EXAM #1: THYROID & PARATHYROID GLANDS Flashcards
What is the embryological origin of the thyroid gland?
- Endoderm of the primitive pharynx
- Thyroid diverticulum
What does the thyroglossal duct connect?
Developing thyroid gland to tongue
What does the adult foramen cecum indicate?
Site of the former thyrogossal duct
Normally the thyroglossal duct regresses. If it does NOT regress, what may it form?
The adult pyramidal lobe of the thyroid.
When does a thyrogossal duct cyst occur?
Occurs when part of the thyroglossal duct fails to obliterate, subsequently forming a cyst.
Where are thyrogossal duct cysts commonly located?
Ventral to the hyoid bone and laryngeal cartilage.
Where are the parathyroid glands normally located?
Posteriorly in the thyroid capsule
What is the function of the thyroid gland i.e. what does the thyroid gland synthesize?
1) T3
2) T4
3) Calcitonin
What is the basic structural and functional unit of the thyroid gland
Thyroid follicle
What is the embryological origin of the thyroid follicle?
Endoderm
What is the normal histological appearance of the thyroid gland?
Simple cuboidal
What surround the thyroid follicles?
Basal lamina and a capillary plexus
What is contained within the thyroid follicles? How does this substance stain?
Colloid- pink
*This contains thyroglobulin
What cells are located adjacent to the thyroid follicular cells?
Parafollicular cells
What is the embryological origin of parafollicular cells?
Neural crest cells
What is the function of the parafollicular cells?
Secretion of calcitonin
What is the stimulus for the secretion of calcitonin?
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
What is the effect of calcitonin on osteoclasts and osteoblasts?
Calcitonin
- Inhibits osteoclasts
- Activates osteoblasts
Outline the HPA pathway that leads to the secretion of thyroid hormone.
- Hypothalamus= TRH
- Anterior pituitary= TSH
- Thyroid= T3 and T4
Outline the synthesis of thyroid hormone.
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
How does iodide get into the thyroid follicular cell?
Na+- I symporter
What enzyme is responsible for the oxidation of iodide to iodine in the calloid?
thyroid peroxidase
Where does iodination take place?
Adjacent to the apical membrane of the follicular cell
What is organification?
Attaching Iodine to thryoglobulin
What enzyme couples MIT and DIT?
Thyroid peroxidase
How is thyroid hormone released from the thyroid gland?
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
What is the alternative to the lysosomal pathway of thyroid hormone release?
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
What is the mnemonic to remember the major functions of the thyroid hormones?
Remember the four B’s:
- BMR (increase)
- Bone growth
- Brain maturation
- Beta-adrenergic
What is the pathological basis for Grave’s Disease?
This is toxic goiter/ hyperthyroidism
- Thyroid-stimulating immunoglobulin i.e. autoantibody to TSH is formed
- Induces hyperthyroid state
What causes the exopthalamous seen in Grave’s Disease?
- 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
What is the embryological origin of the parathyroid glands?
Derivatives of the pharyngeal pouches:
- Pouch 3= inferior
- Pouch 4= superior
Histologically, how does the parathyroid gland differ from the thyroid gland?
Parathyroid gland contains densely packed cells vs. follicular cells
What are the major cells of the parathyroid gland?
1) Chief
2) Oxyphil
What is the function of chief cells?
Secretion of parathyroid hormone
What are the physiological effects of parathyroid hormone?
1) Increase serum [Ca2+]
2) Decrease serum [phosphate]
3) Increase urine [phosphate]
Describe the effect of PTH on bone.
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.
Describe the effect of PTH on the kidneys.
1) Conserves Ca++
2) Excretes phosphate
Describe the effect of PTH on the GI tract.
- PTH regulates the formation of Vitamin D
- Vitamin D stimulates Ca++ absorption in the GI tract
Compare the effects of PTH and Calcitonin?
PTH= INCREASES low blood Ca++
Calcitonin= DECREASE high blood Ca++
What is the clinical consequence of hypoparathyroidism?
Hypocalcemic tetany
B/c no PTH to increase low blood Ca++
