Table of Contents
Introduction
Methimazole is an antithyroid drug used primarily in the treatment of hyperthyroidism, especially in conditions such as Graves’ disease. It belongs to the thionamide class and works by inhibiting the synthesis of thyroid hormones in the thyroid gland. It is preferred over propylthiouracil in most cases due to its longer duration of action and better safety profile.
Mechanism of Action (Step-wise)
- Methimazole accumulates in the thyroid gland.
- It inhibits the enzyme thyroid peroxidase (TPO).
- TPO normally catalyzes oxidation of iodide (I⁻) to iodine (I⁰).
- Methimazole blocks this oxidation step.
- It also inhibits iodination (organification) of tyrosine residues in thyroglobulin.
- This prevents formation of monoiodotyrosine (MIT) and diiodotyrosine (DIT).
- Methimazole further inhibits coupling of MIT and DIT to form T3 (triiodothyronine) and T4 (thyroxine).
- As a result, synthesis of thyroid hormones is reduced.
- It does not affect already formed or stored thyroid hormones, so onset of action is delayed.
A key exam point is that methimazole inhibits thyroid hormone synthesis by blocking thyroid peroxidase.
Pharmacokinetics
Methimazole is administered orally and is well absorbed from the gastrointestinal tract. It has a relatively long half-life, allowing once-daily dosing. It accumulates in the thyroid gland, which prolongs its effect beyond its plasma half-life. It is metabolized in the liver and excreted in urine. Compared to propylthiouracil, methimazole has a longer duration of action and better patient compliance.
Clinical Uses
Methimazole is used in the treatment of hyperthyroidism, particularly Graves’ disease. It is used for long-term management, preoperative preparation before thyroidectomy, and to control hyperthyroidism before radioactive iodine therapy. It is generally preferred over propylthiouracil except in the first trimester of pregnancy and in thyroid storm, where PTU is preferred.
Adverse Effects
Common adverse effects include rash, itching, and gastrointestinal discomfort. A serious but rare adverse effect is agranulocytosis, which can lead to life-threatening infections. Hepatotoxicity may also occur, though less commonly than with propylthiouracil. Other effects include hypothyroidism with excessive dosing. Patients should be advised to report symptoms such as sore throat or fever immediately.
Comparative Analysis
| Feature | Methimazole | Propylthiouracil (PTU) | Radioactive Iodine |
|---|---|---|---|
| Class | Thionamide | Thionamide | Radioisotope |
| Mechanism | Inhibits TPO | Inhibits TPO + peripheral T4→T3 | Destroys thyroid tissue |
| Effect on T4→T3 conversion | No | Yes | No |
| Duration | Long | Short | Permanent effect |
| Preferred use | Graves’ disease | Thyroid storm, pregnancy (1st trimester) | Definitive therapy |
| Hepatotoxicity | Lower | Higher | None |
Methimazole differs from PTU in that it does not inhibit peripheral conversion of T4 to T3 but has a longer duration and lower risk of hepatotoxicity. Compared to radioactive iodine, methimazole provides reversible control rather than permanent destruction of thyroid tissue.
MCQs
- Methimazole belongs to which class?
a) Beta blockers
b) Thionamides
c) Corticosteroids
d) Antihistamines
Answer: b) Thionamides
- Methimazole inhibits which enzyme?
a) DNA polymerase
b) Thyroid peroxidase
c) RNA polymerase
d) ATP synthase
Answer: b) Thyroid peroxidase
- Methimazole blocks which step in thyroid hormone synthesis?
a) Iodine uptake
b) Oxidation of iodide
c) Hormone release
d) Peripheral conversion
Answer: b) Oxidation of iodide
- Methimazole inhibits formation of:
a) Insulin
b) MIT and DIT
c) Dopamine
d) Cortisol
Answer: b) MIT and DIT
- Methimazole reduces synthesis of:
a) T3 and T4
b) TSH
c) ACTH
d) GH
Answer: a) T3 and T4
- Methimazole does NOT affect:
a) Hormone synthesis
b) Stored thyroid hormone
c) Iodination
d) Coupling
Answer: b) Stored thyroid hormone
- Methimazole is used in:
a) Hypothyroidism
b) Hyperthyroidism
c) Diabetes
d) Asthma
Answer: b) Hyperthyroidism
- A serious adverse effect is:
a) Hyperglycemia
b) Agranulocytosis
c) Hypokalemia
d) Bradycardia
Answer: b) Agranulocytosis
- Methimazole has which dosing advantage?
a) Multiple daily dosing
b) Once-daily dosing
c) Weekly dosing
d) Monthly dosing
Answer: b) Once-daily dosing
- Compared to PTU, methimazole:
a) Inhibits T4 to T3 conversion
b) Has shorter duration
c) Has longer duration
d) Is more hepatotoxic
Answer: c) Has longer duration
- Methimazole is contraindicated in:
a) Diabetes
b) First trimester pregnancy
c) Hypertension
d) Asthma
Answer: b) First trimester pregnancy
- Methimazole acts in the:
a) Liver
b) Thyroid gland
c) Kidney
d) Brain
Answer: b) Thyroid gland
FAQs
What is the mechanism of action of methimazole?
It inhibits thyroid peroxidase, blocking synthesis of T3 and T4.
Why is methimazole preferred over PTU?
It has a longer duration and lower risk of hepatotoxicity.
Does methimazole affect stored thyroid hormones?
No, it only inhibits new hormone synthesis.
Why is onset of action delayed?
Because preformed thyroid hormones must be depleted.
What is a serious adverse effect of methimazole?
Agranulocytosis.
Can methimazole be used in pregnancy?
It is avoided in the first trimester but may be used later.
References
Goodman & Gilman’s The Pharmacological Basis of Therapeutics – Thyroid Drugs
https://accessmedicine.mhmedical.com/book.aspx?bookid=3191
Katzung: Basic and Clinical Pharmacology – Thyroid & Antithyroid Drugs
https://accessmedicine.mhmedical.com/content.aspx?bookid=3382
Tripathi: Essentials of Medical Pharmacology – Thyroid Drugs
https://www.jaypeedigital.com
Harrison’s Principles of Internal Medicine – Thyroid Disorders
https://accessmedicine.mhmedical.com
