Hyperthyroidism: Turning Down an Overactive Gland
If an underactive thyroid slows everything down, an overactive one speeds everything up — a racing heart, weight loss despite hunger, heat, tremor and anxiety. There are three quite different ways to bring it under control: block the gland with a drug, calm the symptoms while you wait, or destroy the overactive tissue for good. Knowing which does what — and the two rare but deadly emergencies to watch for — is the whole subject.
Hyperthyroidism is the thyroid stuck in overdrive. Too much thyroid hormone winds the whole body up: a fast, sometimes irregular heartbeat, weight loss despite a good appetite, feeling hot and sweaty, tremor, anxiety, restlessness and difficulty sleeping. The commonest cause is Graves' disease, an autoimmune condition in which an antibody mimics TSH and constantly drives the gland — often with a visible goitre and bulging eyes. On the blood test, it's the mirror image of the underactive gland: thyroid hormone is HIGH and, by feedback, TSH is suppressed LOW. The treatment falls into three strategies, and it helps to see them as: block it, calm it, or remove it.
Block it: antithyroid drugs
Carbimazole and propylthiouracil stop the gland making hormone. The antithyroid drugs block the gland's ability to manufacture thyroid hormone. The main drug is carbimazole (which the body converts to its active form, methimazole); the alternative is propylthiouracil (PTU). Because the thyroid stores a reserve of pre-made hormone, these drugs take several weeks to bring levels down — patience is required. Two safety points dominate. First, the rare but serious side effect of carbimazole and PTU is agranulocytosis — a sudden dangerous drop in white blood cells that leaves the patient unable to fight infection. So every patient is warned: if you develop a sore throat, fever or mouth ulcers, stop the drug and get an urgent blood count. Second, the choice between the two shifts in pregnancy: carbimazole is avoided in the first trimester (risk of birth defects), so PTU is preferred early in pregnancy, while carbimazole is generally preferred otherwise (PTU carries a rare risk of severe liver injury). Drugs can be used long-term hoping for remission, or as preparation before one of the definitive treatments.
Calm it, remove it — and the emergencies
While the antithyroid drug slowly takes effect, a beta-blocker (like propranolol) is often added to calm the adrenaline-driven symptoms fast — the racing heart, tremor and anxiety — a neat crossover from the cardiovascular chapter. It doesn't treat the thyroid itself, but it makes the patient comfortable within hours. For a permanent cure, there are two definitive options. Radioactive iodine is elegant: the thyroid greedily takes up iodine to make its hormone, so giving a dose of radioactive iodine concentrates it in the gland and destroys the overactive tissue from within, with little effect elsewhere — though it's avoided in pregnancy and often leaves the patient hypothyroid afterwards (then simply treated with levothyroxine). Surgery to remove the gland (thyroidectomy) is the other definitive route. Finally, two emergencies to respect: a thyroid storm is a rare, life-threatening surge of severe hyperthyroidism (high fever, racing/irregular heart, agitation) needing intensive treatment; and, from the antithyroid drugs, the agranulocytosis already mentioned. Recognise the sore-throat warning and the thyroid storm, and you've covered the dangerous end of this topic.
- Hyperthyroidism speeds the body up; high thyroid hormone with a LOW TSH. Graves' is commonest.
- Antithyroid drugs (carbimazole, PTU) block hormone synthesis — take weeks to work.
- Warn about agranulocytosis: sore throat/fever/mouth ulcers → stop drug, urgent blood count.
- Beta-blocker (propranolol) rapidly calms adrenaline symptoms while the drug takes effect.
- Definitive: radioactive iodine (often → hypothyroid) or surgery; PTU preferred in early pregnancy.
Notice how the two thyroid diseases mirror each other perfectly, right down to the treatment logic. Underactive: hormone low, TSH high, and you REPLACE the hormone. Overactive: hormone high, TSH low, and you BLOCK or REMOVE the source. And there's a satisfying loop back to earlier chapters: the fast, frightening symptoms of an overactive thyroid — pounding heart, tremor, anxiety — are essentially an adrenaline overdrive, which is exactly why a beta-blocker settles them so quickly. It also explains a subtle danger elsewhere: because the thyroid sets the body's metabolic rate, thyroid status changes how the heart behaves and how other drugs are handled — the gland really is the whole body's speed dial.
- Not warning about agranulocytosis — a sore throat on carbimazole/PTU is an emergency.
- Using carbimazole in the first trimester — prefer PTU early in pregnancy.
- Expecting antithyroid drugs to work instantly — the stored hormone means weeks of delay.
- Giving radioactive iodine in pregnancy — contraindicated (destroys fetal thyroid too).
A patient on carbimazole develops a sore throat and fever. What should they do?
- Hyperthyroidism = high thyroid hormone, low TSH; commonest cause is Graves' disease.
- Block it: carbimazole/PTU (weeks to work); warn re agranulocytosis (sore throat → stop, blood count).
- Calm it: a beta-blocker settles the adrenaline symptoms fast.
- Remove it: radioactive iodine (avoid in pregnancy; often → hypothyroid) or surgery. Beware thyroid storm.
- Katzung BG. Basic & Clinical Pharmacology — Thyroid & Antithyroid Drugs.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Antithyroid drugs.
- ATA — Guidelines for the management of hyperthyroidism.
- Whalen K. Lippincott Illustrated Reviews: Pharmacology — Antithyroid drugs.
