Thyroid Gland Flashcards
Where does the thyroid gland lie?
Across trachea at base of larynx - not visible or palpable in health
What are the 2 physiologically active forms of the thyroid hormones?
T3 = triiodothyronine T4 = thyroxine
What 2 cells types are in the thyroid? And what do they do?
- C (clear) cells which secrete calcitonin (Ca regulating hormone)
- Follicular cells which support thyroid hormone synthesis and surround hollow follicles.
What are thyroid follicles and what do they contain?
Spherical structures made of follicular cells. Contain 2-3 months supply of TH.
What do Follicular cells do?
- Manufacture the enzymes that make thyroid hormones as well as thyroglobulin. (transported into colloid)
- Actively concentrate iodide from the plasma and transport it into colloid where is combines with tyrosine residues to form the thyroid hormones.
How does iodide enter the follicular cells?
From plasma via Na/I transporter (symport). Against concentration gradient.
Iodine then transported to colloid via pendrin transporter.
What happens during thyroid peroxidase?
Enzymes exocytosed into the colloid, along with thyroglobulin, catalyses the addition of iodide to tyrosine residues on the thyroglobulin molecule.
- What is made when one iodine is added to tyrosine?
2. What is made when a second iodine is added to tyrosine?
- MIT (monoiodotyrosin)
2. DIT (diiodotyrosine)
What happens when MIT and DIT undergo reactions?
MIT and DIT make T3 ( 2 tyrosine and 4 iodine)
DIT and DIT make T4 (2 tyrosine and 3 iodine)
- reaction catalysed by thyroid peroxidase located on the apical membrane of the follicular cells.
What happens to the colloid in response the TSH?
Portions of the colloid are taken back up into follicular cell by endocytosis. They then form vesicles which contain proteolytic enzymes that cut the thyroglobulin to release thyroid hormones.
Are T3 and T4 lipid soluble?
Yes - so they pass the follicular cell membrane into plasma binding to plasma proteins. Both circulate in plasma
What influences the movement of TH?
TSH release from the pituitary. TSH stimulates follicular cells to endocytose colloidal thyroglobulin. TH stored in colloid when not stimulated.
What does Thyroxine Binding Globulin have a particular high affinity for?
T4 releasing it only slowly into the plasma. Accounts for longer half life (6 days) compared to T3 (1 day)
Most TH circulates in the form of what?
Protein bound T4 about 100nmoles/l compared to T3 about 2.3nmoles/l
Where does 90% of TH binding to TH receptors take place?
Inside cells in T3.
