Hyperthyroidism
An overactive thyroid speeds up nearly every system in the body. Most cases are managed well, but two warning signs on treatment need same-day attention.
Medically reviewed by the RIIMS medical team · Last updated: June 2026
Go to hospital now
- Fever or sore throat while taking anti-thyroid medicine (possible agranulocytosis), needing an urgent white blood cell count the same day
- High fever, a racing heart and confusion or agitation together (possible thyroid storm)
- Chest pain or severe breathlessness
See a doctor soon (not an emergency)
- A new or worsening tremor, weight loss or heat intolerance
- An irregular heartbeat noticed for the first time
- Eye symptoms, such as bulging, redness or double vision, in someone with known thyroid disease
What is hyperthyroidism, and why does fever need urgent attention on treatment?
Hyperthyroidism means the thyroid gland is making too much hormone, speeding up the body's metabolism. Graves' disease, an autoimmune condition, is responsible for roughly 60 to 80% of cases and is the most common cause overall; other causes include toxic nodules and thyroiditis. Symptoms can be dramatic, a racing heart, weight loss despite a good appetite, tremor, heat intolerance, but they can also be mild, especially in older adults, where the main clue may simply be an irregular heartbeat or unexplained weight loss. Treatment usually starts with anti-thyroid medicine, and here one safety point matters more than any other: on these drugs, fever or a sore throat needs an urgent blood test to check the white cell count, not a wait-and-see approach. This is because anti-thyroid drugs carry a rare but serious risk called agranulocytosis, a sudden drop in the white blood cells that fight infection, occurring in roughly 0.3 to 0.6% of people who take them, and it can develop within days. The two most common symptoms of this complication are fever and a sore throat, and getting an urgent blood count checked and contacting your prescribing doctor immediately is the safe response, not waiting to see if it passes. At the far end of severity, poorly controlled hyperthyroidism can tip into thyroid storm, a medical emergency.
Symptoms to watch for
- Rapid or irregular heartbeat
- Unexplained weight loss despite normal or increased appetite
- Tremor or shakiness in the hands
- Heat intolerance and increased sweating
- Anxiety, irritability or trouble sleeping
- A visibly enlarged thyroid (goitre) or bulging eyes, in some cases
When to consult a doctor
A racing heart, unexplained weight loss, tremor or heat intolerance deserve a thyroid blood test. If you are taking anti-thyroid medicine and develop fever or a sore throat, get an urgent blood count check the same day and contact your prescribing doctor immediately.