The Thyroid Gland Flashcards
Describe the shape and position of the thyroid gland
•2 lobes joined by isthmus •“Bow tie” shape and location
•Parathyroid and thyroid are distinct glands
• Lies against and around front larynx
and trachea
• Below thyroid cartilage (Adam’s apple)
• Isthmus joins lobes and extends from 2nd to 3rd rings of the trachea
Describe the embryological development og the thyroid gland
• Thyroid is first endocrine gland to develop.
• At ~3-4 weeks gestation, thyroid gland appears as an epithelial proliferation
in floor of pharynx at base of the
tongue and the takes several weeks to
migrate to final position.
• First descends as diverticulum through thyroglossal duct and migrates
downwards passing in front of hyoid
bone.
• During migration remains connected to tongue by thyroglossal duct which
subsequently degenerates
• Detached thyroid then continues to its final position over following two weeks.
Cysts can form higher up it if it doesn’t mover properly
Describe the histology of thyroid tissue
• Follicular cells arranged
in spheres called thyroid
follicles
• Follicles filled with colloid, a deposit of thyroglobulin
• Colloid is “extracellular” even though it is inside the follicle
What are the cells of the thyroid and parathyroid ad what do they produce
Thyroid follicular cells produce thyroid hormone
Thyroid parafollicular cells produce calcitonin.
Parathyroid principal cells also called chief cells produce Parathyroid hormone
Colloid stores thyroglobulin
Parathyroid principal cells also called chief cells produce Parathyroid hormone
Describe. Teh structure of thyroid hormones
2 tyrosines linked together with iodine at three or four positions on the aromatic rings
In the colloid tyrosines are iodinated
Eg monoiodotyrosine (1 iodine)
Diiodotyrosine (2 indies added)
These 2 added to make
Triiodothyronine
DIT+DIT = T4 = Tetraiodothryonine (thyroxine)
What is thyroglobulin
Thryoglobulin acts as a scaffold on which thyroid hormones are formed
Thyroglobulin protein contains 134 tyrosines, although only a handful of these are actually used to synthesize T4 and T3.
What is thyroid peroxidase
Membrane bound enzyme that regulates 3 separate reactions involving iodide
1) Oxidation of Iodide to Iodine so they can be added to tyrosine (requires the presence of H2O2)
2) Addition of Iodine to tyrosine acceptor residues on the protein throglobulin
3) Coupling of MIT or DIT to generate thyroid hormones within the thyroglobulin protein
How is iodine absorbers
- Dietary iodine reduced to iodide before absorption principally in the small intestine.
- Thyroid hormones and precursors are the only molecules in the human body that contain iodine
- Thyroid gland contains 90-95% of iodine in body
- Iodide (I-), is taken up from blood by thyroid epithelial cells, which have a sodium- iodide symporter or “iodine trap” - balanced by Na+/K+ ATPase
What are common sources of dietary iodine
Dairy products, grains, meat, vegetables, eggs, iodised salt
Give an overview of thyroid hormone synthesis
See slide
What are differences between T3 and 4
Most T4 is converted to T3 outside thyroid
• 90% of thyroid hormone secreted is T4
• Biological activity of T3 is 4 times that of T4
• Most T4 is converted to T3 in liver and kidney
• 80% of circulating T3 is derived from T4
T3 and 4 are transported in blood bound to the
&T protein thyroxine-binding globulin
What cells to thyroid hormones affect
Thyroid hormones effect virtually every cell in the body and have two interconnected responses:
•Effects on cellular differentiation and
development
•Effects on metabolic pathways
Describe the negative feedback loops of thyroidd hormone
Slide
Describe the composition of TSH
• Glycoprotein hormone (carbohydrate groups linked by Glycosylation) composed of 2 non-
covalently bound subunits ( α and β).
• The α subunit is also present in FSH and LH.
• β subunit provides unique biological activity.
What type of receptor does TSH bind to and what are teh pathways this activates
GPCR • Iodide uptake • Iodide oxidation • Thyroglobulin synthesis • Thyroglobulin iodination • Colloid pinocytosis into cell • Proteolysis of thyroglobulin • Cell metabolism & Growth