Mechanism of Action and Pharmacological Effects
Thiamazole (also known as methimazole, mercasolyl, or thyrozol) is a classic antithyroid medication from the thiourea derivative group. Its action aims to interrupt the biochemical cascade of thyroid hormone production.
The primary target is the enzyme thyroid peroxidase. Blocking this enzyme inhibits the iodination of tyrosine residues on thyroglobulin. As a result, hormone assembly is disrupted, leading to a sustained decrease in the synthesis of triiodothyronine ($T_3$) and thyroxine ($T_4$). Unlike radioactive iodine therapy, thiamazole does not destroy thyroid follicular cells; it only temporarily halts their hormonal activity.
Indications for Use
In clinical practice, the management of hyperthyroidism (thyrotoxicosis) relies on three approaches: surgical, medical (conservative), and combined. Thiamazole is the cornerstone of conservative therapy.
The primary indication is Graves' disease (diffuse toxic goiter). The drug is administered orally to achieve a euthyroid state by suppressing excessive hormone production.
Side Effects and Complications
Antithyroid therapy requires strict monitoring due to the risk of serious adverse reactions:
- Goitrogenic effect: A characteristic side effect presenting as enlargement of the thyroid gland (goiter formation). This is mediated by negative feedback: the drug-induced reduction in $T_3$ and $T_4$ levels is detected by the hypothalamus-pituitary axis, leading to compensatory hypersecretion of thyroid-stimulating hormone (TSH) by the pituitary gland. Excess TSH stimulates thyroid tissue growth.
- Hematotoxicity: The drug can suppress hematopoiesis, requiring regular blood count monitoring.
- Gastrointestinal disturbances: Nausea and vomiting may occur during treatment.
Clinical Considerations and Combination Therapy
The use of thiamazole has several strict limitations and requires careful combination with other medications:
Pregnancy and Lactation Thiamazole use during this period is severely restricted. The drug of choice for pregnant patients is another thiourea derivative, propylthiouracil. Its advantage is greater plasma protein binding, which significantly reduces placental transfer and lowers fetal risk.
Managing the Goitrogenic Effect To prevent thyroid enlargement caused by thiamazole (as well as potassium perchlorate), inorganic non-radioactive iodine preparations are added to the regimen. They suppress TSH production by the anterior pituitary, breaking the negative feedback loop.
Adjuvant Symptomatic Therapy Because thiamazole reduces hormone synthesis gradually, beta-blockers (e.g., propranolol) are prescribed to rapidly control thyrotoxic symptoms such as tremor, tachycardia, arrhythmias, and agitation. While not true antithyroid drugs, they provide marked sympatholytic effects and are widely used in preoperative preparation and radioactive iodine therapy to alleviate clinical symptoms.
Sample Prescription
According to reference data, the drug is prescribed as oral tablets to be taken after meals.
Rp.: Tab. Mercazolyli 0,5 D.t.d. N. 50 S. Take orally after meals, 1 tablet 3–4 times a day.