Thyroid Gland Flashcards
What is the location of the thyroid gland
- Lies across the trachea at the base of the larynx
- Between the C5 and T1 vertabrae
What are the two active thyroid hormones
- T3 (Triiodothyroinine)
- T4 (Thyroxine)
Describe the 2 thyroid cell types
- C (Clear) cells which secrete calcatonin
- Follicular cells which support thyroid hormone synthesis and surround hallow follicles
Describe thyroid follicles
- Thyroid follicles are spherical structures made of follicular cells
- Centre of follicule is made of colloid, which is a stickyglycoprotein matrix contain TH
What are the functions of thyroid follicules
- Manufacture enzymes that make thyroid hormones (Thyroid peroxidase)
- Manufacture thyroglobulin, large protein rich in tyrosine
- Activley concentrate iodide from the plasma and transport it into the colloid
The enzymes are thyroglobulin are packaged into vesicles and transported into the colloid
Describe the transportation of iodine into the colloid
- Enters the follicular cell through the Na/I symporter (this coupling allows iodine to enter against concentration gradiant
- Iodine then enters the colloid via the pendrin transporter on the basolateral surface of the cell
What is the function of thyroid peroxidase and where does it come from
- Thyroid peroxidase is the enzyme that catalyses the addition of iodine to tyrosine
- It comes from the follicular cells
The addition of one iodine to tyrosine creates what
Iodine + tyrosine = Monoiodotyrosine
The addition of two iodine + tyrosine creates what
2 iodine + tyrosine = Diiodotyrosine
The creation of triiodothyronine is the addition of which two compound
MIT (moniodotyrosine) + Diiodotyrosine =
The creation of thyroxine is made by what?
DIT + DIT = Thyroxine
What is the effect of TSH?
- TSH triggers endocytosis of portions of colloid back into the follicular cells
- Within the cells they form vesicles containing proteolytic enzymes that cut the thyroglobulin to release thyroid hormones into circulation
Describe the hormone characteristics of thyroid hormones and the binding
- Lipid soluble amine hormones which easily diffuse across the membrane
- Bind to thyroxine binding globulin to circulate in plasma
Describe the binding and half lives of T3 and T4
- T3 and T4 are both bound to thyroxine binding globulin
- Thyroxine binding globulin has a higher affinty for T4 than T3
- 50x more T4 bound to protein than there is T3
- Because of the higher affinity T4 has a far higher half life than T3
- T4 half life 6 days, T3 half life 1 day
Describe the physiological activity of T3 and T4
Despite being far more T4, T3 is far more active
- 90% of TH binding is done by T3
- T3 is 3-5x more physiologically active than T4
- T4 is deiodinated to T3 by deiodinase enzymes
-
