Thyroid Hormones

Question 1

What hormones does the thyroid produce?

Answer 1

• thyroxine (T4)
• triodothyronine (T3)

Question 2

What are the overall functions of T3 and T4?

Answer 2

• increase rate of basal oxygen use, basal metabolism, and rate of heat production
• modulate delivery of substrates and oxygen by cardiovascular and respiratory systems to sustain metabolic rate

Question 3

Thyroid hormone levels change based on what?

Answer 3

• energy need
• calorie supply
• environmental temperature

Question 4

What do you need for thyroid hormone synthesis?

Answer 4

• 2 tyrosines
• iodine

Question 5

What is the major thyroid hormone produced?

Answer 5

thyroxine (T4)

Question 6

What is the biologically active thyroid hormone secreted in lesser quantities?

Answer 6

triiodothyonine (T3)

Question 7

What enzyme converts T4 to T3?

Answer 7

5’ deiodinase

Question 8

What is the alternative product of T4 (inactive form of T3) that is produced when less thyroid hormone action is needed?

Answer 8

Reverse T3

Question 9

The parafollicular cells (C cells) of the thyroid gland secrete what?

Answer 9

calcitonin

Question 10

Under stimulation, colloidal thyroid hormone is absorbed where?

Answer 10

into follicular cells

Question 11

How is iodine supplied for thyroid hormone synthesis?

Answer 11

supplied by diet via iodide

Question 12

Thyriod hormones are stored where until stimulated to secrete?

Answer 12

extracellularly in follicular lumen

Question 13

What is the ratio of T4:T3 within the thyroid gland?

Answer 13

10:1

Question 14

What is the Wolff-Chaikoff effect?

Answer 14

transient reduction in thyroid hormone levels cause by ingestion of a large amount of iodine and lasts about 10 days

Question 15

Excessively high levels of dietary iodide (like in the Wolff-Chaikoff effect) will suppress the activity of what?

Answer 15

2 Na/1 I pump

Question 16

Thyroglobulin is stored in follicles as what until stimulation for release of thyroid hormones?

Answer 16

colloid

Question 17

When stimulus of thyroid gland is received (thyroid stimulating hormone), what happens?

Answer 17

• retrieval of thyroglobulin from follicle lumen into endocrine cell via endocytosis
• lysosomes fuse with colloid droplet and release T3, T4, free MIT, and free DIT from thyroglobulin
• T3 and T4 enter blood

Question 18

How are thyroglobulins retrieved from follicle lumen and into endocrine cell via endocytosis?

Answer 18

cell membrane forms pseudopods which engulf a pocket of colloid

Question 19

What happens to MIT and DIT when thyroid hormones are secreted into circulation?

Answer 19

rapidly deiodinated by deiodinase to conserve iodide for recylcing

Question 20

What are the effects of hepatic failure on circulating thyroid hormones?

Answer 20

• decreased liver protein synthesis
• decreased thyroxine binding globulin
• increase in circulating thyroid hormone
• decrease in hormone synthesis

Question 21

What proteins does circulating T3 and T4 bind to?

Answer 21

• thyroxine binding globulin
• transthyretin
• albumin

Question 22

What conditions cause sick euthyroid syndrome?

Answer 22

• fasting/starvation
• stress (cortisol)
• catabolic diseases - cancer
• hepatic diseases
• renal diseases

Question 23

What enzyme is downregulated in sick euthyroid syndrome?

Answer 23

5’ deiodinase

Question 24

Sick euthyroid syndrome is due to dysregulation of what?

Answer 24

thyrotropic feedback control

Question 25

Describe the regulation of the thyroid gland.

Answer 25

- thyrotropin-releasing hormone is released from hypothalamus - causes release of thyroid stimulating hormone from anterior pituitary - causes release of T4 and T3 from thyriod gland - negative feedback of T4 and T3 to anterior pituitary and hypothalamus

Question 26

Since T4 cannot cause negative feedback itself, what must occur?

Answer 26

it has to be converted to T3 via 5' deiodinase since T3 is the active form

Question 27

Thyroid releasing hormone is a _____ hormone.

Answer 27

tripeptide

Question 28

Thyroid stimulating hormone is a ____ hormone with ____ subunits.

Answer 28

glycoprotein, 2

Question 29

What are the biologically active sites of TSH?

Answer 29

beta subunit

Question 30

Describe the steps in the synthesis of thyroid hormones.

Answer 30

- need to synthesize protein molecule called thyroglobulin using tyrosine - iodide transported from blood into follicular epithelial cells via 2 Na+ / 1 symporter - iodide transported across apoical membrane into colloid of follicle via pendrin and oxidized to iodine via thyroid peroxidase - iodine incorporated into tyrosines at specific sites in thyroglobulin via thyroid peroxidase - coupling reaction -- results in MIT and DIT whcih remain attached to thyroglobulin until stimulus for secretion

Question 31

Negative feedback exerted predinantly at the ______ to decrease TSH.

Answer 31

anterior pituitary

Question 32

What enzyme converts T4 to T3?

Answer 32

5' deiodinase

Question 33

What enzyme converts T4 to rT3?

Answer 33

5 deiodinase

Question 34

What are the major sites of degradation of thyroid hormones?

Answer 34

- liver - kidneys - skeletal muscle

Question 35

What hormones provide tonic inhibition for TSH release?

Answer 35

- dopamine - somatostatin

Question 36

How does T4 enter cells?

Answer 36

facilitated diffusion

Question 37

T4 is deiodinated to T3 where?

Answer 37

inside cells

Question 38

T3 binds the receptors within what of the cell?

Answer 38

nucleus

Question 39

Why does T3 bind to the receptor within the nucleus of the cell?

Answer 39

- T3 receptor complex binds to thyroid regulatory element on DNA to stimulate transcription of genes

Question 40

What are the effects of thyroid hormones on basal metabolic rate and metabolism?

Answer 40

- increased oxygen consumption and increased Na/K ATPase - increased cardiac output and pulse pressure - increased thermogenesis - increase supply of substrates for energy production - increase appetite, gut absorption, motility - increase mobilization of carbs and fat

Question 41

Protein synthesis vs degradation is dependent on thyroid hormone levels. Explain.

Answer 41

- if thyroid hormone levels are normal then protein synthesis predominates - if thyroid hormone levels are high then protein degradation predominates to use amino acids for energy

Question 42

Throid hormones act with what to promote growth of bone, cartilage, teeth, epidermis, and hair?

Answer 42

- growth hormone - IGFs

Question 43

What effects do thyroid hormones have on the central nervous system?

Answer 43

- increases blood flow and glucose metabolism in the brain - enhances peripheral nerve reflexes

Question 44

What effects do thyroid hormones have on the autonomic nervous system?

Answer 44

interact with sympathetic nervous system to have similar effects on increasing cardiac output, heat production, etc

Question 45

What is hypoerthyroidism?

Answer 45

overactive thyroid causing excess thyroxine

Question 46

If hyperthyroidism is in thyroid gland, TSH is high or low?

Answer 46

low

Question 47

If hyperthyroidism is in hypothalamus or pituitary, TSH is high or low?

Answer 47

high

Question 48

What is hypothyroidism?

Answer 48

underactive thyroid causing low thyroid hormones

Question 49

If hypothyroidism is in thyroid gland, TSH is high or low?

Answer 49

high

Question 50

If hypothyroidism is in hypothalamus or pituitary gland, TSH is high or low?

Answer 50

low