What is the mechanism of action of propylthiouracil (PTU)?

Propylthiouracil (PTU) acts primarily by inhibiting the enzyme responsible for the conversion of thyroxine (T4) to triiodothyronine (T3) in peripheral tissues. This mechanism is crucial because T3 is the more biologically active form of thyroid hormone, and its levels directly affect metabolism and energy regulation. By reducing the availability of T3, PTU helps to control conditions like hyperthyroidism, where there is excessive production of thyroid hormones.

In addition to its action on the conversion of T4 to T3, PTU also inhibits the synthesis of thyroid hormones in the thyroid gland by blocking the enzyme thyroid peroxidase, which is necessary for the iodination of tyrosine residues in thyroglobulin—a key step in the production of T4 and T3. However, the primary focus in this context is the inhibition of T4 to T3 conversion, aligning with the selected answer.

Other options do not align with PTU's mechanism of action. For instance, PTU does not stimulate thyroid hormone production or inhibit iodine absorption, both of which are unrelated to its therapeutic effects. This distinction helps clarify the specific and effective role PTU plays in managing hyperthyroid states.