Question

1.

A) The effects thyroxine secretion on the body are increased metabolic rate and heat production. Are these two responses related or are they independent of one another? Explain your answer.

B) When thyroxine levels in the blood are low, the level will return to normal due to negative feedback. Describe the events in the negative feedback loop that returns thyroxine to normal levels. Include names of the organs and hormones involved.

C) Compare the BMR of a normal rat, a thyroidectomized rat, and a hypophysectomized rat (highest to lowest) before and after an injection of an equal amount of TSH.   What accounts for the differences or similarities?

Answer 1

A)Many metabolic reactions are exothermic reactions ,resulting in the production of heat.So,the increased metabolic rate leads to the increased heat production .The two are related.

B)When the level of thyroxine is low, the negative feedback exerted by thyroxine on hypothalamus and anterior pituitary are removed.So hypothalamus secretes TRH (Thyrotropin releasing hormone) .This stimulates anterior pituitary to secrete TSH(Thyroid stimulating hormone ).The TSH acts on thyroid to stimulate the secretion of thyroxine.

So the level of thyroxine comes back to normal.

C)Hypothalamus secretes TRH(Thyrotropin releasing hormone),which in turn stimulates the secretion of TSH from anterior pituitary.

So in thyrectomized rat, BMR will be less than normal rat after injection of TSH because of the absence of receptors for TSH to act .

In hypophysectomized rat; TSH is not secreted due to the absence of thyroid stimulation by pituitary.So after TSH stimulation, BMR will increase.

So before the injection , BMR of normal rat > BMR of hypophysectomized rat=BMR of thyrectomized rate

After injection of TSH ,BMR of normal rat=BMR of hypophysectomized rat > BMR of thyrectomized rat