Treatment Approaches and Classification
The management of hyperthyroidism (thyrotoxicosis) relies on three main approaches: conservative (pharmacotherapy), surgical, and combined. Pharmacotherapy involves agents with distinct sites of action.
Based on their mechanism of action, antithyroid agents are divided into five groups:
- Follicular cell-destroying agents: radioactive iodine ($^{131}\text{I}$) preparations.
- Iodide uptake inhibitors: transport system inhibitors (potassium perchlorate).
- Hormone synthesis inhibitors: thiourea derivatives (thiamazole).
- TSH production suppressors: high-dose iodine preparations.
- Symptomatic agents: beta-blockers, which do not affect the gland itself but alleviate clinical manifestations.
Hormone Synthesis Inhibitors and Iodide Uptake Inhibitors
Thiamazole (also known as methimazole) acts by blocking the enzyme thyroid peroxidase. This inhibits the iodination of tyrosine and, consequently, reduces the synthesis of the main thyroid hormones—triiodothyronine ($T_3$) and thyroxine ($T_4$). It is prescribed orally for Graves' disease (diffuse toxic goiter). Notably, the use of thiamazole during pregnancy and lactation is strictly limited; propylthiouracil becomes the drug of choice during this period. Its main advantage is higher plasma protein binding and significantly lower placental transfer.
Potassium perchlorate works differently: it inhibits the thyroid gland's ability to uptake and concentrate iodide. The mechanism is based on the drug competing with ionized iodide for follicular cell transport systems. Potassium perchlorate is administered orally for mild to moderate toxic goiter, and the course of therapy can last up to 12 months.
Both drugs have serious side effects. Thiamazole may cause gastrointestinal disturbances (nausea, vomiting). Potassium perchlorate is notable for high toxicity, particularly hepatotoxicity. Both groups share hematotoxicity—bone marrow suppression that can manifest as leukopenia, thrombocytopenia, agranulocytosis, and aplastic anemia. Additionally, due to reduced hormone levels via negative feedback, these agents exert a goitrogenic effect (triggering a compensatory enlargement of the goiter).
Radioiodine Therapy and Adjuvant Pharmacotherapy
Radioactive iodine ($^{131}\text{I}$) preparations are widely positioned as first-line therapy. Following administration, the isotope is actively taken up by the thyroid gland and incorporated into thyroglobulin. The isotope emits $\gamma$-rays and $\beta$-particles, but it is primarily the $\beta$-particles that exert a marked cytotoxic effect on follicular cells, destroying them. The half-life of the isotope is 8 days, but radiation can be detected in the body for about two months. The antithyroid effect persists for 1–2 months after a single dose. The outcome of such therapy in 80% of patients is the development of hypothyroidism.
Adjuvant agents are used for combination therapy and to minimize side effects:
- Inorganic non-radioactive iodine preparations: They suppress thyroid-stimulating hormone (TSH) production by the anterior pituitary. Their clinical significance lies in preventing the goitrogenic effect caused by thiamazole and potassium perchlorate.
- Beta-blockers (e.g., propranolol): While not true antithyroid drugs, they exert sympatholytic effects. Their purpose is to control the symptoms of thyrotoxicosis. They effectively eliminate tremor, tachycardia, arrhythmias, and agitation. Propranolol is widely used for preoperative preparation and for symptom control during radioactive iodine therapy.