The key difference between carbimazole and methimazole is that carbimazole is an inactive form of drug, whereas methimazole is an active form of drug.
Carbimazole and methimazole are very important drug forms that are used in treating conditions related to the thyroid gland. When we take carbimazole drugs for the treatment of hypothyroidism, it converts into its active form methimazole inside the body.
CONTENTS
1. Overview and Key Difference
2. What is Carbimazole
3. What is Methimazole
4. Carbimazole vs Methimazole in Tabular Form
5. Summary – Carbimazole vs Methimazole
What is Carbimazole?
Carbimazole is a prodrug that is useful in treating hyperthyroidism. After absorption, this drug is converted into the active form (which is why it is categorized as a prodrug). The active form of carbimazole is methimazole. This active form, methimazole, can prevent thyroid peroxidase enzyme from iodinating and coupling the tyrosine residues on thyroglobulin. Therefore, it can reduce the production of the thyroid hormones known as T3 and T4 (thyroxine). This prodrug comes under the World Health Organization’s List of Essential Medicines.
Figure 01: Carbimazole
Hyperthyroidism is a condition caused by the excessive production of thyroid hormones by the thyroid gland. Medical therapy for this condition usually includes either titrating the dose of carbimazole until we can observe the euthyroid condition in the patient or maintaining a high dose of carbimazole for the suppression of endogenous thyroid production, followed by replacing the thyroid hormone with levothyroxine. This process is named as “block and replace” mechanism. Usually, this treatment is performed for about 18-24 months.
However, there can be some side effects as well. Common side effects are rashes and pruritus. We can treat these conditions using antihistamines while taking carbimazole continuously. Moreover, for sensitive patients, we can use propylthiouracil as a substitute.
What is Methimazole?
Methimazole or thiamazole is a medication useful in treating hyperthyroidism. It can treat Graves disease, toxic multinodular goiter, and thyrotoxic crisis. It has an oral route of administration. The maximum effect can be obtained after a week of administration.
However, there can be some side effects of using this drug, which include itchiness, hair loss, nausea, muscle pain, swelling, and abdominal pain. There can be some severe side effects as well, including low blood cell counts, liver failure, and vasculitis. Therefore, it is not recommended to use during the first trimester of pregnancy. But if unavoidable, we can use it in the second trimester of the pregnancy or later, as well as during breastfeeding.
Figure 02: Methimazole
The bioavailability of methimazole is about 93%. Its protein binding is negligible, and the metabolism of the drug occurs in the liver. Moreover, the elimination half-life is about 5-6 hours, and the excretion occurs in the kidney. The chemical formula of this methimazole is C4H6N2S, and the molar mass is about 114.17 g/mol. The melting point is about 146 degrees Celsius, and it is slightly soluble in water.
When considering the mechanism of action of methimazole, it can inhibit the enzyme thyroperoxidase that can act in thyroid hormone synthesis via oxidizing the anion iodide to iodine, hypoiodous acid, and enzyme-linked hypoiodate. This facilitates the addition of tyrosine residues to the hormone precursor thyroglobulin. It is an essential step in the synthesis of triiodothyronine and thyroxine.
What is the Difference Between Carbimazole and Methimazole?
Carbimazole and methimazole are very important drug forms that are used in treating conditions related to the thyroid gland. The key difference between carbimazole and methimazole is that carbimazole is an inactive form of a drug, whereas methimazole is an active form of a drug.
The below infographic presents the differences between carbimazole and methimazole in tabular form for side-by-side comparison.
Summary – Carbimazole vs Methimazole
Carbimazole is a prodrug that is useful in treating hyperthyroidism. Methimazole or thiamazole is a medication useful in treating hyperthyroidism. The key difference between carbimazole and methimazole is that carbimazole is an inactive form of drug, whereas methimazole is an active form of drug.
Reference:
1. “Carbimazole.” Uses, Interactions, Mechanism of Action | DrugBank Online.
Image Courtesy:
1. “Carbimazole” By Fvasconcellos – Own work (Public Domain) via Commons Wikimedia
2. “Methimazole” By Fvasconcellos – self-made by Fvasconcellos. (Public Domain) via Commons Wikimedia