Endocrine 5 - Thyroid

Question 1

Describe the steps of thyroid hormone synthesis.

Answer 1

1. Follicle cells make thyroglobulin and exocytose it out to the colloid.
2. Iodine is pumped into the follicle cell.
3. Thyroid peroxidase (TPO) converts I- to free radical iodine.
4. Free radical iodine is pumped out to the colloid.
5. Free radical iodine attacks tyrosine (organification) and coupling occurs –> di-tyrosines.
6. TSH stimulates pinocytosis of thyroglobulins –> fusion with lysozome intracellularly –> T3 and T4 are released and secreted.

Question 2

What are the relative reactivities of T3, T4, and reverse T3?

Answer 2

T3 > T4 > rT3

-rT3 isn’t active at all

Question 3

What are the three major thyroid hormone binding proteins? Of these, which accounts for the majority of thyroid hormone binding?

Answer 3

Thyroxine-binding globulin (binds the majority of thyroid hormones), transthyretin, and albumin.

Question 4

Does thyroxine-binding globulin have a lower affinity for T3, or T4? What is the consequence of this?

Answer 4

Lower affinity for T3, which allows for higher circulating levels of T3.

Question 5

Does the body make more T3 or T4?

Answer 5

T4

Question 6

Describe what happens to T3, T4, and rT3 levels during starvation.

Answer 6

T3 levels drop a lot.
T4 levels drop a little.
rT3 levels increase a lot.

Question 7

What does somatostatin do to thyroid hormone synthesis?

Answer 7

It decreases it by inhibiting release of TSH from the anterior pituitary.

Question 8

What type of feedback does T3 participate in?

Answer 8

Long loop

Question 9

What is the celluar location of thyroid hormone receptors?

Answer 9

In the nucleus

Question 10

Do nuclear thyroid hormone receptors have a higher affinity for T3, or T4?

Answer 10

T3

Question 11

What does a T3 receptor/RXR heterodimer do?

What does a T3 receptor homodimer do?

Answer 11

T3 receptor/RXR heterodimer negatively regulates transcription.

T3 receptor homodimers positively regulate transcription

Question 12

Name three organs that do not increase O2 consumption in the setting of increased thyroid hormone.

Answer 12

1. Brain
2. Testes
3. Spleen

Question 13

What effect does increased thyroid hormone have on protein degradation and synthesis, respectively?

Answer 13

Both degradation and synthesis increase, but degradation is increased more relative to synthesis. That is why hyperthyroidism causes muscle weakness.

Question 14

What are the three cellular mechanisms of controlling basal metabolic rate?

Answer 14

1. Alter the activity of the Na+/K+ ATPase
2. Synthesize uncoupling proteins (mitochondrial uncoupling)
3. Stimulate vicious cycles of protein synthesis and breakdown

Question 15

By what mechahism does increased T3 increase heart rate?

Answer 15

Increased T3 levels cause cardiac myocytes to express higher numbers of beta adrenergic receptors.

Question 16

Name the disease: severely stunted physical and mental growth due to untreated congenital deficiency of thyroid hormones.

Answer 16

Cretinism

Question 17

What is Hashimoto disease?

Answer 17

Hypothyroidism, caused by previous TB infection or autoimmune attack on the thyroid.

Question 18

What causes Grave’s disease?

Answer 18

Abnormal Abs bind TSH receptors –> high levels of T3 and T4

Question 19

Explain why patients with goiters may have either hyperthyroidism or hypothyroidism.

Answer 19

Goiterous hyperthyroidism might occur in the setting of an anterior pitiutary tumor secreting excess TSH, causing hypertrophy of the thyroid AND elevated thyroid hormones.

Goiterous hypothyroidism can occur in the setting of iodine deficiency, causing high levels of TSH but no ability to synthesize T3 and T4.

Question 20

The most active thyroid hormone is:

a) T4
b) T3
c) rT4
d) rT3
e) TSH

Answer 20

b) T3

Question 21

Most adult tissues respond metabolically to thyroid hormones, but one that does NOT is:

a) liver
b) skeletal muscle
c) adipocytes
d) cardiac muscle
e) brain

Answer 21

e) brain

Question 22

Symptoms of hyperthyroidism include:

a) weight gain
b) incrased basal metabolic rate
c) hypotension
d) hypoglycemia
e) reduced tissue sensitivity to epinephrine

Answer 22

b) increased basal metabolic rate