Final answer:
Propylthiouracil did not affect the metabolic rate of thyroidectomized and hypophysectomized rats because they lacked a thyroid gland or the pituitary gland to produce thyroid stimulating hormone (TSH), making the inhibition by propylthiouracil irrelevant. The correct option is C.
Step-by-step explanation:
Propylthiouracil did not affect the metabolic rate of thyroidectomized and hypophysectomized rats because C. The rats lacked a thyroid gland. Propylthiouracil is a medication that inhibits the production of thyroid hormones by interfering with the incorporation of iodine into tyrosine residues of thyroglobulin, which is the precursor to thyroid hormones.
In rats that have undergone thyroidectomy, there is no thyroid gland to produce thyroxine (T4) and triiodothyronine (T3), regardless of whether propylthiouracil is present or not. Similarly, in hypophysectomized rats, which lack a pituitary gland, there is no production of thyroid stimulating hormone (TSH), which is necessary for stimulating the thyroid gland to produce T3 and T4.
Therefore, without a thyroid gland or the pituitary gland to produce TSH, the application of propylthiouracil would have no effect on the metabolic rate of these rats as there is no synthesis of thyroid hormones to be inhibited.