What is the mechanism of action of propylthiouracil in treating thyroid storm?

Propylthiouracil (PTU) is effective in treating thyroid storm primarily due to its mechanism of inhibiting the conversion of thyroxine (T4) to triiodothyronine (T3). In the context of hyperthyroidism and particularly thyroid storm, there's an excessive release of thyroid hormones leading to severe symptoms. Propylthiouracil acts by blocking the enzyme deiodinase, which is responsible for this conversion. By reducing the levels of the more active hormone, T3, the drug helps in mitigating the physiological effects of hyperthyroidism, thus stabilizing the patient.

Moreover, PTU also has an additional mechanism where it inhibits the synthesis of thyroid hormones by blocking the enzyme thyroid peroxidase, but its role in reducing T3 levels is crucial during acute management, particularly in life-threatening situations like thyroid storm. Understanding this mechanism underscores why propylthiouracil is a preferred choice in acute scenarios involving hyperthyroidism.