Thyrotoxicosis: The Evidence Behind the Pathway

A 32-year-old woman comes to the medicine OPD with three weeks of palpitations, tremor and weight loss. Her heart rate is 112/min. Her neck hurts, and the front of it is tender to touch. She had a sore throat and a cold about a month ago. Her TSH is suppressed, below 0.1 mIU/L, and her free T4 and T3 are both high. The intern has already written carbimazole on the prescription. This is an illustrative case, not a real patient.

By the end of this article you will know why that prescription should wait, and which three findings tell you so.

1. Two Questions, in This Order

Thyrotoxicosis means too much thyroid hormone acting on the tissues, whatever the cause. Hyperthyroidism is narrower: the gland itself is making too much. The tool asks two questions, one after the other. Is this thyrotoxicosis? And if it is, what is causing it?

The first question is answered by the blood test. The second is where the work is. In the tool’s own words, destructive thyroiditis mistaken for Graves’ disease earns the patient an anti-thyroid drug (ATD) that cannot work on it, for as long as the mistake stands.

“Her T4 is high. Isn’t that enough?” Check which T4 it is first. Total T4 and total T3 measure the hormone bound to its carrier protein as well as the free hormone. Thyroid binding globulin (TBG) rises in pregnancy, with an oral contraceptive and in acute hepatitis. Then the total hormone reads high in a patient who is not thyrotoxic at all. Where the free and the total disagree, decide on the free hormone. Our patient’s values are free hormones, and they are high. She is thyrotoxic.

2. Making Hormone, or Leaking It?

There are two ways for a thyroid to flood the blood with hormone. It can make too much, as in Graves’ disease or a toxic nodule. Or it can be damaged, and leak the hormone it has already made and stored, as in thyroiditis. Think of a tank. In the first case the tap is stuck open. In the second the tank has cracked.

This matters because of how the anti-thyroid drugs work. Methimazole, carbimazole and propylthiouracil (PTU) block the making of new hormone. They turn the tap off. They do nothing for a cracked tank. In subacute thyroiditis, silent thyroiditis and the toxic phase of Hashimoto’s thyroiditis (Hashitoxicosis), the hormone is already made, so there is nothing for the drug to block.

3. Three Causes, Four Clues

Beside the blood test, the tool uses three clues to sort the common causes. They are the examination and history, the TSH receptor antibody (TRAb), and the radioactive iodine uptake (RAIU) scan. In the tool it is shown as two narrower tables; here it is as one. Read across each row:

TRAb is the antibody that stimulates the TSH receptor in Graves’ disease. A positive result means the gland is being driven to make hormone. The uptake scan shows how much iodine the gland is taking up to make new hormone. A gland that is making hormone takes up a lot. A gland that is leaking takes up almost none.

“Do I need all three clues every time?” Not always. A diffuse goitre with eye signs points strongly to Graves’ disease. The tool then asks for a TRAb, or an uptake scan, to confirm it. And in India, a diffuse, smooth goitre in a younger patient is Graves’ disease until it is proved otherwise. TRAb settles the question without radiation, and it can be done in pregnancy.

4. Back to Our Patient

Look at her row. A painful, tender goitre after a viral upper respiratory infection is the history of subacute thyroiditis. Her TRAb comes back negative. Her uptake scan shows almost no uptake. All three clues say the same thing: the tank has cracked.

So the carbimazole is struck off. The tool prints a contraindication: do not use anti-thyroid drugs, because they block synthesis and the release here is already under way. Instead she gets propranolol for the symptoms. Her heart rate is above 100 and she has lost weight, so the tool gives 40 mg four times a day. For the pain, a high-dose NSAID such as ibuprofen or naproxen; oral prednisolone is considered if the pain is severe and does not settle. Then she is watched closely. The tool warns that a hypothyroid phase usually follows on its own within 4 to 8 weeks.

5. Which Beta-Blocker?

Propranolol is the preferred beta-blocker in thyrotoxicosis. It is non-selective. At high doses, typically above 160 mg/day, it also does a second job. It blocks the 5′-deiodinase enzyme in the tissues, which turns T4 into the more active T3.

“And if she has asthma?” Then propranolol is the wrong drug, because it can cause severe bronchospasm. Use a cardioselective agent instead: the tool gives metoprolol 25 to 50 mg daily or atenolol 25 to 50 mg daily.

6. Which Anti-Thyroid Drug?

When the gland is making the hormone, an anti-thyroid drug works. Methimazole, or carbimazole, is first-line. It is taken once a day, and it carries a lower risk of fulminant hepatic necrosis than PTU. PTU is kept for two situations only. The first is the first trimester of pregnancy, because methimazole carries an embryopathy that includes aplasia cutis. The second is thyroid storm, where PTU blocks the conversion of T4 to T3 in the tissues as well as the making of hormone.

Two more points from the tool’s plan for Graves’ disease are worth carrying to the ward. Before the first dose, warn the patient. If a fever, a sore throat or jaundice appears, she stops the drug and has a white cell count and liver tests the same day. Agranulocytosis is rare and is not predicted by routine counts, which is why every patient is warned rather than watched. And when you review her at 4 to 6 weeks, titrate on the free T4 and T3, not on the TSH. The TSH stays suppressed for weeks after the hormones have come down, and chasing it leads to overtreatment.

The Common Mistake

The common mistake is the one our intern nearly made: to see a suppressed TSH and a high T4, and to reach for carbimazole. It is easy to see why. Graves’ disease is the commonest cause, and the drug is familiar. But the drug only helps a gland that is making hormone. Spend a minute on the neck, and wait for the TRAb or the scan, before you write it.

Try This

A 26-year-old woman has a diffuse, smooth goitre, a stare and mild proptosis. Her TSH is suppressed and her free T4 is high. TRAb is positive. She is not pregnant and has no asthma. What does the tool advise? Illustrative case.

Answer. This is Graves’ disease: the autoantibody is stimulating the gland to make hormone, so an anti-thyroid drug works. The tool gives methimazole (or carbimazole) 20 to 40 mg daily, with propranolol for the symptoms. Record a baseline full blood count and liver function tests. Warn her about fever, sore throat and jaundice. Reassess at 4 to 6 weeks on free T4 and T3. Refer her to ophthalmology for the eye disease, avoid radioactive iodine, which can worsen it, and ask about smoking.

Take-Home Points

• Confirm thyrotoxicosis on the free hormones, then find the cause: the cause decides the drug.
• Anti-thyroid drugs block the making of hormone; in thyroiditis the hormone is leaking, so they have nothing to block.
• The examination, TRAb and the uptake scan, read together, separate Graves’ disease, thyroiditis and the toxic nodule.
• Propranolol for symptoms; a cardioselective agent (metoprolol or atenolol) in asthma.
• Methimazole or carbimazole first; PTU only in the first trimester and in thyroid storm.

For the underactive gland, read Levothyroxine Dosing: The Evidence Behind the Tool. For the beta-blockers as a class, read Beta-Blockers: Clinical Pharmacology & Practice Pearls.

Declaration: the author built and maintains the tool. It is free to use. The figures in this article are those the tool carried on 5 November 2026, in its Evidence tab and in the plan it prints, each from the sources the tool names. The illustrative cases follow the tool’s rules; check the tool’s result for a real patient.

References

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