Neo-Mercazole compared: methimazole, propylthiouracil, radioactive iodine and surgery
How Neo-Mercazole (carbimazole) compares with methimazole (thiamazole) and propylthiouracil (PTU), and how antithyroid tablets compare with radioactive iodine and thyroid surgery.
Two different comparisons matter. First, which antithyroid tablet: carbimazole (Neo-Mercazole), methimazole or propylthiouracil. Second, whether to rely on tablets at all, or choose radioactive iodine or surgery. Your endocrinologist will weigh these with you; this page lays out what the evidence says.
Key points
- Carbimazole and methimazole are effectively the same drug. Carbimazole turns into methimazole; about 10 mg of carbimazole matches 6 mg of methimazole.
- PTU is a second choice because of rare severe liver injury, except in the first trimester of pregnancy, when it is usually preferred.
- Tablets keep the thyroid but about half of people with Graves’ disease relapse after a standard course.
- Radioactive iodine and surgery relapse far less but usually leave you on lifelong levothyroxine; radioiodine can worsen thyroid eye disease.
Carbimazole vs methimazole vs PTU
| Carbimazole (Neo-Mercazole) | Methimazole (thiamazole) | Propylthiouracil (PTU) | |
|---|---|---|---|
| What it is | Prodrug, converted to methimazole | The active drug | A different antithyroid drug |
| Where you meet it | UK, France, Australia, New Zealand, India | US; Germany and Austria as thiamazole; not licensed in the UK | Most markets, as a second line |
| Typical adult start | 20 to 60 mg a day | 15 to 60 mg a day | 300 mg a day |
| Typical maintenance | 10 to 15 mg a day | 5 to 15 mg a day | 100 to 150 mg a day |
| Doses per day | Often once daily once stable | Often once daily once stable | Usually three |
| Rough equivalence | 5 mg | about 3 mg | 50 mg |
| Main serious risks | Agranulocytosis, liver injury (mostly cholestatic), pancreatitis | Same as carbimazole | Agranulocytosis, severe liver failure, ANCA vasculitis |
| First trimester of pregnancy | Usually avoided | Usually avoided | Usually preferred |
Dose ranges come from the New Zealand Neo-Mercazole data sheet and the US methimazole and PTU labels. The PTU equivalence (50 mg PTU to 5 mg carbimazole) is from the Neo-Mercazole data sheet; the carbimazole to methimazole ratio is from a pharmacokinetic study that found the two equipotent molecule for molecule.
Neomercazole or methimazole: which should I take?
The dose conversion table and a step-by-step guide to switching are on carbimazole vs methimazole.
Clinically they are interchangeable: the same active compound reaches your thyroid. Which one you get mostly depends on what is licensed where you live: the UK licenses carbimazole but not methimazole, while in the US methimazole is the standard tablet and carbimazole is not sold. If you move between countries or switch brands, your doctor converts the dose and checks your blood tests after the change.
Why PTU is second line
PTU works, but in a Japanese randomised trial methimazole 30 mg a day normalised free T4 in more people by 12 weeks (96.5%) than PTU 300 mg a day (78.3%), and the authors did not recommend PTU for first use. The US PTU label carries a boxed warning about severe liver injury and acute liver failure, sometimes fatal and sometimes needing a transplant, and says PTU should be reserved for people who cannot take methimazole and for whom radioiodine or surgery are not suitable. The exception is early pregnancy: the European Thyroid Association advises switching to PTU when planning pregnancy and during the first trimester, because carbimazole and methimazole carry a higher risk of certain birth defects.
Cross-reactions
The Neo-Mercazole data sheet warns of possible cross-allergy between carbimazole, methimazole and PTU. Someone who had a serious reaction such as agranulocytosis to one should not be given another antithyroid drug. A minor rash is different and is sometimes managed with an antihistamine or a switch.
Tablets vs radioactive iodine vs surgery
| Antithyroid tablets | Radioactive iodine | Thyroidectomy | |
|---|---|---|---|
| How it works | Blocks hormone production while the immune process may settle | Destroys thyroid tissue over weeks to months | Removes the gland |
| Relapse (network meta-analysis) | 52.7% | 15% | 10% |
| Usual long-term result | Normal thyroid if remission lasts | Underactive thyroid, lifelong levothyroxine | Underactive thyroid, lifelong levothyroxine |
| Eye disease | Neutral | Can trigger or worsen it | Neutral |
| Main risks | Side effects of the drug; relapse | Eye disease; avoid in pregnancy and breastfeeding | Surgical risks to voice and calcium control |
Relapse figures are from a 2013 network meta-analysis of eight comparative studies (1,402 patients). In a Swedish randomised trial, relapse was 34% to 42% after 18 months of tablets, 21% after radioiodine and 3% to 8% after surgery, and 90% of patients in every group were satisfied with their treatment. In an Italian trial, eye disease appeared or worsened in 15% of people after radioiodine, against 3% on methimazole; a short course of prednisone after radioiodine prevented it.
Who tends to choose which
- Tablets first suits most newly diagnosed Graves’ disease, people with active eye disease, and anyone who wants to keep a working thyroid and is willing to take medicine for 12 to 18 months or longer.
- Radioactive iodine suits people who relapsed after tablets or who prefer a one-off treatment and accept lifelong levothyroxine. It is not used in pregnancy or with active moderate to severe eye disease.
- Surgery suits large goitres pressing on the neck, suspicious nodules, active eye disease when definitive treatment is needed, or people planning pregnancy soon.
For toxic nodular goitre, tablets control levels but rarely give lasting remission, so radioiodine or surgery are usually the definitive options.
Where to find each medicine
Carbimazole, methimazole and PTU are prescription medicines. With a prescription, carbimazole is available online from PockDose, Yasabai and VGR Australia, and methimazole from PockDose and Yasabai. See where to fill a prescription for pharmacies in other countries.
Sources
Checked 15 October 2026.
- Neo-Mercazole New Zealand data sheet — Medsafe (New Zealand)
- Methimazole tablets prescribing information — DailyMed, US National Library of Medicine
- Propylthiouracil tablets prescribing information (boxed warning) — DailyMed, US National Library of Medicine
- Comparative bioavailability of carbimazole and methimazole (Jansson et al., 1983) — Int J Clin Pharmacol Ther Toxicol
- Comparison of methimazole and propylthiouracil in Graves' disease (Nakamura et al., 2007) — Journal of Clinical Endocrinology & Metabolism
- Comparative effectiveness of therapies for Graves' hyperthyroidism (Sundaresh et al., 2013) — Journal of Clinical Endocrinology & Metabolism
- Graves' hyperthyroidism: antithyroid drugs, surgery or radioiodine, a randomized study (Törring et al., 1996) — Journal of Clinical Endocrinology & Metabolism
- Relation between therapy for hyperthyroidism and the course of Graves' ophthalmopathy (Bartalena et al., 1998) — New England Journal of Medicine
- 2018 European Thyroid Association guideline for the management of Graves' hyperthyroidism — European Thyroid Journal
- 2016 American Thyroid Association guidelines for hyperthyroidism and other causes of thyrotoxicosis — Thyroid