Lecture 62-Thyroid Hormones 2

Question 1

How does T3 provide negative feedback?

Answer 1

Suppressing TSH release, decreasing transcription of TSH, down regulating TRA receptors

Question 2

Where is negative feedback predominatantly exerted? What is mostly decreased?

Answer 2

Anterior pituitary

Decrease TSH

Question 3

What are two ways to inhibit TSH?

Answer 3

1. Tonically inhibited by dopamine and somatostatin from hypothalamus
2. Cortisol and GH

Question 4

What happens to thyroid hormone secretion during fasting?

Answer 4

TASH response to TRH will diminish

T3 falls

Metabolic rate decreases

Question 5

What happens to thyroid hormone secretion during meal or cold exposure?

Answer 5

Increases T3 availability

Question 6

What does 5’ deiodinase convert?
What does 5 deiodinase convert?

Answer 6

5 prime converts T4 -> T3
5 converts T4 -> rT3

Question 7

How does starvation effect conversion of T4 to T3?

Answer 7

5’ deiodinase in most tissues is inhibited, lower basal metabolic rate of O2 consumption

Brain 5’ deiodinase not affected

Question 8

Where are major sites of degradation of thyroid hormones?

Answer 8

Liver, kidneys, skeletal muscle

Question 9

Where are SOME T4 excreted?

Answer 9

In bile

Question 10

How does T4 enter the cell to be deiodinated?

Answer 10

Enters cell via facilitated diffusion

Question 11

Where does the T3-receptor complex bind to in order to stimulate transcription of genes?

Answer 11

Binds to thyroid-regulatory element on DNA

Question 12

What effect do thyroid hormones have on cardiovascular effects?

Answer 12

Increases CO, PP and synthesis of adrenergic receptors

Question 13

What effect do thyroid hormones have on thermogenesis?

Answer 13

Increase

Question 14

What effect do thyroid hormones have on oxygen consumption?

Answer 14

Increase and increased Na/K ATPase

Increase in cell activity

Question 15

What effect do thyroid hormones have on metabolism? (4)

Answer 15

1. Increase supply of substrates for production of energy (increase gluconeogenesis, increase absorption of glucose from gut)
2. Increase appetite, gut absorption and motility
3. Increase mobilization of carbohydrates and fat (increase lipolysis)
4. Protein synthesis (if thyroid hormone levels are physiological) and protein degradation (if thyroid hormone levels are high)

Question 16

What effect do thyroid hormones have on growth?

Answer 16

Required for normal growth

Question 17

What effect do thyroid hormones have on central nervous system?

Answer 17

-essential for normal maturation of CNS
-increases blood flow and glucose metabolism in brain
-enhances peripheral nerve reflexes

Question 18

What effect do thyroid hormones have on autonomic nervous system ?

Answer 18

Increase cardiac output

Similar to sympathetic nervous system

Question 19

What is hyperthyroidism?

Answer 19

Overactive thyroid , excess thyroxine

Question 20

What causes hyperthyroidism in cats?

Answer 20

Adenoma, adenocarcinoma, thyroid disrupters in environment

Question 21

What causes hyperthyroidism in horses and dogs?

Answer 21

Adenocarcinoma

Question 22

If hyperthyroidism is in the thyroid gland what do the TSH levels look like? What if the disorder is in the hypothalamus or pituitary?

Answer 22

thyroid gland -> TSH is low (more common) due to negative feedback, high T3/T4 constantly suppressing TSH release

Hypothalamus/pituitary -> TSH is low, overproducing TSH

Question 23

What is hypothyroidism defined as?

Answer 23

Underactive thyroid, low thyroid hormone

Question 24

What is congenital cretinism? (Aka thyroid dwarf)

Answer 24

The lack of enzymes in thyroid gland to make hormone

Question 25

If hypothyroidism is in the thyroid gland what do the TSH levels look like? What if the disorder is in the hypothalamus or pituitary?

Answer 25

Thyroid gland -> TSH is high
Hypothalamus/pituitary gland -> TSH low

Question 26

What is measured in blood tests to check the thyroid?

Answer 26

TSH, and T4 (T3 is typically NOT measured)