Final answer:
Injected propylthiouracil (PTU) lowers the metabolic rate by reducing thyroid hormone production in a normal rat, would have no effect on the thyroidectomized rat's absent hormone production, and may reduce the efficacy of supplemented thyroid hormones in a hypophysectomized rat.
Step-by-step explanation:
The effect of injected propylthiouracil (PTU) on the metabolic rate of rats differs depending on the physiological status of each rat's thyroid system. Propylthiouracil is a medication that reduces the production of thyroid hormones such as thyroxine (T4) and triiodothyronine (T3). These hormones are crucial in regulating the basal metabolic rate (BMR).
For the normal rat, PTU would decrease the production of thyroid hormones, leading to a lowered BMR since thyroid hormones are key BMR-regulators. The thyroidectomized rat, which lacks a thyroid gland, would not produce thyroid hormones and thus PTU would have no effect on its nonexistent hormone production, but since it relies on supplemental thyroid hormones, if any are being given, PTU could reduce their efficacy. Lastly, for the hypophysectomized rat, which lacks a pituitary gland, PTU would also not directly affect its metabolic rate since it does not produce TSH (thyroid stimulating hormone); however, if the rat is being supplemented with thyroid hormones, PTU could again potentially reduce their efficacy.