Thiamazole

Mechanism of action and pharmacologic effects

Thiamazole (also known by the synonyms methimazole, mercazolil, thyrozol) is a classic antithyroid agent from the thiourea derivative group. Its action is aimed at interrupting the biochemical cascade of thyroid hormone production.

The main target of the drug is the enzyme thyroperoxidase (peroxidase). The blockade of this enzyme leads to the inhibition of tyrosine iodination (tyrosine residues of thyroglobulin). As a result, the assembly of hormone molecules is impaired, leading to a sustained decrease in the synthesis of triiodothyronine ($T_3$) and thyroxine ($T_4$). Unlike radioactive iodine preparations, thiamazole does not destroy thyroid follicular cells, but only temporarily halts their hormonal activity.

Indications for use

In clinical practice, the treatment of hyperthyroidism (thyrotoxicosis) is based on three approaches: surgical, pharmacotherapeutic (conservative), and combination therapy. Thiamazole forms the basis of conservative therapy.

The main indication for prescribing the drug is diffuse toxic goiter. The drug is administered orally and helps achieve a euthyroid state by suppressing excessive hormone production.

Side effects and complications

Thyrostatic therapy requires strict monitoring due to the risk of developing serious adverse reactions:

• Strumogenic (goitrogenic) effect: a characteristic side effect manifested by an enlargement of the thyroid gland (goiter formation). The mechanism is related to the negative feedback principle: the drug-induced reduction in $T_3$ and $T_4$ levels is detected by the hypothalamic-pituitary system, which causes compensatory hypersecretion of thyroid-stimulating hormone (TSH) by the pituitary gland. Excess TSH stimulates the growth of thyroid tissue.
• Hematotoxicity: the drug is capable of causing suppression of hematopoiesis, which requires regular monitoring of blood counts.
• Dyspeptic disorders: nausea and vomiting may occur during treatment.

Special prescribing considerations and combination therapy

The use of thiamazole has a number of strict limitations and requires proper combination with other drugs:

Pregnancy and lactation The prescription of thiamazole during this period is extremely limited. The drug of choice for pregnant women is another thiourea derivative, propylthiouracil. Its advantage lies in the fact that it binds better to plasma proteins and, consequently, crosses the placental barrier significantly less, reducing risks to the fetus.

Correction of the strumogenic effect To prevent the enlargement of the thyroid gland caused by thiamazole (as well as potassium perchlorate), inorganic non-radioactive iodine preparations are introduced into the treatment regimen. They inhibit TSH production by the adenohypophysis, breaking the vicious cycle of negative feedback.

Adjuvant symptomatic therapy Since thiamazole reduces hormone synthesis gradually, beta-blockers (e.g., propranolol) are prescribed for the rapid relief of thyrotoxicosis symptoms (tremor, tachycardia, arrhythmias, agitation). They are not true antithyroid agents, but they exert a pronounced sympatholytic effect. Propranolol is actively used in preoperative preparation and radioactive iodine therapy to reduce clinical symptoms.

Sample prescription

According to reference data, the drug is prescribed in tablets for oral administration after meals.

Rp.: Tab. Mercazolyli 0,5 D.t.d. N. 50 S. Orally after meals, 1 tablet 3-4 times a day.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Biochemistry of thyroid hormone synthesis and the role of thyroperoxidase
• The principle of negative feedback in endocrinology (hypothalamic-pituitary-thyroid axis)
• Comparative pharmacokinetics of thiamazole and propylthiouracil
• Hematotoxicity of thyrostatics: pathogenesis and monitoring of agranulocytosis
• Symptomatic therapy of thyrotoxicosis with beta-blockers

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