Propylthiouracil works primarily by:

Propylthiouracil (PTU) is an antithyroid medication primarily used to treat hyperthyroidism by targeting the production of thyroid hormones. It works by inhibiting the enzyme thyroperoxidase, which is crucial in the synthesis of thyroid hormones. By blocking this enzyme, PTU effectively reduces the production of both thyroxine (T4) and triiodothyronine (T3), leading to decreased levels of these hormones in the bloodstream. This action helps to manage the symptoms and effects of hyperthyroidism.

The other options do not apply to the mechanism of action of propylthiouracil. For example, stimulating thyroid hormone production contradicts the drug's purpose of reducing hormone levels. Enhancing insulin release and inhibiting glucagon secretion are mechanisms related to diabetes and glucose metabolism, which are not relevant to the function of PTU in thyroid hormone regulation. Thus, the ability of propylthiouracil to block thyroid hormone synthesis is precisely why it is effective in treating conditions associated with excessive thyroid hormone production.