EXAM #1: THYROID & PARATHYROID GLANDS

Question 1

What is the embryological origin of the thyroid gland?

Answer 1

• Endoderm of the primitive pharynx

- Thyroid diverticulum

Question 2

What does the thyroglossal duct connect?

Answer 2

Developing thyroid gland to tongue

Question 3

What does the adult foramen cecum indicate?

Answer 3

Site of the former thyrogossal duct

Question 4

Normally the thyroglossal duct regresses. If it does NOT regress, what may it form?

Answer 4

The adult pyramidal lobe of the thyroid.

Question 5

When does a thyrogossal duct cyst occur?

Answer 5

Occurs when part of the thyroglossal duct fails to obliterate, subsequently forming a cyst.

Question 6

Where are thyrogossal duct cysts commonly located?

Answer 6

Ventral to the hyoid bone and laryngeal cartilage.

Question 7

Where are the parathyroid glands normally located?

Answer 7

Posteriorly in the thyroid capsule

Question 8

What is the function of the thyroid gland i.e. what does the thyroid gland synthesize?

Answer 8

1) T3
2) T4
3) Calcitonin

Question 9

What is the basic structural and functional unit of the thyroid gland

Answer 9

Thyroid follicle

Question 10

What is the embryological origin of the thyroid follicle?

Answer 10

Endoderm

Question 11

What is the normal histological appearance of the thyroid gland?

Answer 11

Simple cuboidal

Question 12

What surround the thyroid follicles?

Answer 12

Basal lamina and a capillary plexus

Question 13

What is contained within the thyroid follicles? How does this substance stain?

Answer 13

Colloid- pink

*This contains thyroglobulin

Question 14

What cells are located adjacent to the thyroid follicular cells?

Answer 14

Parafollicular cells

Question 15

What is the embryological origin of parafollicular cells?

Answer 15

Neural crest cells

Question 16

What is the function of the parafollicular cells?

Answer 16

Secretion of calcitonin

Question 17

What is the stimulus for the secretion of calcitonin?

Answer 17

Elevated blood Ca++

*Think of as an “osteoclast stop sign” i.e. tells the osteoclasts to STOP breaking down bone and releasing Ca++ into the serum

Question 18

What is the effect of calcitonin on osteoclasts and osteoblasts?

Answer 18

Calcitonin

• Inhibits osteoclasts
• Activates osteoblasts

Question 19

Outline the HPA pathway that leads to the secretion of thyroid hormone.

Answer 19

• Hypothalamus= TRH
• Anterior pituitary= TSH
• Thyroid= T3 and T4

Question 20

Outline the synthesis of thyroid hormone.

Answer 20

1) TSH binds TSH receptors on basal membrane of follicular cells
2) RER synthesizes THYROGLOBULIN
3) Golgi apparatus glycosylates and packages into secretory vesicles
4) Released into the colloid of the follicular lumen
5) Iodine is added in the colloid

Question 21

How does iodide get into the thyroid follicular cell?

Answer 21

Na+- I symporter

Question 22

What enzyme is responsible for the oxidation of iodide to iodine in the calloid?

Answer 22

thyroid peroxidase

Question 23

Where does iodination take place?

Answer 23

Adjacent to the apical membrane of the follicular cell

Question 24

What is organification?

Answer 24

Attaching Iodine to thryoglobulin

Question 25

What enzyme couples MIT and DIT?

Answer 25

Thyroid peroxidase

Question 26

How is thyroid hormone released from the thyroid gland?

Answer 26

Lysosomal pathway - Uptake of colloid by endocytosis - Lysosome and colloid droplet fuse - Digestion by proteases in the lysosome releases thyroid hormones - Hormones diffuse into capillaries near the basal lamina

Question 27

What is the alternative to the lysosomal pathway of thyroid hormone release?

Answer 27

Transepithelial pathway - Megalin receptor causes uptake of colloid - T3 and T4 attached to thyroglobulin is released via exocytosis on the basal aspect of the follicular cell *Note that this bound T3/4 is INACTIVE

Question 28

What is the mnemonic to remember the major functions of the thyroid hormones?

Answer 28

Remember the four B's: - BMR (increase) - Bone growth - Brain maturation - Beta-adrenergic

Question 29

What is the pathological basis for Grave's Disease?

Answer 29

This is toxic goiter/ hyperthyroidism - Thyroid-stimulating immunoglobulin i.e. autoantibody to TSH is formed - Induces hyperthyroid state

Question 30

What causes the exopthalamous seen in Grave's Disease?

Answer 30

- Retro-orbital fibroblasts contain TSH receptors - TSI stimulates retro-orbital fibroblasts - Fibroblasts ynthesize hydrophilic glycosaminoglycans Consequently infiltration of fat and T cells in the retro-orbital space, and edema of the extraocular muscles causes exopthalamous

Question 31

What is the embryological origin of the parathyroid glands?

Answer 31

Derivatives of the pharyngeal pouches: - Pouch 3= inferior - Pouch 4= superior

Question 32

Histologically, how does the parathyroid gland differ from the thyroid gland?

Answer 32

Parathyroid gland contains densely packed cells vs. follicular cells

Question 33

What are the major cells of the parathyroid gland?

Answer 33

1) Chief | 2) Oxyphil

Question 34

What is the function of chief cells?

Answer 34

Secretion of parathyroid hormone

Question 35

What are the physiological effects of parathyroid hormone?

Answer 35

1) Increase serum [Ca2+] 2) Decrease serum [phosphate] 3) Increase urine [phosphate]

Question 36

Describe the effect of PTH on bone.

Answer 36

PTH increases bone resorption of calcium and phosphate: PTH binds to receptors on osteoblasts, which secrete macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-kB ligand (RANK-L) in response. RANK-L binds RANK on osteoclast precursors which promotes their differentiation into mature osteoclasts that are capable of bone resorption and the release of calcium.

Question 37

Describe the effect of PTH on the kidneys.

Answer 37

1) Conserves Ca++ | 2) Excretes phosphate

Question 38

Describe the effect of PTH on the GI tract.

Answer 38

- PTH regulates the formation of Vitamin D | - Vitamin D stimulates Ca++ absorption in the GI tract

Question 39

Compare the effects of PTH and Calcitonin?

Answer 39

PTH= INCREASES low blood Ca++ Calcitonin= DECREASE high blood Ca++

Question 40

What is the clinical consequence of hypoparathyroidism?

Answer 40

Hypocalcemic tetany | B/c no PTH to increase low blood Ca++