Block and Replace easier during coronavirus (COVID-19)

Block and replace regimen for hyperthyroidism, needs fewer blood tests than titration.



Many of us are used to the titration regimen of Carbimazole or PTU for hyperthyroidism, but this relies on regular monitoring of thyroid function. Block and replace outcomes are no different in the long run but require fewer blood tests. If we lose access to blood tests for thyroid function tests during the next few months due to Covid, the use of block and replace should be considered.

The cost of carbimazole means that long term block and replace (£685) is more expensive than low dose carbimazole alone (£78 if on 5mg daily). However this difference in cost only applies to licenced carbimazole in the UK. It is cheaper in many other countries.

Patients who are stable on low dose carbimazole (5 to 10mg) who have been stable for some months do NOT need to change. Any change will probably result in more blood tests. However NEW patients will need many fewer blood tests with block and replace than with titration in the long term.

The standard protocol for Graves disease is to start carbimazole 40mg once daily in all patients who are hyperthyroid. The rate of fall is variable, but as a best guess, patients will be euthyroid or hypothyroid in 4 to 8 weeks.

One blood test at this point confirming hypothyroidism is very useful. Once this has occurred, add thyroxine 100mcg daily (best guess for most patients at around 6 weeks after starting the carbimazole).

From this point on, treat the patient as someone who is hypothyroid (always stay on at least carbimazole 40mg daily, and never change this dose). If you can get blood tests, a test 6 weeks after start of the thyroxine will guide your change in replacement dose. Do NOT change the dose of carbimazole.

If you are able to get blood tests, use them to modify the dose of thyroxine only, as you would for any other patient with hypothyroidism.

If the patient is biochemically hyperthyroid then reduce the thyroxine. Increasing the carbimazole is unlikely to be helpful, as the thyroid should be fully blocked, and they just need a small reduction in the thyroxine dose.

If the patient is biochemically hypothyroid, then give more thyroxine. Do NOT reduce the carbimazole. Doctors who are familiar with the titration regimen will need to recognise that the dose of carbimazole is FIXED on the block and replace regimen, and the commonest error is to try and mix and match the titration regimen with the block and replace one.

Patients who are currently on a titration regimen who are unstable, and who are on less than 40mg carbimazole can switch to block and replace by increasing the dose of carbimazole to 40mg and then adding 100mcg thyroxine.

UK costs of anti thyroid drugs.

Source: BNF, Date, August 2019.141414(BMJ Group and the Royal Pharmaceutical Society of Great Britain)

(a) Maintenance dose reported, initial dose of 200-400 mg daily in divided doses until patient becomes euthyroid.

(b) Maintenance dose reported based on 5mg tablets, initial dose of 15-40 mg daily in divided doses until patient becomes euthyroid, usually 4 to 8 weeks.

(c) Blocking- replacement regimen; combination of carbimazole 40- 60 mg daily, with levothyroxine 100μg daily, usually given for 18 months.

(d) Cost of carbimazole 40mg (2 * 20mg tablets) and levothyroxine 100μg; £55.97 + £1.03

(e) Cost of carbimazole 40mg (2 * 20mg tablets) and levothyroxine 100μg; £671.67 + £13.43