Thyroid Hormones

Question 1

Describe the physiological effects of thyroid hormones

Answer 1

• Growth and development
• Metabolic effects
• Thermogenesis
• Cardiovascular effects

Question 2

Thyroid hormones are _________ at normal levels, but _________ at the levels of hyperthyroidism.

Answer 2

• anabolic at normal
• catabolic at hyperthyroidism

Question 3

Describe the metabolic effects of thyroid hormones:

Answer 3

• increase BMR
• increase oxygen consumption
• starvation lowers both T3 and thyroid receptor

Question 4

How do thyroid hormones impact thermogenesis?

Answer 4

• increase resting heat production
• inability to adjust to environmental temperature is characteristic symptom of thyroid issues

Question 5

Explain the effects of thyroid hormones on growth and development:

Answer 5

• Essential for brain development (absent T3 –> cretinism)
• Promotes protein synthesis (skeletal/muscle growth)

Question 6

Explain the cardiovascular effects of thyroid hormones:

Answer 6

• increase catecholamine sensitivity

Question 7

Describe the differences between T3 and T4 in their structures

Answer 7

• Thyroxine (T4) has four iodide substituents.
• Triiodothyronine (T3) has three iodide substituents.

Question 8

What are the differences between the activities of T3 and T4?

Answer 8

T4 is produced more, but T3 is more potent.

Question 9

Identify the locations of particular reactions in thyroid hormone synthesis:

Answer 9

• Dietary iodine (I2) is converted to iodide (I-) in the stomach
• Iodide is actively transported into the cell by NIS
• In the follicular cell, Iodide passes down its electrochemical gradient and into the follicular colloid (partially through pendrin)
• It is oxidized by thyroid peroxidase to I at the apical membrane
• Iodide-free thyroglobulin is transported to the apical membrane
• Thyroglobulin is iodinated by thyroid peroxidase at one or two positions, forming MIT and DIT
• MIT + DIT = T3
• DIT + DIT = T4
• Coupling is dependent on thyroid peroxidase
• Thyroid hormone-containing thyroglobulin is retrieved back into the cytosol of the follicular cell as colloid droplets by pinocytosis
• Lysosomal exopeptidases cleave T4 (or T3) from thyroglobulin and the hormones are released into circulation where T4 can be converted to the more active T3 by 5’ deiodinase

Question 10

Distinguish toxic goiter from nontoxic goiter.

Answer 10

Toxic goiter is the enlargement of the thyroid due to Graves’.

Non-toxic occurs with hypothyroidism.

Question 11

List common symptoms of hypothyroidism:

Answer 11

• decrease in BMR
• defective thermoregulation (cold)
• dwarfism and mental retardation in infants and children
• increased vascular resistance
• myxoedema coma –> death
• nontoxic goiter
• weight gain

Question 12

List common symptoms of hyperthyroidism:

Answer 12

• increased BMR
• defective thermoregulation (hot)
• increased heart rate and cardiac output
• nervousness
• weight loss

Question 13

Describe the pathology of Hashimoto’s thyroiditis

Answer 13

• cause: autoimmune
• effects:
• hypothyroidism

Question 14

Describe the pathology of Graves’ disease

Answer 14

• cause: autoimmune
• effects:
• increased thyroid hormone production and IgG antibodies that bind to activate TSH receptor
• hyperthyroidism

Question 15

Which drugs are used for thyroid hormone replacement therapy?

Answer 15

• Levothyroxine (T4)
• Liothyronine (T3)
• Desiccated thyroid & thyroglobulin
• Liotrix

Question 16

Identify treatment options for hyperthyroidism

Answer 16

• thioamides
• radioactive iodine (RAI)
• Iodide (Lugol’s solution)

Question 17

1) Dietary iodine (I2) is converted to __________ in the stomach.

Answer 17

iodide (I-)

Question 18

2) Iodide is actively transported into the cell by ______________.

Answer 18

Na+ I- Symporter (NIS)

Question 19

3) In the follicular cell, iodide passes down its electrochemical gradient and into the follicular colloid (partially through the apical transporter, _________). It is oxidized by _______________ to I0 at the apical membrane.

Answer 19

• pendrin
• thyroid peroxidase

Question 20

4) Iodide-free __________ is transport to the apical membrane

Answer 20

thyroglobulin

Question 21

5) Thyroglobulin is ___________________________ at one or two positions, forming the hormone precursors mono-iodotyrosine (MIT), and di-iodotyrosine (DIT)

Answer 21

• iodinated by thyroid peroxidase
• iodide organification

Question 22

6) MIT + DIT = T3
- DIT + DIT = T4
- Coupling is dependent on ____________.

Answer 22

thyroid peroxidase

Question 23

7) Thyroid hormone-containing _______________ us retrieved back into the cytosol of the follicular cell as colloid droplets by pinocytosis

Answer 23

thyroglobulin

Question 24

Lysosomal exopeptidases _____________ from thyroglobulin and the hormones are released into circulation where T4 can be converted to the more active T3 by ______________.

Answer 24

• cleave T4 (or T3)
• 5’ deiodinase

Question 25

Explain the hypothalamus-pituitary-thyroid axis:

Answer 25

Hypothalamus secretes thyrotropin releasing hormone (TRH) which stims the anterior pituitary to secrete thyrotropin (TSH). This stims the thyroid to secrete T4 and T3.

Question 26

What inhibits the hypothalamus from producing TRH?

Answer 26

T4 and T3

Question 27

severely stunted physical and mental growth

Answer 27

cretinism

Question 28

an enlargement of the thyroid gland due to a persistent rise in TSH

Answer 28

Goiter

Question 29

Which drug inhibits thyroid peroxidases?

Answer 29

methimazole

Question 30

Which is more potent, T3 or T4?

Answer 30

T3 is more potent than T4

Question 31

Which is produced more, T3 or T4?

Answer 31

T4 is produced 10x more than T3

Question 32

Transport proteins have a greater affinity for ______.

Answer 32

T4