Trade names Tapazole, others MedlinePlus a682464 Routes of
administration by mouth CAS ID 60-56-0 Bioavailability 93% | AHFS/Drugs.com Monograph Pregnancy
category D (US) ATC code H03BB02 (WHO) Molar mass 114.17 g/mol | |
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Thiamazole (INN) or methimazole (USAN) is an antithyroid drug, and part of the thioamide group. Like its counterpart propylthiouracil, a major side effect of treatment is agranulocytosis.
Contents
Medical uses
Thiamazole is a drug used to treat hyperthyroidism such as in Graves' disease, a condition that occurs when the thyroid gland begins to produce an excess of thyroid hormone. The drug may also be taken before thyroid surgery to lower thyroid hormone levels and minimize the effects of thyroid manipulation. Additionally, thiamazole is used in the veterinary setting to treat hyperthyroidism in cats.
Adverse effects
It is important to monitor any symptoms of fever or sore throat while taking thiamazole; this could indicate the development of agranulocytosis, an uncommon but severe side effect resulting from a drop in the white blood cell count (to be specific, neutropenia, a deficiency of neutrophils). A complete blood count (CBC) with differential is performed to confirm the suspicion, in which case the drug is discontinued. Administration of recombinant human granulocyte colony-stimulating factor (rhG-CSF) may increase recovery.
Other known side effects include:
Drug interactions
Adverse effects may occur for individuals who:
Thiamazole has been demonstrated to potently inhibit all CYP450 enzymes, meaning that the plasma concentration of all hepatically metabolized drugs will be significantly increased when taken with, or after, methimazole. This must be considered when starting a patient on methimazole, increasing their dose, or starting a thiamazole patient on another drug. Many drugs will need their doses lowered and dosing-time intervals increased when co-administered with thiamazole. For some hepatically metabolized drugs with very low therapeutic indexes, co-administration with thiamazole may not be possible at all.
Mechanism of action
Thiamazole inhibits the enzyme thyroperoxidase, which normally acts in thyroid hormone synthesis by oxidizing the anion iodide (I−) to iodine (I2), hypoiodous acid (HOI), enzyme linked hypoiodate (EOI) facilitating iodine's addition to tyrosine residues on the hormone precursor thyroglobulin, a necessary step in the synthesis of triiodothyronine (T3) and thyroxine (T4).
It does not inhibit the action of the sodium-dependent iodide transporter located on follicular cells' basolateral membranes. Inhibition of this step requires competitive inhibitors such as perchlorate and thiocyanate.
It acts at CXCL10.