The Thyroid Gland Flashcards
What is the function of thyroid hormone generally
Regulate basal metabolic rate
What are the cell types of the thyroid gland
C -cells
Follicular cells
What do C cells do
Secrete CALCITONIN (Ca2+ regulating hormone).
What do follicular cells do
Make the enzymes that make thyroid hormones
Make thyroglobulin, a large protein
Actively concentrate iodide from the plasma and transport it into the colloid
What is the enzyme that make thyroid hormones called
Thyroid peroxidase
What is important about thyroglobulin
Rich in tyrosine residues
What are thyroid follicles
Thyroid follicles are spherical structures whose walls are made of follicular cells.
What is centre of a follicle filled with
Colloid (sticky glycoprotein matrix)
How much thyroid hormone does the colloid contain
Contain 2-3 mths supply of TH (in precursor form)
What the process of concentration of iodine in the follicles
Iodide enters the follicular cells from the plasma via a Na+/I- transporter (symport).
It couples to Na+ enabling the follicular cells to take up iodide against a concentration gradient.
Iodide is then transported into the colloid.
What the name of the transporter of the iodide into the colloid
Pendrin transporter
What does the thyroid peroxidase do once secreted into the colloid
Catalyses the oxidation of Iodide to Iodine
Adds iodine to tyrosine residues
Conjugates MIT and DIT
What is added to tyrosine to create Monoiodotyrosine
1 iodine added
What is added to tyrosine to create diiodotyrosine
2 iodine added
What is made when MIT + DIT
triiodothyronine or T3
What is made when DIT + DIT
tetraiodothyronine (Thyroxine T4)
What stimulates thyroid hormone release into the blood
Thyroid stimulating Hormone
What happens when the thyroid is stimulated by Thyroid stimulating Hormone
Portions of the colloid are taken back up into the follicular cell by endocytosis.
Within the cells they are packaged into vesicles containing proteolytic enzymes that cut the thyroglobulin to release thyroid hormones
Both T3 and T4 are lipid soluble and so pass across the follicular cell membrane into the plasma where they bind to plasma proteins
Why are thyroid hormones bound to thyroglobulin
The hormones are attached to thyroglobulin in order to remove their lipophilic nature and keep them in the follicle
What are the plasma proteins that bind thyroid hormones
Thyroxine-binding globulin.
What kind of hormones are thyroid hormones
Thyroid hormones are amine hormones but act like steroid hormones
What % of of thyroid hormones released from the thyroid gland is normally thyroxine
93%
What % of of thyroid hormonesreleased from the thyroid gland is normally thrioiodine
7%
How do the half life of T4 and T3 differ and why
TBG has a higher affinity for T4 meaning it releases it slowly into the plasma (preventing it from degradation and prolonging its half life)
What thyroid hormone is used in the cells
T3
How is T4 converted to T3
T4 is deiodinated to T3 by deiodinase enzymes
Where T4 is deiodinated to T3 by deiodinase enzymes
- Around half the T4 is deiodinated in plasma,
- The rest is deiodinated inside target cells.
What controls thyroid release
Thyroid releasing hormone from the hypothalamus.
What increases the Thyroid releasing hormone from the hypothalamus.
Cold
Exercise
Pregnancy
What can inhibit Thyroid releasing hormone from the hypothalamus.
Glucocorticoids
Somatostatin (GHIH)
How does glucocorticoid inhibit TH release
Inhibits TSH and conversion of T4 to T3
How does Somatostatin inhibit TH release
Inhibits TSH (TH required for GH action so if we are wanting to stop the effects of GH (purpose of GHIH release) then it makes sense that it also inhibits TSH.
What are the specific functions of thyroid hormones
Raises metabolic rate and promotes thermogenesis
Increase hepatic gluconeogenesis
Net increase in proteolysis
Net increase in lipolysis
Critical for growth -
stimulates GH receptor expression
Essential for brain development in utero
