Last updated: October 2026
An underactive thyroid (hypothyroidism) is when your thyroid gland doesn’t make enough hormones. Common symptoms are extreme tiredness, feeling cold, weight gain, constipation, low mood, brain fog, dry skin and hair, and heavy or irregular periods. It’s more common in women. A simple blood test diagnoses it, and daily levothyroxine tablets usually treat it well.
This guide covers the full list of symptoms, signs in women, causes, the thyroid blood test, treatment on the NHS, the link with menopause and mood, diet and supplements, and when to see a GP.
Key point: thyroid symptoms come on slowly and overlap with lots of other things, such as stress, menopause, low iron and depression. If you’ve felt exhausted, cold and sluggish for weeks, ask your GP for a blood test rather than guessing.
What is an underactive thyroid?
Your thyroid is a small, butterfly-shaped gland at the front of your neck. It makes hormones, mainly thyroxine (T4) and triiodothyronine (T3), that control how fast your body uses energy. They affect your heart rate, body temperature, digestion, weight, mood and energy.
When the thyroid doesn’t make enough of these hormones, everything slows down. That’s hypothyroidism. It’s common: a large review of European studies found around 3 in 100 people have it, and many more have mildly low thyroid function that hasn’t been diagnosed.
Underactive thyroid symptoms
According to the NHS, symptoms may be mild and easy to miss, and they usually develop slowly. Common symptoms include:
| Area | Symptoms |
|---|---|
| Energy | Extreme tiredness (fatigue), feeling slow and sluggish. See why am I always tired? |
| Temperature | Feeling cold more than usual |
| Weight | Putting on weight |
| Digestion | Constipation |
| Brain | Difficulty concentrating or thinking clearly. See brain fog |
| Mood | Low mood or depression. See low mood vs depression |
| Skin and hair | Dry skin, dry hair or hair loss |
| Voice | A croaky (hoarse) voice |
| Periods | Irregular or heavy periods |
Other symptoms people often report include aching muscles and joints, cramps, a slow heart rate, puffy face, and slower thinking or speech. See joint pain.
Underactive thyroid symptoms in women
Hypothyroidism is much more common in women than men. The most common cause, Hashimoto’s disease, usually starts between the ages of 30 and 50, which overlaps with the perimenopause. Things women often notice:
- Period changes: heavier, longer or irregular periods.
- Fertility: an underactive thyroid can make it harder to get pregnant.
- Pregnancy: if you have an underactive thyroid and are pregnant or planning a pregnancy, tell your GP, as your dose usually needs adjusting.
- After birth: thyroid problems sometimes start in the year after having a baby (postpartum thyroiditis).
- Hair thinning and dry skin.
- Weight gain that’s hard to shift. See menopause weight gain.
Underactive vs overactive thyroid
An overactive thyroid (hyperthyroidism) is the opposite problem: the thyroid makes too much hormone and everything speeds up. It’s less common. The two are easy to mix up by name, so here’s a quick comparison.
| Underactive (hypothyroidism) | Overactive (hyperthyroidism) | |
|---|---|---|
| Energy | Tired, sluggish | Restless, but often tired too |
| Weight | Weight gain | Weight loss |
| Temperature | Feel cold | Feel hot, sweaty |
| Heart | Slower heart rate | Fast or irregular heartbeat |
| Mood | Low mood | Anxiety, irritability |
| Bowels | Constipation | Diarrhoea |
| Common cause | Hashimoto’s disease | Graves’ disease |
Both are diagnosed with the same blood test. If you have anxiety, a racing heart and weight loss, mention it to your GP. See anxiety symptoms.
What causes an underactive thyroid?
In the UK, the main cause is Hashimoto’s disease, an autoimmune condition where the immune system attacks the thyroid. Other causes include:
- other thyroid conditions, such as thyroiditis
- surgery or radiotherapy on the neck or thyroid
- treatment for an overactive thyroid
- certain medicines
- not getting enough iodine, which is rare in the UK
You’re more likely to develop it if someone in your family has a thyroid condition, or if you have another autoimmune condition such as type 1 diabetes or coeliac disease.
Conditions that feel similar
Many thyroid symptoms are shared with other common conditions, which is why a blood test matters.
| Condition | Overlapping symptoms | Read more |
|---|---|---|
| Perimenopause and menopause | Tiredness, weight gain, low mood, period changes, brain fog | Perimenopause symptoms |
| Iron deficiency | Tiredness, hair loss, feeling cold | Iron deficiency |
| Vitamin B12 deficiency | Tiredness, brain fog, low mood | B12 deficiency |
| Low vitamin D | Tiredness, aches | Vitamin D in winter |
| Depression | Low mood, tiredness, slowed thinking | Low mood vs depression |
| Stress and burnout | Exhaustion, poor concentration | Burnout symptoms |
| Seasonal low mood | Tiredness and low mood in winter | SAD |
The thyroid blood test
Your GP will ask about your symptoms and family history and may examine your neck. A blood test checks:
- TSH (thyroid-stimulating hormone): made by the pituitary gland. A high TSH usually means the thyroid is underactive, because your body is “shouting” for more hormone.
- T4 (thyroxine): a low T4 alongside a high TSH confirms hypothyroidism.
