The Thyroid Gland Flashcards
What does the thyroid gland weigh?
15-20g
What are the 2 physiologically active forms of thyroid hormone?
T3 (thiiodothyronine)
T4 (thyroxine)
What is T3 also known as?
Thiiodothyronine
What is T4 also known as?
Thyroxine
What are the 2 cell types of the thyroid gland structure?
C (clear cells)
Follicular cells
What is the function of C cells?
Secrete calcitonin (calcium regulating hormone)
What is the function of follicular cells?
Support thyroid hormone synthesis and surround hollow follicles
What are thyroid follicle walls made of?
Follicular cells
What is found inside thyroid follicles?
Colloid (sticky glycoprotein) matrix, contains 2 to 3 months supply of TH
How long a supply of TH is contained in thyroid follicles?
2 to 3 months
What do the follicle cells produce?
Enzymes that make thyroid hormones as well as thyroglobulin (large protein rich in tyrosine residues)
What happens to the enzymes and thyroglobulin that is produced by the follicle cells?
- packaged into vesicles
- exported from follicular cells into the colloid
As well as producing enzymes and thyroglobulin, what else do follicular cells do?
- concentrate iodide from plasma
- transport it into the colloid
- where it combines with tyrosine residues to form thyroid hormones
Where is iodide and tyrosine derived from?
Diet
What forms together to form the thyroid hormones?
Iodide and tyrosine residues
Explain the process of iodide entering the follicular cell?
enters follicular cell from plasma via Na+/I- transporter (symport):
- coupling to Na enables follicular cells to take up iodide against a concentration gradient
- iodide is then transported into the colloid via the pendrin transport
What transporter is used to transport iodide into colloid?
Pendrin transporter
What is thyroid peroxidase also known as?
Thyroperoxidase
Explain the process of iodide and tyrosine residues forming thyroid hormones?
Enzymes catalyse addition of iodide to tyrosine residues on the thyroglobulin molecule (process in which iodide loses an electron to become iodine)
What is formed from the addition of one iodine to tyrosine?
MIT (monoiodotyrosine)
What is formed from the addition of a second iodine to tyrosine (one iodine added to MIT)?
DIT (diiodotyrosine)
From MIT/DIT, what forms T3?
MID + DIT
From MIT/DIT, what forms T4?
DIT + DIT
What enzyme catalyses the formation of thyroid hormones and where is it located?
Thyroid peroxidase located on the apical membrane of follicular cells
How much tyrosine and iodine forms T4?
2 tyrosine and 4 iodine
How much tyrosine and iodine forms T3?
2 tyrosine and 3 iodine
What is the thyroid glands response to TSH?
- portions of colloid are taken back up into follicular cells by endocytosis
- within cell they form vesicles which contain proteolytic enzymes that cut the thyroglobulin to release the thyroid hormones
- T3 and T4 are lipid soluble so pass across follicular membrane into plasma where the bind with plasma proteins, mainly thyroxine-binding globulin
What is release of TH into the plasma under the influence of?
TSH released from pituitary
When not stimulated, TH are stored in colloid
Does thyroxine binding globulin (TBG) have a greater affinity for T3 or T4?
T4
- releasing it slowly into the plasma, partly accounting for the longer half life of T4
What does TBG stand for?
Thyroxine binding globulin
What is the half life of T3 and T4?
T3 - about 1 day
T4 - about 6 days
Does all TH exert an inhibitory effect on TSH and TRH?
No, only free TH that is not bound to proteins
Is most circulating TH T3 or T4?
- T4: 100nmol/L
- T3: 2-3nmol/L
Is most binding of TH to TH receptors done by T3 or T4?
- T4 is 50x greater in concentration than T3
- 90% of TH binding to TH receptors inside cells is T3
- TH receptor has a higher affinity for T3
- T3 is more physiologically active)
Does TH receptor have a greater affinity for T3 or T4?
T3
making T3 3-5 times more physiologically active than T4
T4 can be deiodinated to T3 by what enzymes?
Deiodinase enzymes
What happens to circulating T4 that is not used?
50% is deiodinated in plasma, the rest is deiodinated inside target cells
For regulation of thyroid hormone release, what things stimulate continuous secretion due to the release of TRH from hypothalamus?
Cold, exercise, pregnancy
What are some functions of TH once it binds to receptors in target cells?
change transcription and translation to alter protein synthesis
- raises metabolic rate and promotes thermogenesis
- increases hepatic gluconeogenesis
- (although no effect on BG as pancreas is releasing adequate insulin therefore not a counter-regulatory hormone)*
- net increase in proteolysis
- net increase in lipolysis
- critical for growth -> stimulates GH receptor expression
- essential for brain development in utero (maternal iodine deficiency = congenital hypothyroidism)
For regulation of thyroid hormone release, what things are inhibitory to the continuous secretion of TH?
Glucocorticoids and somatostatin:
- somatostatin inhibits TSH (TH required for GH action)
- glucocorticoids inhibit TSH and conversion of T4 to T3
why is thyroid hormone not a glucose-counter regulatory hormone as it increases hepatic gluconeogenesis
no effect on BG as pancreas releases adequate insulin
