Methimazole is a drug used to treat hyperthyroidism, a condition that usually occurs when the thyroid gland is producing too much thyroid hormone. It may also be taken before thyroid surgery to lower thyroid hormone levels and minimize the effects of thyroid manipulation. Methimazole is also used in the veterinary setting to treat hyperthyroidism in cats.

Mechanism of action:

Methimazole inhibits the enzyme thyroperoxidase, which normally acts in thyroid hormone synthesis by oxidizing the anion iodide (I-) to iodine (I0), facilitating iodines 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.

Adverse effects:

It is important to monitor any symptoms of fever or sore throat while taking methimazole; 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 side effects include:

skin rash
itching
abnormal hair loss
upset stomach
vomiting
loss of taste
abnormal sensations (tingling, prickling, burning, tightness, and pulling)
swelling
joint and muscle pain
drowsiness
dizziness
decreased white blood cells
decreased platelet
Aplasia cutis congenita (prenatal exposure)

Adverse effects may occur for individuals who:

Take anticoagulants (blood thinners) such as warfarin (Coumadin), diabetes medications, digoxin (Lanoxin), theophylline (Theobid, Theo-Dur) and vitamins
Have ever had any blood disease, such as decreased white blood cells (leukopenia), decreased platelets (thrombocytopenia) or aplastic anemia, or liver disease (hepatitis, jaundice)
Are pregnant, or going to become pregnant, or are breast-feeding. An alternative anti-thyroid drug, propylthiouracil, is often substituted during pregnancy or breast-feeding. If pregnancy occurs while taking methimazole, switching to propylthiouracil may be an alternative. Early studies suggested that methimazole may harm the fetus; however, more recent studies suggest this may not be the case.
Are going to have surgery, including dental surgery.