Thyroid

Question 1

What are T3 and T4 synthesized from and what is required in the diet for their synthesis?

Answer 1

Synthesized from the amino acid tyrosine on thyroglobulin

Requires iodide from the diet

Question 2

What is the inactive form of thyroid hormone?

Answer 2

reverse T3

Question 3

What helps iodide get in the cell and does it require ATP?

Answer 3

2Na/I- symporter–> not a pump so doesn’t require ATP

Question 4

Where is the majority of organic iodide in the body?

Answer 4

residing in the thyroid gland in association with colloidal thyroglobulin

Question 5

Where does the majority of T3 and reverse T3 come from?

Answer 5

T4

Question 6

What is most circulating TH bound to?

Answer 6

Thyroid-binding globulin

also transthyretin and albumin

Question 7

What TH is more active and which is considered a pro hormone?

Answer 7

T3 is more biologically active/potent

T4 is often referred to as a pro hormone

Question 8

What metabolizes THs in tissues such as the liver and kidney?

Answer 8

5’ peripheral deiodinases

Question 9

Which TH is used to treat hypothyroidism and why?

Answer 9

T4– longer half-life and greater stability compared to T3

Question 10

What does cytoplasmic 5’/3’-monodeiodinase do?

Answer 10

Converts T4–> T3

Question 11

What does free TH do when it gets into the cell?

Answer 11

bind to nuclear DNA at thyroid response elements in the promoter region of genes regulating transcription of T3 and T4 genes

Question 12

What does injection of T3 and T4 do to the basal metabolic rate? What is the difference in the effects btwn T3 and T4?

Answer 12

Increases BMR–THs have a slow onset and long duration of action

T3 acts 4x as rapidly as T4–2-3days for T3 and several days for T4

Question 13

What role does THs play in bone formation?

Answer 13

promote ossification and fusion of bone plates and bone maturation

Question 14

What does TH deficiency in infants cause?

Answer 14

mental retardation (cretinism)–slowed only if treatment is initiated shortly after birth and growth retardation–slowed by treatment with thyroxine(T4)

Question 15

What effects does T3 have on organs in the body?

Answer 15

• -Growth–bone formation and maturation
• -CNS- maturation of CNS
• -BMR- increase Na/K ATPase/O2 consumption/heat production/BMR
• -Metabolism- Increase glucose absorption/glycogenolysis/gluconeogenesis/lipolysis/protein synthesis and degradation(net catabolic)
• -Cardiovascular- increase CO

Question 16

How does hyperthyroidism effect BMR? Hypothyroidism?

Answer 16

BMR increases in hyperthyroidism

BMR decreases in hypothyroidism

Question 17

What do hypothyroid women have to lookout for when supplementing with thyroxine?

Answer 17

osteoporosis

Question 18

What does hyperthyroidism increase expression of and what effect does this have? What is used to treat?

Answer 18

B-adrenergic receptors–leads to enhanced sensitivity to circulating epi and NE

B-adrenergic antagonists-propranolol used to treat symptoms

Question 19

What increases in hypothyroidism and what does this cause?

Answer 19

increase in serum cholesterol which increases risk for atherosclerosis

Question 20

How does TRH stimulate TSH release?

Answer 20

TRH activates G-protein coupled receptor linked to PLC–leading to IP3 generation and mobilization of intracellular Ca

Question 21

How does TSH stimulate TH synthesis/release?

Answer 21

activating GPCR linked to adenylate cyclase–generating intracellular cAMP

Question 22

What 2 substances inhibit TSH release?

Answer 22

Dopamine–on anterior pituitary and somatostatin– on hypothalamus

Question 23

What do the levels of serum TSH look like in hypothyroidism? Hyperthyroidism?

Answer 23

Hypothyroidism- TSH is significantly elevated due to lack of negative feedback by low circulating T3 and T4

Hyperthyroidism- TSH is reduced due to excessive negative feedback by high circulating T3 and T4