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Flashcards in PBL 3 - Thyroid Deck (22)
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At which level is the thyroid found?

Tracheal rings 2, 3, 4


List the potential embryological remnants of the thyroid. (3)

1. Patent thyroglossal duct (from foramen caecum)

2. Pyramidal lobe

3. Lingual thyroid


Describe the size of the thyroid gland.

Weight: 15-25g
Length of lobes: 4-6cm
Thickness of lobes: 1.3-1..8cm


What is the thyroid gland anterior to? (3)

1. Trachea
2. Oesophagus
3. Longus colli muscles


What is the thyroid gland posterior to? (4)

1. Sternocleidomastoid muscle
2. Sternohyoid muscle
3. Sternothyrohyoid muscle
4. Omohyoid muscle


Describe the blood supply of the thyroid gland.

1. External carotid artery
a. Superior thyroid artery
---Anterior glandular branch
---Posterior glandular branch

2. Thyrocervical trunk of subclavian artery
a. Inferior thyroid artery
---Inferior branch
---Ascending branch

3. Brachiocephalic trunk
a. Thyroid ima artery (only occasionally present)


Describe the venous drainage of the thyroid gland.

1. Internal jugular vein
a. Superior thyroid vein
b. Middle thyroid vein

2. Left/right brachiocephalic veins
a. Inferior thyroid vein


What is the lymphatic drainage of the thyroid gland? (2)

Paratracheal lymph nodes

Deep cervical lymph nodes


Which important nerves pass underneath the thyroid gland?

External laryngeal nerve

Recurrent laryngeal nerve


Describe the histology of the thyroid gland. (4)

Describe the follicles at different stages of activity.

1. Fibrous capsule

2. Collagenous septa
a. Divide thyroid into lobules

3. Follicles
a. Epithelium when active: columnar
b. Epithelium when inactive: cuboidal
c. Lumen contains colloid (containing thyroglobulin)

4. Parafollicular C cells
a. Found individually scattered through the follicle linings


How is the thyroid regulated? (3)

1. Hypothalamic-pituitary axis
a. TRH from hypothalamus
b. TSH from anterior pituitary

2. Negative feedback of T3/T4
a. T3/T4 inhibit the hypothalamus and ant pituitary gland

3. Blood calcium levels
a. Control calcitonin secretion


What are the effects of TSH on the thyroid? (5)

1. Increased proteolysis of thyroglobulin
2. Increased activity of the iodide pump
3. Increased iodination of tyrosine
4. Increased size and secretory activity of thyroid cells
5. Increased number of thyroid cells


Briefly list the steps in the synthesis of T3/T4. (6)

1. Iodide trapping in the thyroid follicular cell via the sodium iodide transporter

2. Iodide diffuses through the cell to the luminal side

3. Iodide is transported out of the cell and into the lumen via pendrin

4. Oxidation of iodide to iodine via hydrogen peroxidase

5. Organification of tyrosine amino acids in thyroglobulin via tyrosine peroxidase action

6. MIT and DIT molecules are coupled to each other
a. 2 DIT makes T4
b. 1 DIT and 1 MIT makes T3


What is the structure of thyroglobulin?

Large glycoprotein

~70 amino acids


Describe the process of iodine recycling in thyroid follicular cells. (3)

1. 75% of MIT/DIT molecules are not converted to T3/T4

2. Iodine is removed from MIT/DIT via deiodinase enzymes

3. This is then recycled to make more T3/T4


Describe the storage of T3/T4 in the thyroid.

Stores last 2-3 months

T3/T4 stored as part of the thyroglobulin


How are T3/T4 secreted? (4)

1. Thyroglobulin molecules in the colloid re-enter the follicular cell
a. Pseudopod extensions enclose small amounts of colloid
b. This forms pinocytic vesicles
c. Pinocytic vesicles enter the cell via endocytosis

2. Pinocytic vesicles fuse with lysosomes in the thyroid follicular cell
a. Proteases in the lysosome break down the thyroglobulin molecule, releasing T3 and T4

3. T3 and T4 diffuse out of the cell on the basolateral side and into blood capillaries


What is the alternative method of T3/T4 secretion? (4)

1. Some of the thyroglobulin enters the cell without being packaged into pinocytic vesicles
a. Via membrane transporter megalin
b. Megalin-thyroglobulin complex is carried across the cell via transcytosis
c. It is then released into the capillaries


How are thyroid hormones transported in the blood? (4)

1. Thyroxine binding globulin (70%)
2. Thyroxine binding prealbumin/transthyretin (10-15%)
3. Albumin (15-20%)
4. Free T3/T4 (0.03-0.3%)


What are the 3 enzymes that convert T4 into T3 in the periphery?

5' deiodinase 1 (D1)
a. Location: liver, kidney

5' deiodinase 2 (D2)
a. Location: everywhere

5' deiodinase 3 (D3)
a. Location: placenta, brain, skin
b. Removes iodine from the inner thyronine ring to form reverse T3 (inactive)


Describe T3 signalling.

1. T3 binds to nuclear receptors
a. Thyroid hormone receptor is often associated with the retinoid X receptor

2. Hormone-receptor complexes act as transcription factors
a. Bind to the thyroid hormone response element of DNA to cause new protein synthesis


List the effects of thyroid hormones on different body processes. (6)

1. On growth:
a. Stimulates skeletal growth
b. Growth and development of foetal brain

2. On carbohydrate metabolism:
a. Increased glucose uptake
b. Increased glycolysis
c. Increased gluconeogenesis
d. Increased absorption of glucose from GI tract

3. On fat metabolism:
a. Increased lipolysis
b. Increased free fatty acids
c. Decreased plasma cholesterol levels

4. CNS development

5. On cardiovascular system:
a. Increased CO
b. Increased HR
c. Increased blood flow

6. Increased basal metabolic rate
a. Increased Na+/K+ ATPase activity