5.3 Metabolic Rate and the Thyroid and Parathyroid Glands

Question 1

What is the thyroid gland?

Answer 1

large gland located in the neck

Question 2

What is the parathyroid gland?

Answer 2

embedded behind the thyroid gland

Question 3

What does the thyroid gland contain?

Answer 3

a large number of follicles

Question 4

What does each follicle have?

Answer 4

small spherical structure made of thyroid cells that produce triiodothyronine (T3) and thyroxine (T4)

Question 5

What is triiodothyronine (T3)?

Answer 5

contains three iodine atoms

Question 6

What is thyroxine (T4)?

Answer 6

contains 4 iodine atoms

Question 7

How is T3 and T4 produced?

Answer 7

thyroid gland actively acquires iodine from the bloodstream

Question 8

What is the concentration of iodine in the thyroid gland?

Answer 8

~25 times that found in blood

Question 9

What is the primary source for iodine in our diets?

Answer 9

iodized salt

Question 10

What do the thyroid hormones T3 and T4 do?

Answer 10

• increase the metabolic rate
• don’t have a single target organ, instead stimulate most cells of the body to metabolize more glucose and use more energy

Question 11

Does T3 or T4 have a more potent activity?

Answer 11

T3, although the thyroid releases about ten times more than T4

Question 12

What does the liver do?

Answer 12

convert most of the T4 produced by the thyroid gland to T3

Question 13

Why are T3 and T4 considered peptide hormones?

Answer 13

because they are synthesized from the amino acid tyrosine

Question 14

Even though T3 and T4 are considered peptide hormones, where are receptors located?

Answer 14

their receptor is actually located inside cells, more like a steroid hormone receptor

Question 15

What other hormone does the thyroid gland produce?

Answer 15

calcitonin, which helps control blood calcium levels

Question 16

What processes does calcium play an important role in?

Answer 16

nerve conduction, muscle contraction, blood clotting

Question 17

When does the thyroid gland secrete calcitonin?

Answer 17

when the blood calcium level rises

Question 18

What is the primary effect of calcitonin?

Answer 18

to bring about the deposition of calcium in the bones

Question 19

What happens when the blood calcium level decreases to normal?

Answer 19

thyroid stops releasing calcitonin

Question 20

What is PTH?

Answer 20

parathyroid hormone

hormone produced by the parathyroid glands that causes the blood phosphate level to decrease and the blood calcium level to increase

Question 21

What does PTH promote?

Answer 21

• the activity of bone cells and the release of calcium from the bones
• the reabsorption of calcium by the kidneys where it helps activate vitamin D

Question 22

What does vitamin D do after it is activated?

Answer 22

stimulates the absorption of calcium from the intestine

these effects bring the blood calcium level back to the normal range

Question 23

What happens as soon as blood calcium is back to normal?

Answer 23

parathyroid glands no longer secrete PTH

Question 24

Describe the regulation of blood calcium level.

Answer 24

• when blood calcium level is high, thyroid gland secretes calcitonin
• calcitonin promotes the uptake of calcium by the bones, and therefore the blood calcium level returns to normal
• when the blood calcium level is low, the parathyroid glands release PTH
• PTH causes the bones to release calcium and the kidneys to reabsorb calcium and activate vitamin D
• thereafter, intestines absorb calcium
• therefore, blood calcium level returns to normal