Thyroid Hormones

Question 1

Thyroid Gland

Answer 1

Produces hormones essential for growth, development and metabolism
Below larynx and above trachea
Secretes T4 (thyroxine) and T3 (triiodothyronine) via follilcles
- Secretion controlled by TSH which is secreted by anterior pituitary
- Secretes calcitonin

Question 2

Follicle

Answer 2

Functional unit

- Lined with cuboidal epithelial cells

Question 3

Colloid

Answer 3

What the follicle is filled with; secretory substance

- Thyroglobulin, glycoprotein containing thyroid hormones

Question 4

Thyroxine (T4)

Answer 4

Most abundant
Converted to T3
Four iodinations on tyrosine
4-12 mg/dL

Question 5

Triiodothyronine (T3)

Answer 5

Actual "active" form
Has three iodinations on tyrosine
3 times more potent than T4
Shorter half life
78-195 ng/dL

Question 6

Iodine Requirement for Formation

Answer 6

1 mg/week
Absorbed from GIT and rapidly excreted by kidneys
Iodide pump: traps circulating iodide via active transport –> sodium-iodide symporter –> concentrates iodide ions in blood
Regulated by concentration of TSH

Question 7

Formation of Thyroglobulin

Answer 7

ER and Golgi synthesize and secrete into the thyroid follicles
~70 tyrosines –> combines with iodine to form hormones which are stored as colloid
One T3 for every 17 T4
- Stored for 2-3 months (defect won’t be detected early)

Question 8

Oxidation of Iodide Ion

Answer 8

Essential in forming thyroid hormones

• Enables coupling to the tyrosine
• Oxidation facilitated by peroxidase and H2O2
• Oxidized iodine binds to tyrosine
• Binding of iodidne with thyroglobulin is called organification of thyroglobulin

Question 9

T3 and T4 formation

Answer 9

Tyrosine is iodized to MIT and then to DIT
T4 is formed by coupling two DIT molecules
T3 is formed by one MIT and one DIT
- Remain in throglobulin until taken up via pinocytosis and cleaved by enzymes in vesicles
- MIT and DIT do not enter circulation –> cleaved by deiodinase enzyme –> iodine is recycled

Question 10

T3 and T4 secretion

Answer 10

Pinocytosis: apical membrane sends out a little projection and pinches off a droplet of colloid and takes it within the cell –> digestion cleaves off T3 and T4 –> Released into circulation
– Hypothyroidism with a lack of deiodinase enzyme

Question 11

T3 and T4 transport

Answer 11

Binds to plasma protein TBG
T4 with greater affinity
Bind to intracellular proteins in the target cells for storage, thus response is slow in onset with long duration of action (T3 mroe rapid)

Question 12

Thyroid Hormone activation of target cells?

Answer 12

T3 and T4 readily diffuse
- T4 –> T3 –> nuclear reveptor –> dimer formation with throid hormone receptor and retinoid X receptor –>. binds to thyroid hormone response element of the DNA –> causes transcription/translation of specific genes

Question 13

Function of Thyroid Hormone

Answer 13

Bind to intracellular receptor and increase transcriptiono f target genes
Increase BMR 60-100% above normal (increases number & activity of mitochondria in cells and active transport of ions through membrane – ATPase) –> increase in appetite and food intake but decreased body weight

Question 14

Hyperthyroid

Answer 14

Excessive hormone

Decrease weight

Question 15

Hypothyroid

Answer 15

Decreased hormones

Increased weight

Question 16

Increase in metabolism?

Answer 16

Increase in by-products to be eliminated from body
Vasodilation and increased blood flow (increased CO)
Increased rate and depth of respiration due to greater oxygen use and formation of CO2