What is the embryological origin of the thyroid gland
It is derived from endoderm from the floor of the pharynx (foramen cecum)
It then descends in front of the gut connected to the thyroglossal duct which later disappears
When does the thyroid reach its final destination
Week 7
The thyroid is composed of
Thyroid follicles (spherical) which are filled with colloid
What are thyroid follicles lined by
Simple epithelium
Simple squamous epithelium
Simple columnar epithelium
Depending on its activity
What is another name for C-cells and what are they derived from
"Parafollicular cells"
Derived from neural crest cells that migrate to the body of the 4th pharyngeal pouch and eventually to the thyroid gland
Thyroid follicular cells secrete
Thyroxine (T4) and Triiodothyronine (T3)
Where is T3 and T4 stored
Colloid in the lumen of the thyroid follicles
Parafollicular cells secrete
Calcitonin
What effect does calcitonin have
It acts on osteoclasts by inhibiting bone resorption, increasing Ca++ absorption, and decreasing Ca++ and PO4- in blood
"CalcitonIN keeps Calcium IN bones"
What is the effect of high Ca+
Increases Calcitonin secretion and decreases PTH secretion
What is the only thyroid cell that has contact with the hormones T3 and T4
Follicular cells
TSH is secreted from what and has what effect
Secreted from the Pituitary gland
It binds to receptors on the basal surface of the follicular cells and triggers the synthesis and secretion of T3 and T4
Thyroglobulin (TGB)
Synthesized in the RER and glycosylated in the golgi, it is then released into the lumen of the thyroid follicular cells via secretory vesicles
How does Iodide get into the cytosol of the follicular cells from the capillaries
It is transported from the capillaries on the basal side by Na/I symporters
(ATP is required)
Peroxidase has what role on the apical membrane of the follicular cells
It oxidizes Iodide to Iodine (I2) which requires the presence of H2O2
Pendrin
Iodide/Chloride transport in the follicular cells on the apical side
What happens to the iodine in the lumen of the follicular cells
It binds to tyrosine residues of TGB to form MIT and DIT
What does coupling of one MIT and one DIT form
T3
What does the joining of two DIT make
T4
How long can the thyroid store hormones
Months (in colloid)
MIT
Monoiodotyrosine
DIT
Diiodotyrosine
MIT and DIT or unique in that
They are reused in the cell
T4 makes up about how much of the released hormone in comparison to T3
80-90%
What happens in the liver to the T4
It is converted to T3
How is the secretion of Thyroid Hormone controlled
By the level of iodine in the blood (low iodine hinders synthesis)
By negative feedback (presence of T3 or T4 stops the thyroid from producing more)
Cretinism
Results in children with a hyposecretion of thyroid hormones due to an Iodine deficiency
Causes retardation and dwarfism
Graves disease
Results from thyroid hyperfunction due to immunological dysfunction. Antibodies bind TSH receptors in the follicular cells elevating levels of T4 and T3
Why is TSH low in Graves disease
Because the high levels of T3 and T4 are shutting the off the release of TSH from the Pituitary gland