Oral Adjunct to radio-iodine therapy, Graves' disease, Hyperthyroidism, Preparation for thyroidectomy
Adult: Initially, 15-60 mg daily in 3 divided doses, depending on the severity of the disease; gradually reduce to a maintenance dose of 5-15 mg daily once patient is euthyroid. Usual treatment duration: 6 months to 2 years (for conservative treatment of hyperthyroidism). Child: Initially, 0.3-0.5 mg/kg daily in 3 divided doses. Maintenance: Approx 1/2 of the initial dose.
Administration
Should be taken with food.
Contraindications
Previous damage to bone marrow or other serious adverse effects associated with treatment of thiamazole or other thioamides (e.g. carbimazole, propylthiouracil).
Special Precautions
Patient with moderate to severe granulocytopenia, pre-existing cholestasis (not caused by hyperthyroidism). Hepatic and renal impairment. Children. Pregnancy (particularly during 1st trimester) and lactation.
Adverse Reactions
Significant: Bleeding, hypoprothrombinaemia, aplastic anaemia, thrombocytopenia, leucopenia, dermatologic reactions, drug fever, hypothyroidism, lupus-like syndrome, vasculitis resulting in severe complications, pancreatitis, hepatotoxicity including acute liver failure. Cardiovascular disease: Periarteritis. Blood and lymphatic system disorders: Lymphadenopathy. Ear and labyrinth disorders: Vertigo. Endocrine disorders: Insulin autoimmune syndrome. Gastrointestinal disorders: Dysgeusia, ageusia, acute salivary gland swelling, nausea, vomiting, epigastric distress. Hepatobiliary disorders: Hepatitis, jaundice. Musculoskeletal and connective tissue disorders: Arthralgia, myalgia. Nervous system disorders: Neuritis, paraesthesia, headache, drowsiness. Renal and urinary disorders: Rarely, nephritis. Skin and subcutaneous tissue disorders: Rash, pruritus, urticaria, alopecia, skin pigmentation. Potentially Fatal: Agranulocytosis.
Perform CBC with differential prior to initiation of therapy, during febrile illness and at the onset of pharyngitis. Monitor thyroid-stimulating hormone (TSH), serum free T4 and total T3 levels; prothrombin time (during treatment particularly before surgical procedures), LFT (at baseline and if symptoms of liver injury occur), thyrotropin receptor antibodies (before discontinuing treatment in patients with Graves’ disease).
Overdosage
Symptoms: Epigastric pain, nausea, vomiting, headache, oedema, fever, joint pain, pruritus, aplastic anaemia, agranulocytosis; hepatitis, exfoliative dermatitis, nephrotic syndrome, neuropathies, CNS stimulation or depression may also occur. Management: Supportive treatment.
Drug Interactions
Concomitant use with β-blockers, cardiac glycosides, oral anticoagulants (e.g. warfarin) and theophylline may require dosage adjustments according to changes in patient’s thyroid status.
Action
Description: Mechanism of Action: Thiamazole inhibits the production of thyroid hormones by blocking the oxidation of iodine and its ability to combine with tyrosine to form thyroxine (T4) and triiodothyronine (T3).
Synonym: methimazole. Onset: 12-18 hours. Duration: 36-72 hours. Pharmacokinetics: Absorption: Readily and almost completely absorbed from the gastrointestinal tract. Bioavailability: Approx 93%. Time to peak plasma concentration: 1-2 hours. Distribution: Accumulates in the thyroid gland. Readily crosses the placenta and enters breast milk. Metabolism: Metabolised in the liver. Excretion: Mainly via urine; faeces (minimal). Elimination half-life: 5-13 hours.
Chemical Structure
Thiamazole Source: National Center for Biotechnology Information. PubChem Compound Summary for CID 1349907, Methimazole. https://pubchem.ncbi.nlm.nih.gov/compound/Methimazole. Accessed Oct. 28, 2020.
H03BB02 - thiamazole ; Belongs to the class of sulfur-containing imidazole derivative agents. Used in the management of thyroid diseases.
References
Anon. Methimazole. AHFS Clinical Drug Information [online]. Bethesda, MD. American Society of Health-System Pharmacists, Inc. https://www.ahfscdi.com. Accessed 07/10/2020.Anon. Methimazole. Lexicomp Online. Hudson, Ohio. Wolters Kluwer Clinical Drug Information, Inc. https://online.lexi.com. Accessed 05/10/2020.Buckingham R (ed). Thiamazole. Martindale: The Complete Drug Reference [online]. London. Pharmaceutical Press. https://www.medicinescomplete.com. Accessed 05/10/2020.Methimazole Tablet (ECI Pharmaceuticals, LLC). DailyMed. Source: U.S. National Library of Medicine. https://dailymed.nlm.nih.gov/dailymed/. Accessed 05/10/2020.Methimazole Tablets, USP (PAR Pharmaceutical Companies, Inc.). U.S. FDA. https://www.fda.gov/. Accessed 05/10/2020.
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