Thiamazole
Thiamazole is an antithyroid drug that blocks the enzyme thyroid peroxidase, the step by which the thyroid gland attaches iodine to build its hormones. Production falls over the following weeks while hormone already stored in the gland is used up, so the effect is gradual rather than immediate. It is used in hyperthyroidism, an overactive thyroid.
It is taken with food, which is easier on the stomach. TSH and free T4 are checked repeatedly, since the aim is to bring hormone levels down without pushing the thyroid into underactivity, and TSH lags behind the actual thyroid state early on. Thiamazole can rarely suppress the bone marrow's production of white cells, so white cell count and absolute neutrophils are checked when fever, a sore throat or infection appears rather than on a routine schedule; ALT is watched because the liver can also be affected.
A sore throat, mouth ulcers, fever or any infection that appears out of nowhere needs a blood count promptly rather than watchful waiting. Yellowing of the skin or eyes, dark urine, persistent nausea or right-sided abdominal pain also mean see a doctor.
Reference information, not medical advice. Discuss any change with your doctor.