Thionamides have thyrostatic effects and are utilised to treat hyperthyroidism. They compete with iodide ions for the enzyme thyroid peroxidase. So, thionamides inhibit the iodide coupling in thyroid hormone synthesis, thereby decreasing T3 and T4 levels. Propylthiouracil (PTU) and thiamazol belong all to this group of thionamides.
In general, the principal of block-replacement therapy is followed: maximal inhibition of thyroid hormone secretion by fixed dosages of thionamides and subsequent replacement with L-thyroxine. Plasma TSH levels are used to titrate L-thyroxine dose.
I. Thionamides do NOT have an immediate effect on thyroid hormone levels in the blood, because the thyroid follicles conatin a reservoir of thyroid hormones.
II. Agranulocytosis is a feared complication of thionamides.
Extra info: Patients on thionamides should always be warned to consult a physician, when they have fever and a sore throat. This is the initial presentation of agranulocytosis.