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Health condition

Underactive thyroid patient guide

An underactive thyroid (hypothyroidism) means the thyroid gland does not make enough hormone, slowing the body down and causing fatigue, weight gain and low mood.

Key points

What matters most, at a glance.

The essentials in plain English - symptoms, when to seek help, who to see and what treatment looks like.

  • Common symptoms: tiredness, weight gain, feeling cold, dry skin, constipation and low mood

  • See your GP for a blood test (TSH and free T4) if symptoms persist for more than a few weeks

  • Managed by your GP, with referral to an endocrinologist for complex or unstable cases

  • Standard treatment is daily levothyroxine, adjusted using regular blood tests

  • Untreated hypothyroidism can raise cholesterol and affect heart health, so diagnosis matters

Understanding it

About Underactive thyroid patient guide, and what to do next.

An underactive thyroid develops when the thyroid gland in the neck produces too little thyroxine, most often because of autoimmune damage (Hashimoto’s thyroiditis). Because thyroid hormone sets the pace of almost every organ, symptoms are wide-ranging: persistent tiredness, unexplained weight gain, sensitivity to cold, dry skin and hair, constipation, brain fog and low mood. Symptoms usually build slowly, which is why many people put them down to age or stress.

Diagnosis is a simple blood test measuring TSH and free T4, and treatment is daily levothyroxine with doses fine-tuned over a few months. Most people feel substantially better once levels are stable, although getting the dose right can take time. Most people start with their GP, who can refer to an endocrinologist. NHS routine referrals often take 20-45+ weeks depending on where you live, while a private consultation is typically available within about two weeks.

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, see them.

Frequently asked

Common questions about Underactive thyroid patient guide.

Quick, plain-English answers to what people ask us most.

  • What is an underactive thyroid (hypothyroidism)?

    It is a condition where the thyroid gland doesn't produce enough thyroid hormone, slowing down the body's metabolism and affecting many bodily functions.

  • What causes an underactive thyroid?

    The most common cause is autoimmune thyroiditis (Hashimoto's disease), where the immune system gradually damages the thyroid; other causes include previous thyroid surgery, radioactive iodine treatment, certain medications, and iodine deficiency.

  • What are the symptoms of an underactive thyroid?

    Fatigue, weight gain, feeling cold, dry skin, hair thinning, low mood, and constipation are common, often developing gradually over months.

  • Is an underactive thyroid dangerous?

    If left untreated for a long time, severe hypothyroidism can lead to serious complications including heart problems and, very rarely, a life-threatening condition called myxoedema coma, but it is easily managed once diagnosed and treated.

  • Is an underactive thyroid genetic?

    There is a genetic predisposition to autoimmune thyroid disease, and it often runs in families alongside other autoimmune conditions, though environmental factors also play a role.

  • Can an underactive thyroid be cured?

    Autoimmune hypothyroidism is usually a lifelong condition managed with daily thyroid hormone replacement (levothyroxine) rather than cured, though some causes, like certain medication-related hypothyroidism, may resolve if the trigger is removed.

  • How is an underactive thyroid treated?

    Treatment is with daily levothyroxine tablets to replace the missing thyroid hormone, with the dose adjusted based on regular blood tests until levels are stable.

  • How long does it take to feel better on treatment?

    Some symptoms improve within a few weeks of starting the correct dose, but it can take several months for energy levels, weight, and other symptoms to fully normalise, with dose adjustments made along the way.

  • Underactive vs overactive thyroid - what's the difference?

    An underactive thyroid (hypothyroidism) slows metabolism causing fatigue and weight gain, while an overactive thyroid (hyperthyroidism) speeds up metabolism causing weight loss, rapid heartbeat and anxiety; blood tests clearly distinguish between the two.

  • When should I see a doctor about thyroid symptoms, and how quickly can I be seen privately?

    See a doctor if you have persistent fatigue, weight gain or other symptoms of an underactive thyroid. NHS endocrinology referral can take time, though GPs can often start initial testing quickly; private endocrinologists can often see you within about two weeks.