PBL 3 - Thyroid

Question 1

At which level is the thyroid found?

Answer 1

Tracheal rings 2, 3, 4

Question 2

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

Answer 2

1. Patent thyroglossal duct (from foramen caecum)
2. Pyramidal lobe
3. Lingual thyroid

Question 3

Describe the size of the thyroid gland.
Weight?
Length?
Thickness?

Answer 3

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

Question 4

What is the thyroid gland anterior to? (3)

Answer 4

1. Trachea
2. Oesophagus
3. Longus colli muscles

Question 5

What is the thyroid gland posterior to? (4)

Answer 5

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

Question 6

Describe the blood supply of the thyroid gland.

Answer 6

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)

Question 7

Describe the venous drainage of the thyroid gland.

Answer 7

1. Internal jugular vein
   a. Superior thyroid vein
   b. Middle thyroid vein
2. Left/right brachiocephalic veins
   a. Inferior thyroid vein

Question 8

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

Answer 8

Paratracheal lymph nodes

Deep cervical lymph nodes

Question 9

Which important nerves pass underneath the thyroid gland?

Answer 9

External laryngeal nerve

Recurrent laryngeal nerve

Question 10

Describe the histology of the thyroid gland. (4)

Describe the follicles at different stages of activity.

Answer 10

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

Question 11

How is the thyroid regulated? (3)

Answer 11

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

Question 12

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

Answer 12

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

Question 13

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

Answer 13

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

Question 14

What is the structure of thyroglobulin?

Answer 14

Large glycoprotein

~70 amino acids

Question 15

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

Answer 15

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

Question 16

Describe the storage of T3/T4 in the thyroid.

Answer 16

Stores last 2-3 months

T3/T4 stored as part of the thyroglobulin

Question 17

How are T3/T4 secreted? (4)

Answer 17

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

Question 18

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

Answer 18

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

Question 19

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

Answer 19

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

Question 20

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

Answer 20

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)

Question 21

Describe T3 signalling.

Answer 21

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

Question 22

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

Answer 22

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