Final answer:
Propylthiouracil decreases thyroid hormone synthesis and release by inhibiting the production of T4 and T3 in the thyroid gland, making B) Decreased thyroid hormone synthesis and release the correct answer.
Step-by-step explanation:
An injection of propylthiouracil into an otherwise normal animal will affect thyroid hormone synthesis. Propylthiouracil is a drug that inhibits the production of thyroid hormones by interfering with the iodination of tyrosine residues in thyroglobulin and by inhibiting the conversion of thyroid hormones T4 (thyroxine) to the more active form T3 (triiodothyronine).
Considering the mechanism of action of propylthiouracil, the correct answer is B) Decreased thyroid hormone synthesis and release. This drug is typically used to treat hyperthyroidism, a condition where the thyroid gland produces too much thyroid hormone, by reducing the production and release of thyroid hormones from the thyroid gland. As a result, the overall activity of the thyroid gland is reduced, not increased or inhibited completely, and thyroid hormone degradation is not directly affected by propylthiouracil.