Tiamazol: What is it? Formula, Presentation, Mechanism of Action, Indications, Dosage and Side Effects

It is an antithyroid drug that works by blocking the synthesis of thyroid hormones T3 and T4.

This blockage is achieved by obstructing the association of iodine in tyrosyl residues of thyroglobulin, preventing iodization.

Chemical formula

  • C4H6N2S.
  • 1-metil-3H-imidazol-2-tiona.


  • 5 mg tablets for oral use.

Mechanism of action

The thiamazole (methimazole) decreases the concentration of inorganic iodine by reducing the formation of the precursors of the thyroid hormones triiodothyronine and thyroxine.

In this way, thiamazole (methimazole) acts in a way that alters the synthesis of thyroid hormones but without modifying the action of thyroid hormones that are already synthesized.

The antithyroid drugs do not hinder the actions of the exogenous thyroid hormones, nor in their release; their effects become evident when the reserves of thyroid hormones are depleted.


The tirodril is indicated for the treatment of pathologies in adults and children of 3 years or more related to hyperthyroidism, such as:

  • Treatment of thyrotoxic crises (thyroid storm).
  • Preparation for thyroidectomy.
  • Preparation before and after the application of radioactive iodine.


In adults:

  • Hyperthyroidism: Dosage of 10 to 40 mg/day, divided into 3 to 4 doses. Exceeding the euthyroid state usually takes 1 to 2 months, then it is recommended to reduce the dose to a maintenance level of 5 to 15 mg/day. In some patients, a single daily intake has been adequate.
  • Pre-operative treatment: Dosage of 10 to 15 mg daily, divided into several doses per day.


  • Initially: Dose of 0.45 to 1 mg / kg / day, in several doses.


  • Neonatal hyperthyroidism: 1.5 mg dose every 8 hours, gradually reducing the dose in case clinical improvement is observed.

In case of renal failure, the need to adjust the dose to achieve renal function should be evaluated.


In patients older than 65 years, a periodic dose adjustment is recommended under strict monitoring.

The initial dose for the treatment of children and adolescents from 3 years of age will be adjusted by the doctor according to the child’s body weight.

The usual treatment begins with a daily dose of 0.5 mg/kg of body weight divided into two or three equal amounts.

The daily dose is adjusted according to the child’s response to the doctor’s treatment for maintenance therapy.

Additional treatment with levothyroxine may be necessary to prevent hypothyroidism.

A total daily dose of 40 mg of thiamazole should not be exceeded.

Side effects

This medicine can cause side effects in some people.

About 2 to 4% of the cases of treatment have reported the following symptoms and usually occur in the first two months of treatment, classifying them by the rate of occurrence in:

Very common affects more than 1 in 10 patients:

Allergic skin reactions such as redness, hives, rashes, and itching.

But generally, they are mild and recurrent conditions with the continued treatment of thiamazole.

Common, affects up to 1 in 10 patients.

Diarrhea, vomiting, nausea, muscle pain, joint pain.

These can progressively develop and appear after the treatment has started.

Uncommon, they affect up to 1 in every 100 patients.

Inflammation of the mucous membranes, fever, or pharyngitis is evidence of the abnormality of the blood cells (symptomatology of agranulocytosis ).

Rarely do they affect up to 1 in 1,000 patients?

Headaches, and alteration of the sense of taste, such as ageusia and dysgeusia, occur after weaning, so it may take several weeks to normalize these values.

Very rare, they affect up to 1 in 10,000 patients

Reduction of platelets or thrombocytopenia, inflammation of the lymph nodes or generalized lymphadenopathy, a severe decrease of blood cells of all systems, or pancytopenia.

Perturbation of hormones that regulate blood sugar levels, causing a high blood sugar decrease or autoimmune insulin syndrome.

As a result of an overdose, thyroid hyperthyroidism and swelling in the neck (struma) can occur due to an increase in the thyroid-stimulating hormone, goiter, and thyroid hypofunction.

Inflammation of the optic nerve ( neuritis ), peripheral nerve disease (polyneuropathy), inflammation of the blood vessels ( vasculitis ), and acute salivary swelling.

Jaundice, liver dysfunction such as inflammation of the liver, and liver poisoning, due to the alteration of the biliary secretion.

Allergic severe skin reactions can affect the whole body, such as generalized dermatitis and Stevens-Johnson syndrome.

Alopecia, autoimmune inflammatory diseases of the skin and connective tissue such as systemic lupus erythematosus, inflammation of the joints, and nephritis.

The frequency, type, and severity of side effects are similar in children and adults.

Precautions and contraindications

It should not be used if:

  • You are allergic to thiamazole, other antithyroid medications, or any other components of this drug.
  • There is a reduced number of specific cells in the blood (granulocytopenia).
  • In case of bile obstruction before starting treatment.
  • There has been damage to the bone marrow after previous therapy with thiamazole and carbimazole.
  • When you are in adjuvant treatment with thyroid hormones during pregnancy.
  • In case of an enlarged thyroid, large goiter, swelling in the front of the neck hinders breathing.
  • He has inflammation of the mouth, pharyngitis, or fever.


Deficiency and excess iodine affect the effectiveness of thiamazole; it is necessary to adjust the dose according to the individual supply of iodine and thyroid function.

No direct interactions with other drugs are known.

It should be monitored in the case of anticoagulant treatments.