Final answer:
Injected thyroxine would increase the metabolic rate of both the thyroidectomized and hypophysectomized rats by substituting for their missing thyroid hormones, while its effect on a normal rat should be minimal since the normal rat's thyroid already regulates its BMR.
Step-by-step explanation:
The effect of injected thyroxine on metabolic rate varies among the three types of rats due to their differing thyroid statuses. In a normal rat, injected thyroxine likely increases the rat’s metabolic rate, although the normal rat should already be producing thyroxine at a level to maintain its basal metabolic rate (BMR). A thyroidectomized rat, which has had its thyroid gland removed, has a reduced metabolic rate due to lack of thyroxine production; therefore, injected thyroxine would increase its metabolic rate toward normal. A hypophysectomized rat, with its pituitary gland removed, would also have a reduced metabolic rate because it cannot produce thyroid stimulating hormone (TSH), which is necessary for thyroid hormone production. Injected thyroxine would substitute for the missing thyroid hormones, raising the hypophysectomized rat’s metabolic rate.
Thyroxine (T4) and triiodothyronine (T3) are hormones produced by the thyroid gland that control the BMR by regulating the rate of metabolism in cells. These hormones increase the rate of glucose oxidation and energy production, leading to a rise in body temperature, a process known as the calorigenic effect.