- Thyroid antibodies may be checked to see if Hashimoto’s is the cause.
If TSH is raised but T4 is normal, this is called subclinical hypothyroidism. It often doesn’t need treatment straight away, but your GP may repeat the test. NICE has guidance for GPs on testing and treatment.
Tip: the NHS notes that biotin (vitamin B7), found in some hair, skin and nail supplements, can affect thyroid test results. Tell your GP about any supplements you take before the test.
Treatment for an underactive thyroid
The usual treatment is levothyroxine, a daily tablet that replaces the missing hormone. Most people take it for life and can live completely normally.
- How to take it: the NHS advises taking it on an empty stomach, 30 to 60 minutes before breakfast, caffeine drinks or other medicines, at the same time each day.
- Check-ups: you’ll have regular blood tests to get the dose right. It can take a few months to feel the full benefit.
- Interactions: some medicines and supplements, including calcium and iron supplements, indigestion remedies and St John’s wort, can affect how levothyroxine works. Always check with your pharmacist before starting any new supplement.
- Free prescriptions: in England, people who take levothyroxine for an underactive thyroid can usually get a medical exemption certificate for free prescriptions. Ask your GP.
Diet, supplements and lifestyle
No diet or supplement can cure an underactive thyroid or replace levothyroxine. But looking after yourself helps with energy and mood, especially while your dose is being adjusted.
- Eat a balanced diet with plenty of fibre, which also helps constipation. See food and mood and gut health.
- Iodine: iodine contributes to the normal production of thyroid hormones and normal thyroid function. Most people in the UK get enough from milk, yoghurt, eggs and fish. Don’t take high-dose iodine or kelp supplements unless a doctor advises it, as too much can cause thyroid problems.
- Move every day, even gently. It helps energy, mood and weight. See how to boost energy naturally.
- Prioritise sleep. See how to sleep better.
- Get your vitamin D, iron and B12 checked if you’re still tired once your thyroid levels are right.
Some people with tiredness like an evening drink as part of their wind-down. Our Magnesium Glycinate Sleep Complex is a berry bedtime drink, and magnesium contributes to the reduction of tiredness and fatigue and to normal muscle function. It isn’t a treatment for thyroid problems. If you take levothyroxine, check with your pharmacist how far apart to take it from any supplement, including magnesium.
Living well with an underactive thyroid
Once your dose is right, most people feel much better and live completely normally. A few practical tips:
- Be patient. It can take a few months of dose changes to feel your best. Keep a simple symptom diary to share at check-ups.
- Be consistent. Take levothyroxine the same way every day. A pill organiser and phone reminder help.
- Tell your GP about changes, such as pregnancy, new medicines, big weight changes or new symptoms.
- Don’t stop or change your dose without speaking to your GP.
- Look after your mood. If you still feel low once your levels are normal, ask about other causes and support.
- Connect with others. The British Thyroid Foundation and Thyroid UK have support groups and reliable information.
Risks if an underactive thyroid isn’t treated
An underactive thyroid isn’t usually serious if it’s treated properly. Left untreated, or on the wrong dose, the NHS says it can increase the risk of heart problems, such as coronary heart disease and heart failure, and bone problems such as osteoporosis. In pregnancy, an untreated underactive thyroid can raise the risk of complications. Very rarely, severe untreated hypothyroidism can cause a medical emergency called myxoedema coma, which is more common in older people.
When to see a GP
See a GP if you think you might have an underactive thyroid, especially if you’ve had tiredness, feeling cold, weight gain and low mood for a few weeks or more, or if thyroid problems run in your family. If you’re already on levothyroxine and still feel unwell, ask for a review of your dose. The British Thyroid Foundation has helpful patient information and support.
FAQs
What are the first signs of an underactive thyroid?
Often tiredness, feeling cold, weight gain, constipation and low mood. Symptoms usually come on slowly, so they’re easy to miss.
How is an underactive thyroid diagnosed?
With a blood test for TSH and T4. A high TSH with a low T4 usually means hypothyroidism.
Can an underactive thyroid cause weight gain?
Yes, it can cause some weight gain, partly from fluid. Treatment usually helps, alongside a balanced diet and activity.
Can an underactive thyroid cause anxiety or depression?
Low mood and depression are common symptoms. If you feel low, ask your GP to check your thyroid as well as your mood.
Is an underactive thyroid the same as Hashimoto’s?
Hashimoto’s disease is the most common cause of an underactive thyroid in the UK. It’s an autoimmune condition where the immune system attacks the thyroid.
Can supplements fix an underactive thyroid?
No. Levothyroxine is the treatment. Some supplements, such as biotin, can affect thyroid tests, and calcium and iron can affect levothyroxine, so check with your GP or pharmacist.
Sources
- NHS. Underactive thyroid (hypothyroidism).
- NHS. Levothyroxine.
- NICE (2019). Thyroid disease: assessment and management (NG145).
- Garmendia Madariaga A et al (2014). The incidence and prevalence of thyroid dysfunction in Europe: a meta-analysis. Journal of Clinical Endocrinology and Metabolism.
- British Thyroid Foundation. btf-thyroid.org.
This guide is general information, not medical advice. Evopure makes the Magnesium Glycinate Sleep Complex.
