Lectures 11 & 12 - Thyroid hormone and Functions Jan 31 & Feb 3, 2025

Question 1

Where is the thyroid gland located?

Answer 1

On the anterior surface of the trachea

The thyroid gland is composed of left and right lobes and a small connecting branch or isthmus.

Question 2

What is the average size range of the thyroid gland?

Answer 2

20-34 g

The size of the gland varies depending on hereditary, environmental, and nutritional factors.

Question 3

What are the two distinct cell types in the thyroid gland?

Answer 3

• Follicular cells
• Parafollicular cells (C-cells)

Follicular cells secrete thyroxine (T4) and triiodothyronine (T3), while parafollicular cells release calcitonin.

Question 4

What essential trace element is required for the production of thyroid hormones?

Answer 4

Iodine

Thyroid hormones are the only ones that require iodine for active forms.

Question 5

How long can thyroid hormone supplies be stored?

Answer 5

Up to three months

Question 6

Where are thyroid hormone receptors located?

Answer 6

In the nucleus

Unlike other peptide hormones, thyroid hormones have receptors in the nucleus.

Question 7

What is the mechanism of action of thyroid hormones similar to?

Answer 7

Steroids

Question 8

What is the first stage of thyroid hormone synthesis?

Answer 8

Synthesis of thyroglobulin (TG)

This occurs in the thyroid follicular cell.

Question 9

What is the role of the Na/I symporter in the thyroid gland?

Answer 9

Facilitates iodide trapping

It is located at the basolateral membrane and is enhanced by thyroid-stimulating hormone (TSH).

Question 10

What occurs during the second stage of thyroid hormone synthesis?

Answer 10

Iodide trapping

Question 11

What is the third stage of thyroid hormone synthesis?

Answer 11

Oxidation of iodide

Question 12

What enzyme oxidizes iodide to I2 in the follicular lumen?

Answer 12

Thyroid peroxidase

Question 13

What process occurs during the fourth stage of thyroid hormone synthesis?

Answer 13

Iodination of thyrosine

Question 14

What are the products of the coupling reaction in thyroid hormone synthesis?

Answer 14

• T4 (tetraiodothyronine)
• T3 (triiodothyronine)

T4 is produced about 10 times more than T3.

Question 15

What happens during the endocytosis stage of thyroid hormone synthesis?

Answer 15

Iodinated thyroglobulin, T4, and T3 are endocytosed into follicular epithelial cells

Question 16

What is the seventh stage of thyroid hormone synthesis?

Answer 16

Proteolysis and hydrolysis of T3 and T4 from thyroglobulin

Question 17

What is the role of lysosomal proteases in thyroid hormone synthesis?

Answer 17

Hydrolyze peptide bonds to release T3 and T4 from thyroglobulin

Question 18

What occurs during the eighth stage of thyroid hormone synthesis?

Answer 18

Deiodination

Question 19

What are the three main functions of the thyroid gland?

Answer 19

• Collect and transport iodine
• Synthesize thyroglobulin and secrete it into the colloid
• Remove thyroid hormones from thyroglobulin and secrete them into systemic circulation

Question 20

True or False: Reverse T3 (rT3) is biologically active.

Answer 20

False

rT3 is devoid of biological activity.

Question 21

What type of deiodinase removes an iodine from the inner benzene ring of T4?

Answer 21

5/3-deiodinase

Question 22

What type of deiodinase removes an iodine from the outer benzene ring of T4?

Answer 22

5’/3’-deiodinase

Question 23

What are the three functions of the thyroid gland?

Answer 23

Collect and transport iodine, synthesis of thyroglobulin and secreting it into the colloid, remove the thyroid hormones from thyroglobulin and secrete them into the systemic circulation.

Question 24

What is the consequence of iodine deficiency?

Answer 24

Hypothyroidism and goiter.

Question 25

What condition may result from infancy hypothyroidism?

Answer 25

Cretinism.

Question 26

What are the physiological effects of thyroid hormones?

Answer 26

Increase basal metabolic rate, stimulate carbohydrate, protein, and lipid metabolism, and stimulate thermogenesis.

Question 27

How do thyroid hormones affect metabolism in muscle, liver, and kidney?

Answer 27

Thyroid hormone-induced increases in oxygen consumption are paralleled by increases in the activity of the Na-K pump in the plasma membrane.

Question 28

What roles do thyroid hormones play in bone, liver, brain, and heart?

Answer 28

* Bone: Increases osteoblastic and osteoclastic activity; promotes normal growth & skeletal development. * Liver: Triglyceride and cholesterol metabolism. * Brain: Axon growth and development. * Heart: Inotropic and chronotropic effects.

Question 29

What are the two active forms of thyroid hormone?

Answer 29

Triiodothyronine (T3) and tetraiodothyronine (T4).

