PHYS Thyroid Gland - Week 12

Question 1

Describe the HPT axis and its role in controlling T3 & T4 secretion. What stimulates T3 & T4 secretion? What pathway is triggered? What percentage of T3 vs T4 is secreted? What form are T3 & T4 in (e.g., active or inactive and where does conversion occur if req)? State how T3 & T4 travel through circulation.

Answer 1

Cold, stress… -> TRH release from nuclei in the hypothalamus into hypothalamo-hypophyseal portal system @ median eminence -> TRH binds to G-protein coupled receptor @ thyrotroph cells in the ant pit -> TSH production -> TSH binds to receptors on surface of follicle cells -> circulating I- actively co-transported w Na+ across basolateral membrane of epithelial cells -> -vely charged I- ions diffuse into colloid -> oxidised to I2 -> follicular cells produce thyroglobulin (140 tyrosine residues ) -> secreted via exocytosis into the colloid -> iodine attached to rings of tyrosine residues in thyroglobulin as mediated by thyroperoxidase enzyme -> thyroglobulin containing T3/T4 are exocytosed back into follicular cells -> exposure to lysosomes -> T3 & T4 isolated -> secreted into blood (most bound to TBG). -> secretion into general circulation -> binds to receptor on thyroid epithelial cells -> thyroid follicle cells secrete T3 (10%) & T4 (90%).
T3 active. T4 pro-hormone w conversion primarily @ kidney &/ liver. T3 & T4 are largely bound to TBG, but also albumin…very small concentrations exist free.

Question 2

What effects do thyroid hormones have in the body?

Answer 2

• Growth
• Bone maturation
• Tooth maturation & erupation
• Maturation of hair follicles & nails
• Growth & development of the CNS
• Increased alertness & responsiveness to stimuli (e.g., hunger & sound)
• Normal emotional tone
• Increased glucose absorption @ intestines
• Increased glycogenolysis
• Increased gluconeogenesis
• Increase lipolusis
• Increased protein synthesis degradation (net catabolic)
• Increased Na+-K+-ATPase
• O2 consumption
• Increased heat production
• Increased BMR
• Increased CO

Question 3

Thyroxine - T3 OR T4?

Answer 3

T4.

Question 4

In what conditions are antibodies or immunoglobulins present?

Answer 4

Autoimmune thyroid conditions (e.g., Graves’ disease & HT).

Question 5

Describe the feedback loop of the thyroid hormones.

Answer 5

+ve stimuli @ hypothalamus -> increased secretion of TRH -> increased release of TSH @ ant. pit -> increased secretion of T3 & T4 @ thyroid. -ve feedback of T3/4 on ant. pit -> reduced TSH secretion & -ve feedback of T3/4 on hypothalamus -> reduced TRH secretion.