What does Carbimazole do?
Carbimazole is an anti-thyroid medicine. It prevents the thyroid forming and releasing thyroid hormone. By doing that, it effectively treats someone suffering from hyperthyroidism.
It starts to act almost immediately - within an hour or two of taking a dose. However, it takes time, usually several weeks, for the excess thyroid hormone that is already in the body (e.g. in the bloodstream) to fall. For this period, Carbimazole dose has to be sufficient to reduce the forming and release of thyroid hormone to near zero. Even a fairly low rate of release can keep you hyperthyroid much longer.
The more severe the hyperthyroidism, the more thyroid hormone you will already have in your body, the longer it is likely to take. During this period, it might not appear to be making much difference.
You should be tested to ensure that the thyroid hormone levels are dropping. If the levels are not dropping, a higher dose might be required.
Long-Term Strategies
Once the excess of thyroid hormone has reduced, a lower dose of Carbimazole may be considered. The ideal would be just sufficient to prevent the thyroid releasing too much thyroid hormone, but maintaining adequate levels in the bloodstream.
Adjusting this dose is difficult but many do manage to find a suitable dose. If that approach doesn’t ensure stability, then it is possible to use a 'block and replace' approach. That is, continue taking sufficient Carbimazole to prevent the release of any new thyroid hormone at all. And take levothyroxine tablets as the sole source of thyroid hormone.
Some endocrinologists try an approach somewhere between the two - taking a dose of Carbimazole which does not quite suppress thyroid hormone release, and make that dose up with levothyroxine.
Crossing the Line
When the level of thyroid hormone in the bloodstream drops, there is a point at which it is probably just about right, followed almost inevitably by becoming hypothyroid. Crossing the line from hyperthyroid to hypothyroid can be very sudden and severe.
Some patients do not appreciate this is likely to happen and simply carry on with the same dose of Carbimazole. They are likely to become more and more hypothyroid.
What should happen is that as soon as they think they are becoming hypothyroid, they should have a blood test. And there should be an adjustment to their dose of Carbimazole. If following a 'block and replace' approach, then a full review is essential.
It seems entirely sensible for the patient to have a blood test request issued when they are prescribed Carbimazole. Ready for the day when they feel they have reached this point. (Changes from paper requests to online handing of blood tests might make this difficult or not necessary. But their notes should indicate that the patient can ring and make an appointment without question.)
That is, you should be able to go and get a test the moment you think it is needed. And (hopefully) the test results will often be ready before your appointment with any doctor.
Dosing
Patients are prescribed a dose of Carbimazole which the prescriber hopes is the lowest dose that will suppress thyroid hormone release. But this is difficult to judge. Too low a dose can leave the patient hypothyroid for a longer time, but in someone only a little hyperthyroid, this might be preferred to prescribing a high dose.
In someone who is severely hyperthyroid, there seems little reason not to go straight to a moderately high dose - typically 30 or 40 milligrams.
Testing is required so that it is clear the dose is sufficient that thyroid hormone levels are actually dropping. If not, the dose can be increased to 60 milligrams and, in extreme, even higher.
Split Dosing
A common issue with people being prescribed Carbimazole is knowing whether they should take it all in one dose. Or split it into two or three doses.
Not only can you split your dose, many would say you absolutely SHOULD split it.
Carbimazole starts to work very soon after taking the tablet(s). It rises rapidly to a peak and then falls quite quickly. By 12 hours, a lot of it has gone. By 24 hours, only a small amount remains.
Its half-life is of the order of 6 hours. If you take 10 milligrams, then 6 hours later you have 5 remaining, at 12 hours, you have 2.5, at 18 there is 1.25, and a full 24 hours later, just 0.58. Effectively, negligible. (This is theoretical. Real world times/remaining could be higher or lower.)
By taking it as a single dose, your thyroid can "escape" the effect of the Carbimazole and release more thyroid hormone. By splitting, you maintain a more even level.
Patients are sometimes advised to split dose when they start Carbimazole, especially if they are severely hyperthyroid, as the effects of escaping its control are particularly significant.
But this possibility is also often not mentioned not mentioned at all.
Even if it is mentioned, prescribers sometimes provide the Carbimazole in inappropriate dosages. In the UK, 20 and 5 milligram tablets are usual. (10 and 15 milligram tablets have been available. But they end up being far, far more expensive.) You should not usually be expected to split individual tablets. If you are on 20 milligrams a day, in two doses, you should get 5 milligram tablets and take two of them at a time. Not 20 milligram tablets and have to split them. If you split dose, your prescriber should write this in your notes and ensure it is stated as the dosing instructions
Not splitting your dose will maintain the amount of Carbimazole at between about 10.6 and 0.625.
Splitting your dose will maintain your effective level at between about 6.6 and 1.66. (Takes two or three days to settle there.) Clearly, the highest level is much lower. But the lowest level is much higher.
In order to illustrate the effect of splitting, I have produced a theoretical chart. In this, the green bars show the level of Carbimazole from a 10 milligram dose taken once a day. And the blue bars show the level of Carbimazole from a 5 milligram dose taken twice a day.
You can see that not splitting results in a much higher peak, but it also has a far wider range. In particular, it has a lower trough. The problem is when the blood level drops to the point at which it fails to keep release of thyroid hormone under control. Splitting maintains your effective dose in a much tighter, more even range.
When the dose is tapered, it is common to end up on as little as 5 or 10 a day and that can (sometimes) be taken in one dose. But experience is king.
If you have been taking Carbimazole in one dose and now switch to split dosing, that might have a similar effect to a dose increase.
If you split dose, you need to remember to take the second dose. Obvious but depending on what sort of person you are, you might need to set an alarm, or a calendar reminder, to make sure you take it at the right time.
Time of day does not matter. Midday and midnight being just as good as 06:00 and 18:00! Choose times that work for you and are 12 hours apart (or 8 hours if splitting in three).
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| Illustration of Carbimazole levels when taken in one dose (green) and two doses (blue). |
The peaks of Carbimazole are very brief and are unlikely to be significant. It is the lowest levels of Carbimazole, the troughs, that matter. If there is always sufficient to suppress release of thyroid hormone, that is sufficient. Having more can’t suppress it any more than totally suppressed!
A typical Patient Information Leaflet:
3. How to take Carbimazole
Always take this medicine exactly as your doctor or pharmacist has told you. Check with your doctor or pharmacist if you are not sure.https://mhraproducts4853.blob.core.windows.net/docs/0d9596325eff6bb5874fd5b16f3bfc88ba6d8883
• The tablets should be taken by mouth.
• You can take the tablets before, after or during meals.
• Each day’s tablets may be divided into two (morning and evening) or three daily doses (morning, noon and night).
Carbimazole Warnings
Every Patient Information Leaflet has detailed information which explains why taking Carbimazole puts you at risk of agranulocytosis, what to look out for and what to do. Please make sure you read - and re-read, this.
[i][b]helvella - Splitting Carbimazole Doses[/b]
A short discussion about Carbimazole primarily focussed on splitting doses but containing other information which may be helpful to anyone taking, or considering, Carbimazole.
Last updated 10/02/2025[/i]
Link to blog:
https://helvella.blogspot.com/p/helvella-splitting-carbimazole-doses.html
If you find anything incorrect, misleading, typos, links that don’t work, etc., please let me know. Go to my profile and use the contact details there.
