Biology, Eighth Edition (Raven)

Chapter 46: The Endocrine System

Mechanism of Thyroxine Action

How does the body respond to thyroxin? Thyroxin is a hormone produced by the thyroid gland. Thyroxin is important in controlling both development and maturation. It also controls metabolism in adult organisms. Excess thyroxin results in overly rapid development or metabolism while a deficiency of thyroxin results in delayed development or a sluggish metabolism. Thyroid diseases are generally the result of the production of inappropriate amounts of thyroid hormones.

View the animation below, then complete the quiz to test your knowledge of the concept.


1.	Which of the following statements about thyroxine is TRUE?

		A)	it is water-insoluble
		B)	it is lipophilic
		C)	it cannot pass through the cell membrane
		D)	A and B
		E)	A, B and C

2.	The thyroid gland secretes

		A)	large amounts of triiodothyronine but no tetraiodothyronine.
		B)	large amounts of tetraiodothyronine but no triiodothyronine.
		C)	large amounts of both triiodothyronine and tetraiodothyronine.
		D)	large amounts of triiodothyronine and small amounts of tetraiodothyronine.
		E)	large amounts of tetraiodothyronine and small amounts of triiodothyronine.

3.	Which of the following statements about thyroxine is TRUE?

		A)	Both the T3 and T4 forms enter target cells.
		B)	Inside the target cell, all T3 is converted to T4.
		C)	Only the T4 form can enter the nucleus.
		D)	A and B
		E)	A, B and C

4.	Thyroxine is brought to target cells via a protein-carrier.

		A)	True
		B)	False

5.	The thyroxine-hormone receptor complex increases the level of transcription of specific genes.

		A)	True
		B)	False