Question 30

Which thyroid hormone is more physiologically active?

Answer 30

T3.

Question 31

What is the half-life of T3?

Answer 31

Approximately 1 day.

Question 32

How much of the hormone secreted by the thyroid is in the form of T3?

Answer 32

About 10%.

Question 33

What enzyme converts T4 to T3 in the liver and kidneys?

Answer 33

5’/3’-deiodinase.

Question 34

What is reverse T3 (rT3) and its significance?

Answer 34

rT3 is physiologically inactive and has two iodines on the outer benzyl ring rather than on the inner ring.

Question 35

What are the two types of deiodinases and their functions?

Answer 35

* 5/3-deiodinase: Removes an iodine from the inner benzene ring of T4 producing rT3. * 5’/3’-deiodinase: Removes an iodine from the outer benzene ring of T4 producing T3.

Question 36

What physiological changes affect the activity of 5’/3’-deiodinase?

Answer 36

Caloric restriction or stress inhibit type-1, reducing T3 and thus metabolic rate.

Question 37

How do thyroid hormones trigger cellular actions?

Answer 37

Thyroid hormones bind to nuclear receptors to trigger DNA transcription, translation, and protein synthesis.

Question 38

What is the role of TRH in thyroid hormone regulation?

Answer 38

TRH stimulates the thyrotrophs of the anterior pituitary to secrete thyrotropin (TSH).

Question 39

What feedback mechanism regulates thyroid hormones?

Answer 39

Circulating thyroid hormones exert feedback control on both TRH and TSH secretion.

Question 40

What is the effect of estrogen during pregnancy on thyroid hormone levels?

Answer 40

Increased levels of estrogen stimulate the synthesis of thyroxine-binding globulin, resulting in reduced levels of free thyroid hormone.

Question 41

What is the primary cause of hypothyroidism?

Answer 41

Hyposecretion of the hormone.

Question 42

What is the function of the anterior pituitary in thyroid hormone regulation?

Answer 42

The anterior pituitary releases thyrotropin (TSH) in response to TRH from the hypothalamus.

Question 43

What is the primary action of TSH on the thyroid gland?

Answer 43

Stimulates T4/T3 synthesis in the thyroid gland.

Question 44

What are the physiological roles of thyroid hormones in the gut?

Answer 44

Increase the rate of carbohydrate absorption.

Question 45

What is the relationship between T4 and T3 in terms of biological activity?

Answer 45

Only T4 and T3 are biologically active, with T3 being far more active than T4 due to a higher affinity for thyroid receptors.

Question 46

What hormones inhibit TSH release?

Answer 46

Somatostatin and dopamine ## Footnote These hormones are released by hypothalamic neurons and influence TSH release in response to T3 levels.

Question 47

What is hypothyroidism caused by?

Answer 47

Hyposecretion of the hormone ## Footnote It can be classified as primary or secondary.

Question 48

What are the primary causes of hypothyroidism?

Answer 48

* Problems with thyroid gland * Immune cell destruction * Radioactive iodine * Surgery * Tumor * Iodine deficiency ## Footnote Primary hypothyroidism is due to direct issues with the thyroid gland.

Question 49

What are the secondary causes of hypothyroidism?

Answer 49

Problems with pituitary or hypothalamus ## Footnote Secondary hypothyroidism occurs when the issue lies in the regulation of TSH production.

Question 50

What is cretinism?

Answer 50

Severe retardation of physical and mental growth due to lack of iodine ## Footnote It is a disorder associated with hypothyroidism.

Question 51

What is myxedema?

Answer 51

A form of cutaneous and/or dermal edema caused by increased deposition of connective tissue elements ## Footnote It is also associated with hypothyroidism.

Question 52

What is iodine-deficient goiter?

Answer 52

An enlargement of the thyroid gland due to iodine deficiency ## Footnote It is a disorder linked to hypothyroidism.

Question 53

What is Graves disease?

Answer 53

An autoimmune condition where B lymphocytes synthesize thyroid-stimulating immunoglobulin (TSI) ## Footnote TSI binds to TSH receptors, mimicking TSH effects.

Question 54

What does thyroid-stimulating immunoglobulin (TSI) do?

Answer 54

Binds to and activates the TSH receptor ## Footnote This leads to increased iodine trapping and hormone synthesis by the thyroid.

Question 55

What is the result of increased TSI production in Graves disease?

Answer 55

* Increased iodide trapping by the thyroid * Increased synthesis and secretion of T3 and T4 * Thyroid enlargement (goiter) ## Footnote This condition leads to hyperthyroidism.

Question 56

Disorders associated with hyperthyroidism include:

Answer 56

TBD ## Footnote The text did not specify the disorders associated with hyperthyroidism.