Thyromazol, also known as methimazole, acts by blocking the production of thyroid hormones. It inhibits the synthesis of thyroid hormones by blocking the oxidation of iodine and its ability to combine with tyrosine to form thyroxine and triiodothyronine (T3).

Thyromazol binds to thyroid peroxidase and thereby inhibits the conversion of iodide to iodine. Thyroid peroxidase normally converts iodide to iodine (via hydrogen peroxide as a cofactor) and also catalyzes the incorporation of the resulting iodide molecule onto both the 3 and/or 5 positions of the phenol rings of tyrosines found in thyroglobulin. Thyroglobulin is degraded to produce thyroxine (T4) and tri-iodothyronine (T3), which are the main hormones produced by the thyroid gland. So Methimazole effectively inhibits the production of new thyroid hormones.

Avoid concurrent use with other drugs known to cause agranulocytosis (e.g. clozapine). Dosage of β-blockers, Digoxin, Warfarin and Theophylline may require adjustments according to changes in thyroid status. Concurrent use may increase clearance of prednisolone; increased risk of QT prolongation with macrolides.

Allergic skin reactions, jaundice, nausea, vomiting, epigastric distress, loss of taste, arthralgia, myalgia, paraesthesia, headache, drowsiness, peripheral neuritis, vertigo, sialodenopathy, lymphadenopathy, drug fever, lupus-like syndrome, periarteritis, hypoprothrombinaemia, hypothyroidism (prolonged therapy). Potentially Fatal: Blood dyscrasias especially agranulocytosis, hepatitis.

Simptoms of Thyromazol overdose: nausea, vomiting, epigastric distress, headache, fever, pruritus, arthralgia, oedema, pancytopenia, agranulocytosis, exfoliative dermatitis, hepatitis, neuropathy, CNS stimulation or depression.

Treatment is symptomatic and supportive. Empty stomach contents by inducing emesis or by gastric lavage. If the patient is comatose, having seizures or lacks the gag reflex, gastric lavage may be performed if an endotracheal tube with cuff inflated is in place to prevent aspiration of gastric contents. If bone marrow depression develops, use anti-infectives, corticosteroids and transfusions of fresh whole blood.