[2] Severe side effects may include low blood cell counts, liver failure, and vasculitis.

[10] 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.

Other known side effects include: Adverse effects may occur for individuals who: Thiamazole inhibits the enzyme thyroperoxidase, which normally acts in thyroid hormone synthesis by oxidizing the anion iodide (I−) to iodine (I2), hypoiodous acid (HOI), and 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).

[citation needed] It does not inhibit the action of the sodium-dependent iodide transporter located on follicular cells' basolateral membranes.

[17] The cyclic thiourea derivative thiamazole is a white to matte brown crystalline powder with a characteristic odour.

[22] When the therapeutic potential of thiamazole was recognised in the late 1940s, a number of alternative routes were developed based, for example, on the use of 2-chloro-1,2-diethoxyethane as starting material, in a reaction with methylamine.

He later published evidence that thiouracil was more effective and began a search for analogues with higher potency and less toxicity.

[25] In July 2024, the US Food and Drug Administration (FDA) approved Felanorm, the first generic methimazole oral solution for the treatment of hyperthyroidism in cats.