FPGEE for National Association of Boards of Pharmacy (NABP) Practice Exam 2025 - Free NABP Practice Questions and Study Guide

Methimazole and propylthiouracil (PTU) are both medications used to manage hyperthyroidism, particularly in conditions like Graves' disease, but they are not interchangeable due to their differing mechanisms of action and specific clinical roles.

Methimazole functions primarily by inhibiting the enzyme thyroperoxidase, which is crucial for the synthesis of thyroid hormones. It effectively reduces the production of thyroxine (T4) and triiodothyronine (T3) by blocking the iodination of tyrosine residues in thyroglobulin. This action leads to a decrease in thyroid hormone levels.

On the other hand, PTU also inhibits thyroperoxidase but has an additional mechanism where it reduces the peripheral conversion of T4 to T3. This distinction is particularly important in certain clinical scenarios. For instance, PTU is often preferred in the management of thyroid storm due to its rapid onset and ability to lower T3 levels effectively.

Thus, while both medications aim to decrease thyroid hormone levels, their different actions and efficacy profiles mean they cannot be used interchangeably without considering the specific clinical situation and patient needs.