S7) The Thyroid Gland Flashcards Preview

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Flashcards in S7) The Thyroid Gland Deck (45)
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1

Briefly, describe the structure of the thyroid gland

2 lobes joined by isthmus

2

Describe the location of the thyroid gland

- Lies against and around front larynx and trachea

- Below thyroid cartilage 

- Isthmus extends from 2nd to 3rd rings of the trachea

3

Thyroid is first endocrine gland to develop.

In 5 steps, describe the embryological development of the thyroid gland

⇒ Appears as an epithelial proliferation in floor of pharynx at base of tongue

⇒ Descends as diverticulum through thyroglossal duct

Migrates downwards in front of hyoid bone

⇒ Remains connected to tongue by thyroglossal duct

⇒ Duct degenerates and detached thyroid continues to final position 

4

Describe the microscopic structure of the thyroid gland

Follicular cells arranged in spheres called thyroid follicles

- Follicles filled with extracellular colloid, a deposit of thyroglobulin

5

The thyroid and parathyroid are distinct glands. 

Describe how their cells vary in terms of hormone production

6

Describe how thyroid hormones are formed

2 tyrosine molecules are linked together with iodine at 3/4 positions on the aromatic rings 

7

What is the role of thyroglobulin in thyroid hormone formation?

Thryoglobulin acts as a scaffold on which thyroid hormones are formed 

8

What is thyroid peroxidase and what does it do?

Thyroid peroxidase is a membrane bound enzyme that regulates thyroid hormone synthesis

9

Thyroid peroxidase regulates 3 separate reactions involving iodide.

Identify these

Oxidation of iodide to iodine (requires H2O2)

- Addition of iodine to tyrosine acceptor residues on the protein thyroglobulin

Coupling of MIT/DIT to generate thyroid hormones within the thyroglobulin protein

10

How is dietary iodine absorbed in the thyroid gland?

- Iodine is oxidised to iodide before absorption in the small intestine

- Iodide (I-) is taken up from blood by thyroid epithelial cells via a sodium-iodide symport

11

Explain the following statement: most of T4 is converted to T3 outside of the thyroid

- 90% of thyroid hormone secreted is T4

- Most T4 is converted to T3 in the liver & kidneys 

- 80% of circulating T3 is derived from T4

12

Which thyroid hormone is more biologically active?

Biological activity of T3 is 4x that of T4

13

How are the thyroid hormones transported in the blood?

T3 & T4 are transported in blood bound to the protein thyroxine-binding globulin

14

Thyroid hormones effect virtually every cell in the body and have two interconnected responses.

What are these?

- Effects on cellular differentiation and development

- Effects on metabolic pathways 

15

Regulation of thyroid hormone secretion is via negative feedback.

Outline this

16

Describe the structure of thyroid stimulating hormone

Glycoprotein hormone composed of 2 non-covalently bound subunits:

- α subunit is also present in FSH and LH

- β subunit provides unique biological activity 

17

TSH is a trigger for thyroid hormone release.

Identify the seven processes stimulated sequentially

⇒ Iodide uptake

⇒ Iodide oxidation (iodine)

⇒ Thyroglobulin synthesis

⇒ Thyroglobulin iodination

⇒ Colloid pinocytosis into cell

⇒ Proteolysis of thyroglobulin

⇒ Cell metabolism & growth 

18

Describe the cellular mechanisms involved in the TSH induced signalling pathway for thyroid hormone secretion

19

Identify the three general effects of thyroid hormones on the body

- Increase in BMR and heat production

- Stimulation of metabolic pathways

- Sympathomimetic effects

20

How do thyroid hormones increase BMR and heat production?

- Increases number and size of mitochondria

- Stimulates synthesis of enzymes in respiratory chain

21

Describe two metabolic pathways stimulated by thyroid hormones 

Catabolic pathways:

- Lipid metabolism – lipolysis and β-oxidation

- Carbohydrate metabolism – insulin-dependent entry of glucose into cells, gluconeogenesis and glycogenolysis

22

What are the sympathomimetic effects of thyroid hormones?

Sympathomimetic effects – increases target cell response to catecholamines by increasing receptor number

23

Identify the tissue specific effects of thyroid hormones in terms of the following:

- Cardiovascular system

- Nervous system

CVS – increases heart’s responsiveness to catecholamines (increased cardiac output and peripheral vasodilation)

- Nervous system – myelination of nerves & development of neurones  

24

Describe the function and action of thyroid hormone receptors

- Function as hormone-activated transcription factors

- Act by modulating gene expression 

25

In four steps, outline the cell signalling pathway induced by thyroid hormones on intracellular receptors

26

Indicate the normal plasma levels of thyroid hormones as well as reasons for variations unrelated to thyroid disease

27

What is goitre?

Goitre is the enlargement of the thyroid gland due to its overstimulation

- It may accompany either hypo- or hyperthyroidism

28

What is hypothyroidism?

Hypothyroidism is an endocrine disorder wherein the thyroid gland is underactive and produces an insufficient amount of T3/T4

29

How does a patient with hypothyroidism present?

- Obesity

- Lethargy

- Intolerance to cold

- Bradycardia

- Dry skin

- Alopecia

30

What are the thyroid function test results for a patient with hypothyroidism?

- Low T3

- Low T4

- Elevated TSH