Physiology - Endocrine Pt 2 (Thyroid gland)

Thyroid hormone synthesis STEPS

1. tyrosine - containing thyroglobulin produced by the ER/golgi apparatus in the thyroid follicular cells are transported into the colloid via exocytosis

2. iodide is carried by secondary active transport from the blood into the colloid by symporters in the basolateral membrane of the follicular cells

3. in the follicular cell, iodide is oxide to active form by thyroperoxidase at the luminal membrane

4. The active iodide exits the cell through a luminal channel to enter the colloid

5. Catalysed by thyroperoxidase, attachment of one iodide to tyrosine within the thyroglobulin molecule yield one MIT, attatchment of two iodide to tyrosine yields DIT.

6. Coupling of one MIT and one DIT yields T3, coupling of two DITS yields T4

7. On appropriate stimulation, the thyroid follicular cells engulf a portion of tg-containing colloid by phagocytosis

8. lysozomes attack engulfed vesicles and split iodinated products from thyroglobulin

9. T3 and T4 diffuse into the blood (secretion)

10. MIT and DIT are deiodinated, freed iodide is recycled for synthesizing more hormone

Thyroid hormone EFFECTS

• Provides substrates for oxidative metabolism

• Provides adequate nutrients for ATP synthesis and protein synthesis

• increases rate of metabolism

• calorigenic affect = increased metabolic activity results in increased heat production

• increases heart rate and force of contraction

• increases target-cell responsiveness to catecholamines (sympathomimetic)

• essential for normal gorwth and development

• myeline and synapse formation requires thyroid hormone

• regulates microtubule assembly

Name the two catecholamines secreted by the adrenal medulla

and describe how they are stored and released.

epinephrine and norepinephrine, stored in chromaffin graules and released via exocytosis on stimulation of post gangliogonic sympathetic fibers

Discuss the major hormonal changes and the purposes served by each change during the stress response

1. increased epinephrine reinforces sympathetic nervous system “fight or flight response”, mobilises energy stores (increase blood glucose and blood fatty acids)

2. increased CRH-ACTH-Cortisol levels, mobilise energy stores and metabolic building blocks (increase blood glucose, fatty acids, amino acids)

3. increased Glucagon and decreased insulin, to increase blood glucose and fatty acids

4. increased Renin-angiotensin aldosterone system and vassopressin : conserve salt and water, ATII + ADH, cause arterial vasoconstriction