Thyroid gland Flashcards
What does hypo release which acts on pituitary
TRH
What does pituitary release which acts on THG
TSH
Thyroid glands
C cells release calcitonin which regulates Ca2+ homeostasis
THG releases thyroid hormones
Thyroid hormones
Thyroxine (T4) Tri iodothyronine (T3)
What’s required for action of THG
Dietary iodine
Thyroid hormones action
Act on nuclear receptors to regulate gene transcription (TRE’s- thyroid response element) in target genes
Effect on metabolism
protein (lipid, carbohydrates) metabolism
Leading to growth
Body weight effects
Regulated BMR
THG structure
L and R lobes
Well vascularised
Has thyroid follicles which store thyroglobulin (tyrosine residues). This is stored in the centre of follicles, and the follicle cavities are surrounded by epithelial cells , where synthesis of hormones takes place.
Core contains protein rich element: colloid
C cells adjacent to follicles
Blood vessel capillaries directly associated with follicle cells, allowing direct release of hormones into the blood
TH synthesis
Iodination of tyrosine occurs at 2 sites either to form MIT (mono-iodotyrosine) or DIT (di-iodotyrosine)
Then there is coupling of MIT and DIT to make T4/T3
Tyrosine residues bolted together to form Thyrogobulin
T4
thyroxine
DIT +DIT
T3
Triiodothyronine
DIT +MIT
What happens after formation of T3 and T4
Iodinated thyroglobulin enters lumen (colloid) by exocytosis, from epi follicle cells where it is synthesised
Stored thyroglobulin re-enters follicle cells by endocytosis
Lysosomal enzymes release T3 and T4
MOst are bound by binding proteins in plasma
‘Free’ fraction of T3 and T4 can enter target tissues
Inside the follicle cells
Tyrosine residues form thyroglobulin
Iodine taken up by iodide transporters into cells- cotransported with NA ions
Peroxidases iodinate tyrosine residues under influence of thyroid peroxidase
When you need thyroid hormone it is endocytosed back into the cell where T3 and T4 are liberated by lysosomal digestion (from thyroglobulin) and released into the blood
Remaining tyrosine residues recycled (ie make throglobulin again)
Whole process requires TSH to be released fro pituitary- lysosomal digestion occurs under influence of TSH
TRH (Hypo) under influence of
GH (-ve)
Cold (+ve)
Sympathetic activation (+ve)
Cortisol (-ve)
TSH (anterior pituitary) under influence of
TRH (+ve)
Somatostatin (-ve)
Incr TSH causes
Incr I uptake, incr T3 and T4 synthesis in thyroid gland- released into blood