly raised TSH with normal T4 is subclinical.
- 03
Recognising
Confirm on repeat
A single abnormal result is usually confirmed by a second test in 6-8 weeks unless symptoms are marked.
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Confirming
TPO antibodies
Positive antibodies confirm Hashimoto’s and increase the likelihood of progression to overt disease.
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Confirming
Rule out other causes
Amiodarone, lithium, radiotherapy or thyroid surgery history all matter.
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Managing
Levothyroxine started
Usual starting dose 50-100 mcg (25 mcg in older adults or cardiac disease). Once daily on an empty stomach.
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Managing
Recheck TSH at 6-8 weeks
Aim to normalise TSH. Adjust dose in 25 mcg increments; symptoms often improve after biochemistry.
Typical timeline: 6-12 weeks from first blood test to a settled dose.
Symptoms
What an underactive thyroid actually feels like.
Symptoms come on slowly and overlap with other common conditions - which is why it is often missed for months or years.
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Persistent tiredness
The most common symptom - unexplained fatigue that rest does not fix.
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Weight gain
Slow, gradual gain - usually modest, and not the sole cause of significant obesity.
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Feeling cold
Cold intolerance - feeling cold when others do not.
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Low mood or brain fog
Slowed thinking, poor concentration and low mood - often improve with treatment.
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Dry skin & hair loss
Coarse hair, thinning eyebrows, dry skin.
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Constipation
Slow bowels are common - and often overlooked.
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Heavier or irregular periods
A common menstrual pattern with untreated hypothyroidism.
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Red-flag features
New neck lump, hoarseness, difficulty swallowing, or severe symptoms with confusion - see the red-flag section.
Treatment
How hypothyroidism is treated in the UK.
Levothyroxine, taken correctly, transforms most people. Here is what each part of the plan does - and why the details matter.
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Levothyroxine
Synthetic T4 - once daily on an empty stomach, 30-60 minutes before food or other medications.
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Right dose, right target
Aim for TSH in the lower half of the reference range for most; slightly higher for older adults.
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Consistent brand where possible
Bioequivalence varies subtly between manufacturers - some people notice a change on switching brand.
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Timing & interactions
Take away from iron, calcium, PPIs, coffee and soy - all reduce absorption.
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T3 (liothyronine)
Occasionally used in specialist care - most people do well on levothyroxine alone.
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Subclinical management
Mildly raised TSH with normal T4 - treated if TSH persistently >10 mIU/L, TPO positive, or symptomatic (especially in pregnancy planning).
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Pregnancy adjustments
Levothyroxine dose increases by ~25-30% early in pregnancy; TSH target is lower. Discuss ahead of conception where possible.
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Annual reviews
TSH check yearly once stable; sooner if symptoms change or you start new medications.
What this guide is based on
The sources behind every number on this page.
UK national guidance and specialist society standards, current at the time of last review.
Key references
Guidelines and standards we relied on.
A quiet reminder
This guide is for information, not medical advice.
Your GP knows your history and can tell you which parts apply to you. If in doubt, ask for a thyroid function test.
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NICE. Thyroid disease: assessment and management (NG145).
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British Thyroid Association. Guidelines on the management of primary hypothyroidism.
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British Thyroid Foundation. Patient information.
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NHS. Underactive thyroid: overview.
Red flags
When hypothyroidism needs urgent attention.
Hypothyroidism is usually a slow, steady condition. These are the situations where it stops being routine - and you should act promptly.
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New or growing neck lump
Any thyroid nodule - urgent GP for ultrasound and specialist review.
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Hoarseness or voice change
With thyroid enlargement - assess urgently.
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Difficulty swallowing
Especially with a goitre - specialist assessment.
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Severe symptoms with confusion
Very rare but serious - myxoedema coma needs A&E.
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Chest pain on starting levothyroxine
Especially in older adults or those with heart disease - contact your GP.
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New palpitations, tremor
May indicate overtreatment (iatrogenic hyperthyroidism) - blood test and dose review.
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Pregnancy planning
Adjust dose and monitor TSH tightly - poor control affects the baby.
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Postnatal exhaustion beyond norm
Postnatal thyroiditis is under-recognised - check thyroid function.
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Amiodarone, lithium, immunotherapy
Any of these medications - baseline and periodic thyroid checks.
Living with it
A lifelong condition, a very manageable one.
Four small things that make the biggest difference day to day - timing, consistency, life-stage changes and yearly reviews.
A quiet reminder
Consistency beats intensity, every time.
Take it at the same time every day, on an empty stomach - and keep the yearly blood test.
- 01 Timing
Own the routine
Take levothyroxine at the same time daily, on an empty stomach, apart from other tablets.
- 02 Consistency
Stick to one brand where you can
Report symptom change if a switch happens - some people notice a real difference.
- 03 Life stages
Doses change through life
Pregnancy, illness, weight change and new drugs all affect requirement.
- 04 Reviews
Annual TSH once stable
A yearly blood test catches drift early - do not stop taking it because you feel well.
Frequently asked
Everything we get asked about hypothyroidism.
Quick answers on TSH targets, levothyroxine, subclinical disease, pregnancy and when to worry.
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How is hypothyroidism diagnosed?
By blood tests - raised TSH with low free T4 confirms overt hypothyroidism. TPO antibodies confirm the autoimmune cause.
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What is subclinical hypothyroidism?
TSH mildly raised with normal T4 - often watched and re-tested. Treated if TSH is persistently over 10, if TPO antibodies are positive with symptoms, or if planning pregnancy.
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Why on an empty stomach?
Food, coffee, iron, calcium and PPIs all reduce levothyroxine absorption - taking it 30-60 minutes before food gives more reliable levels.
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Will I need it for life?
For most people, yes. The thyroid does not usually recover from autoimmune damage. Doses may need adjusting over time.
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Does levothyroxine cause weight gain?
The opposite - untreated hypothyroidism causes modest weight gain. Once treated, weight tends to normalise.
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What if I still feel tired on treatment?
First recheck TSH and free T4. Also consider iron deficiency, low vitamin D, sleep apnoea, depression - all common and often overlap with hypothyroid symptoms.
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Should I try T3?
Most people do well on levothyroxine alone. Adding T3 is a specialist decision, occasionally helpful, and needs monitoring for over-replacement.
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What about natural desiccated thyroid?
Not recommended by NICE or the British Thyroid Association - inconsistent hormone content and higher risk of over-replacement.
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Hypothyroidism and pregnancy?
Dose usually increases by 25-30% early in pregnancy; TSH target is lower. Ideally reviewed before conception.
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When should I see a GP urgently?
A new neck lump, hoarseness, difficulty swallowing, chest pain on starting levothyroxine, or severe symptoms with confusion.