PBL 3 - Thyroid Flashcards
At which level is the thyroid found?
Tracheal rings 2, 3, 4
List the potential embryological remnants of the thyroid. (3)
- Patent thyroglossal duct (from foramen caecum)
- Pyramidal lobe
- Lingual thyroid
Describe the size of the thyroid gland.
Weight?
Length?
Thickness?
Weight: 15-25g
Length of lobes: 4-6cm
Thickness of lobes: 1.3-1..8cm
What is the thyroid gland anterior to? (3)
- Trachea
- Oesophagus
- Longus colli muscles
What is the thyroid gland posterior to? (4)
- Sternocleidomastoid muscle
- Sternohyoid muscle
- Sternothyrohyoid muscle
- Omohyoid muscle
Describe the blood supply of the thyroid gland.
- External carotid artery
a. Superior thyroid artery
- –Anterior glandular branch
- –Posterior glandular branch - Thyrocervical trunk of subclavian artery
a. Inferior thyroid artery
- –Inferior branch
- –Ascending branch - Brachiocephalic trunk
a. Thyroid ima artery (only occasionally present)
Describe the venous drainage of the thyroid gland.
- Internal jugular vein
a. Superior thyroid vein
b. Middle thyroid vein - 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.
- Fibrous capsule
- Collagenous septa
a. Divide thyroid into lobules - Follicles
a. Epithelium when active: columnar
b. Epithelium when inactive: cuboidal
c. Lumen contains colloid (containing thyroglobulin) - Parafollicular C cells
a. Found individually scattered through the follicle linings
How is the thyroid regulated? (3)
- Hypothalamic-pituitary axis
a. TRH from hypothalamus
b. TSH from anterior pituitary - Negative feedback of T3/T4
a. T3/T4 inhibit the hypothalamus and ant pituitary gland - Blood calcium levels
a. Control calcitonin secretion
What are the effects of TSH on the thyroid? (5)
- Increased proteolysis of thyroglobulin
- Increased activity of the iodide pump
- Increased iodination of tyrosine
- Increased size and secretory activity of thyroid cells
- Increased number of thyroid cells
Briefly list the steps in the synthesis of T3/T4. (6)
- Iodide trapping in the thyroid follicular cell via the sodium iodide transporter
- Iodide diffuses through the cell to the luminal side
- Iodide is transported out of the cell and into the lumen via pendrin
- Oxidation of iodide to iodine via hydrogen peroxidase
- Organification of tyrosine amino acids in thyroglobulin via tyrosine peroxidase action
- 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)
- 75% of MIT/DIT molecules are not converted to T3/T4
- Iodine is removed from MIT/DIT via deiodinase enzymes
- This is then recycled to make more T3/T4