Thyroid gland Flashcards
Describe the development of the thyroid
-give week numbers
During week 4 the thyroid diverticulum arises in the midline of the floor of the mouth between the 1st and 3rd branchial arch components of the tongue
Between weeks 4-7 it grows caudally down the thyroglossal tract over the developing larynx to the anterior aspect of the trachea
After week 20 it becomes functional
Describe the development of the parathyroid glands
The inferior glands arise from the 3rd pharyngeal ouch and descend in the neck
The superior glands arise from the 4th pharyngeal pouch but descend less
Gross structure of the thyroid and parathyroid arrangement
Thyroid: Two lateral lobes [like wings] and a central isthmus [lies anterior to 2nd and 4th cartilaginous rings of trachea]
Parathyroid: Superior glands found superior to inferior glands, one of each on each lateral lobe
Blood supply of thyroid
Superior [from external carotid] and inferior [from subclavian] thyroid artery
In short, the arrangement of cells in the thyroid gland
-what hormones do they contain/secrete?
Follicular arrangement of cells - epithelial cells [named follicular cells] arranged in spherical follicles surrounding a lumen into which is stored the hormone precursor thyroglobulin in the colloid, synthesised by the follicular cells
Between the follicles can be found parafollicular cells (C cells) which secrete calcitonin, a calcium lowering hormone
The full pathway by which follicular cells release hormones
TSH from anterior pituitary binds to Gs-protein coupled receptor on basal surface producing cAMP. This process:
1) stimulates the pumping of 1 iodide and 2 sodium into the cell through the NIS protein
2) stimulates production of thyroglobulin
On the apical surface, pendrin transports iodide into the lumen of the follicle, and thyroid peroxidase within the apical membrane iodinates iodide ions to iodine
Iodine then iodinates thyroglobulin
Iodinated thyroglobulin is then endocytosed by the apical membrane and the endosome fuses with lysosomes which digest the iodinated thyroglobulin to T3 and T4
T3 and T4 vesicles are then exocytosed
Follicular cell structure
Cuboidal cells with polarised surfaces (basal and apical)
Extensive RER and prominent Golgi for production and packaging of thyroglobulin
Dense granules containing thyroglobulin visible
Many lysosomes
Many mitochondria for maintenance of the Na/K gradient
Microvilli on apical surface to increase surface area for transport of iodide ions, iodination of iodide ions and for transport of iodinated thyroglobulin
How is the height of the follicular cell related to function??
More columnar hence taller when there is more secretion of T4 and T3 due to greater stimulation from TSH
Enzyme that converts T4 –> T3 in liver and kidney
Type 1 deiodinase
Enzyme that converts T4 and T3 to rT3
Type 3 deiodinase
Which of T3, T4 and rT3 is metabolically inactive?
rT3
Which of T3, T4 and rT3 is metabolically active?
T3
Which of T3, T4 and rT3 is a prohormone?
T4
What are the direct actions of T3?
Acts on a nuclear receptor called the Thyroid Receptor which activates gene expression via hormone response elements of promoter sequences over a few hours
Major contributor to the total energy expenditure of an individual, between 60% and 75%
Which tissues does T3 act on and what are its general effects in adults? Give examples
-think about it’s own production as well!
All tissues
Increases production of proteins which will consequently increase resting energy expenditure, heat production and energy use
Increased production of Na/K pump
Increase production of enzymes for protein breakdown, carb metabolism and lipolysis
Greater cardiac output, heart rate and stroke force due to increased production of myosin, B1 receptors and Ca2+ ATPase
Increased gut motility
Increased bone turnover
NEGATIVE FEEDBACK LOOP - T3 inhibits TRH and TSH secretions from hypothalamus and anterior pituitary respectively - any T4 is converted to T3 by Type 2 deiodinase by the pituitary for this negative feedback to occur
