The thyroid gland

Question 1

Describe the development of the thyroid gland

Answer 1

• Arises in the midline at a point later known as the Foramen caecum
• epithelial cells sink downwards, anterior to the hyoid and the larynx
• The thyroglossal duct connects the developing thyroid to the tongue

Question 2

Where is thyrotropic releasing hormone synthesised and secreted?

Answer 2

• Small bodied neurones in the arcuate nucleus and median eminence

Question 3

What suppresses TSH release?

Answer 3

Dopamine and somatostatin

Question 4

What is the main site of action of TSH?

Answer 4

Thyroid follicular cells

Question 5

What are the thyroid hormones and which is more active?

Answer 5

• T4: thyroxine
• T3: Triiodothyronine
• T3 is more active than T4

Question 6

Describe the synthesis and secretion of T3 and T4

Answer 6

• TSH increases the activity of the na/I co transporter on the basolateral surface of the follicular cells, increasing iodine trapping
• Iodine leaves the cell via pendrin and enters the lumen. • Thyroglobulin ins secreted
• Thyroid peroxidase on the luminal surface of the secretory vesicles oxides I- to I
• TSH stimulates the iodination of thyroglobulin in the follicular lumen
• TSH stimulates the conjugation fo iodinated tyrosine to form T3 and T4 linked to thyroglobulin
• Tsh stimulates the endocytosis of iodinated thyroglobulin into the follicular cells from the thyroid colloid
• TSH stimulates the proteolysis of iodinated thyroglobulin, forming T3 and T4 in the lumen of the lysoendosome
• Secretion: TSH stimulates the secretion of T4 and T£ into the circulation

Question 7

What are the deiodinases?

Answer 7

• 5’3’ monodeiodinase

* 5/3 monodeiodinase

Question 8

What buffers the levels of T3 and T4 in the blood?

Answer 8

TBG - thyroxine binding globulin

Question 9

What are the physiological actions of thyroid hormones/

Answer 9

• Increased glucose uptake from the GI tract
• Increased glucose utilisation
• Increased liver glycogenolysis and gluconeogenesis
• Increased lipolysis in adipose tissue increasing free fatty acids
• General increase in protein turnover with net increase in anabolism
• Increase in specific enzymes/membrane proteins/hormone receptors