Hyperthyroidism: when your thyroid is overactive

What hyperthyroidism actually is

Where an underactive thyroid slows everything down, an overactive one speeds everything up. In hyperthyroidism the gland produces more thyroid hormone than the body needs, and the metabolism runs hot: the heart beats faster, the gut empties quicker, the body burns through fuel, and the nervous system sits permanently on alert.

It is frequently mistaken for anxiety - by patients, and sometimes for a long time by clinicians. The racing heart, the sweating, the sleeplessness, the sense of being unable to settle: these are the symptoms of a panic disorder and they are also the symptoms of a thyroid producing too much hormone. If you have been treated for anxiety and it has not helped, this is a reasonable thing to ask about. Anxiety, palpitations and being believed goes into this properly.

Why it happens

The most common cause in the UK is Graves' disease, an autoimmune condition where antibodies switch the thyroid on and keep it on. It is the only cause associated with thyroid eye disease, which is why eye symptoms point strongly towards it.

Other causes include a thyroid nodule - or several - producing hormone independently of the body's control system, and thyroiditis, where an inflamed thyroid leaks stored hormone. Thyroiditis matters because it behaves completely differently: it is usually temporary, it often follows a viral illness or pregnancy, and it typically resolves on its own, sometimes passing through an underactive phase on the way. Taking an antithyroid drug for it would be pointless, because the gland is not overproducing - it is leaking. Distinguishing the causes is why a scan or antibody test comes before treatment.

Taking more levothyroxine than your body needs produces the same biochemical picture, which is one reason doses are adjusted carefully and rechecked.

How it is diagnosed

The blood test picture is the mirror image of an underactive thyroid. TSH is low or undetectable - the pituitary has stopped asking for hormone because there is plenty - while free T4, free T3, or both are raised.

Working out the cause usually comes next. TRAb antibodies indicate Graves' disease. A radioactive iodine uptake scan can distinguish an overactive gland from a leaking one, and can show whether a single nodule is responsible. A thyroid ultrasound may be used to look at nodules. See thyroid antibodies explained.

Treatment: three routes

Unlike hypothyroidism, there is a genuine choice here, and it is worth understanding before you are asked to make it. All three routes are legitimate. Which suits you depends on the cause, your age, whether you have eye disease, whether you might become pregnant, and how you feel about the trade-offs.

Antithyroid medication. Carbimazole is the usual first choice in the UK, with propylthiouracil used in specific situations including the first trimester of pregnancy. Carbimazole carries a small risk of birth defects if taken in early pregnancy, so if you could become pregnant it is worth using reliable contraception while you are on it, and telling your specialist promptly if you are planning a pregnancy or think you might already be pregnant. These drugs stop the thyroid making hormone. A course typically runs for twelve to eighteen months, after which a proportion of people with Graves' stay well and a substantial proportion relapse. See antithyroid medication: the basics.

There is one safety point that matters more than any other, and it is not optional. Rarely, these drugs can cause a sudden collapse in white blood cells (agranulocytosis), which leaves you unable to fight infection. If you develop a sore throat, mouth ulcers, a fever, or flu-like illness while taking carbimazole or propylthiouracil, stop the drug, seek medical advice immediately - including out-of-hours or A&E if the surgery is closed - and ask for an urgent full blood count. Do not wait to see whether it passes.

Radioactive iodine. A drink or capsule of iodine-131, taken up by the thyroid, which destroys overactive tissue over weeks to months. It is effective and widely used. Most people become hypothyroid afterwards and need levothyroxine for life - this is an expected result, not a failure. It is avoided in pregnancy and breastfeeding, requires a period of contact restrictions afterwards, and can worsen thyroid eye disease, so it is used cautiously or avoided where eye disease is active. Radioactive iodine, explained covers it in full.

Surgery. Thyroidectomy removes part or all of the gland. It is the fastest route to a definitive answer and is chosen for large goitres, pressure symptoms, suspicion of cancer, active eye disease, or when the other options are unsuitable. It means lifelong levothyroxine if the whole gland is removed, and it carries the usual surgical risks plus two specific ones - to the voice and to the parathyroid glands. See thyroidectomy: preparing and recovering.

Beta blockers such as propranolol are often given alongside any of these, early on. They do not treat the thyroid at all - they blunt the symptoms, taking the edge off the racing heart and the tremor while the real treatment gets to work. Feeling better on one does not mean the underlying problem is being addressed.

The bit people find hardest

Treating an overactive thyroid means bringing the levels down, and coming down from hyperthyroid can feel worse before it feels better. Weight comes back on, often more than was lost. The wired energy - which some people, quietly, had come to rely on - goes. It is common to overshoot into an underactive state, and to feel the exhaustion and fog of hypothyroidism having only just escaped the opposite.

None of this means the treatment is failing. It usually means the dose needs adjusting, and it takes time. But it is a genuinely difficult stretch, and being told it is a sign of progress does not make it feel like one. Weight changes, without shame sits with this rather than dismissing it.

When not to wait

Untreated or undertreated hyperthyroidism strains the heart. Call 999 or go to A&E for chest pain, severe breathlessness, fainting, or a very fast heart rate that will not settle. For a persistently fast or irregular heartbeat without those red flags, contact your GP or NHS 111 promptly - don't leave it for a routine appointment.

Thyroid storm is a rare, life-threatening escalation - high fever, a very fast heart rate, agitation or confusion, vomiting, sometimes triggered by infection, surgery or stopping medication abruptly. It is a medical emergency. Call 999.

And, again, because it is the thing most worth memorising: sore throat, mouth ulcers or fever on carbimazole or propylthiouracil means stop the drug and get an urgent blood count the same day - out-of-hours or A&E if you have to. See when to seek urgent help.