Thyroid gland Flashcards
1
Q
What is the structure of the thyroid gland?
A
Two lobes, right and left
Joined together in the middle by the isthmus
2
Q
What is the pyramidal lobe?
A
Extends upwards from the middle of the thyroid
3
Q
How many people have the pyramidal lobe?
A
Only a few
4
Q
What does the thyroid gland lie in front of?
A
The larynx and the trachea
5
Q
What does the thyroid gland lie below?
A
Thyroid and cricoid and cartilage
6
Q
What does the thyroid gland lie above?
A
Suprasternal notch
7
Q
What does the isthmus lie in front of?
A
2nd - 3rd rings of cartilage of trachea
8
Q
What is the pretracheal fascia?
A
Fascia that joins the thyroid gland
to the trachea and larynx
9
Q
Why does the thyroid gland moves upward on swallowing?
A
Because the larynx moves up when swallowing
pulls on pretracheal fascia
pulls up thyroid gland
10
Q
How many parathyroid glands are there?
A
Four
11
Q
Where are the parathyroid glands located?
A
Behind the thyroid gland
12
Q
What are thyroid follicles made up of?
A
Follicular cells
arranged in spheres
Extracellular colloid within the spheres
Parafollicular cells located between follicular cells
13
Q
What do follicular cells do?
A
Produce thyroglobulin, thyroid hormones
14
Q
What does the colloid store?
A
Thyroglobulin
15
Q
How do the follicular cells transport thyroglobulin into the colloid?
A
Exocytosis on their apical domain
16
Q
What do parafollicular cells produce?
A
Calcitonin
17
Q
What does calcitonin have a role in?
A
Calcium homeostasis
18
Q
What is the histology of the parathyroid gland?
A
Principle/chief cells
19
Q
What do principle/chief cells do?
A
Produce parathyroid hormone
20
Q
What happens to iodine in the GI tract?
A
Reduced to iodide I-
21
Q
What happens to iodide in the GI tract?
A
Taken up by epithelial cells in the small intestine
Absorbed into blood
22
Q
Where is iodine found in the body?
A
Mostly in the thyroid gland
23
Q
Which molecules of the body contain iodine?
A
monoiodotyrosine, MIT
diiodotyrosine, DIT
triiodothyronine, T3
thyroxine, T4
24
Q
What is MIT made up of?
A
Tyrosine
One iodine attached to its aromatic ring
25
What is DIT made up of?
Tyrosine
| Two iodines attached to its aromatic ring
26
What is T3 made up of?
MIT and DIT joined together
| Two tyrosines, three iodines
27
What is T4 made up of?
Two DIT molecules joined together
| Two tyrosines, four iodines
28
How is iodide taken up by follicular cells?
By the sodium iodide trap
29
Where is the sodium iodide trap located on follicular cells?
Basolateral domain
30
What happens to iodide in the follicular cells? What type of reaction is this?
I- -----> I2
| oxidation
31
What enzyme catalyses the reaction
I- -----> I
in the follicular cells?
Thyroid peroxidase
32
What additional molecule is required to be present for the oxidation of iodine?
Hydrogen peroxide H2O2
33
What happens to iodine in the follicular cells?
It diffuses into the colloid
34
What happens to iodine and thyroglobulin in the colloid?
Thyroglobulin tyrosine residues are iodinated
| forming MIT and DIT on thyroglobulin
35
What enzyme catalyses the iodination of thyroglobulin in colloid?
Thyroid peroxidase
36
Thyroid peroxidase catalyses oxidation, iodination and....
Coupling
37
What happens in the coupling reaction?
MIT and DIT are joined together
to form T3 and T4
on thyroglobulin
38
How do follicular cells take up thyroglobulin from the colloid?
Pinocytosis in the apical domain
39
What is the pinocytosed thyroglobulin contained in inside the follicular cell?
Vesicle
40
What happens to the vesicle containing thyroglobulin?
Lysosome fuses with it
41
What happens to the thyroglobulin inside the phagolysosome?
It is digested by proteases
| releasing T3 and T4
42
What happens to the isolated T3 and T4 in the follicular cells?
Diffuse across the basolateral domain of the follicular cells
enter the blood
43
Which is secreted into the blood more - T3 or T4?
T4
44
Why do T3 and T4 not dissolve in the blood?
Are hydrophobic
| Insoluble in water
45
How are T3 and T4 carried in the blood?
By binding to thyroxine-binding globulin!
46
Which is more biologically active - T3 or T4?
T3
47
Where is T4 converted into T3?
Liver
| Kidneys
48
Where are thyroid hormone receptors located?
Inside the cell
| Inside the nucleus - are nuclear receptors
49
How does thyroid hormone enter cells?
Through thyroid hormone transporters
50
How does thyroid hormone enter the nucleus?
Through the nuclear pores
51
Where is the thyroid hormone receptor inside the nucleus?
Is pre-bound to specific DNA sequences
52
What are the specific DNA sequences that the thyroid hormone receptor is pre-bound to? What are they called?
Promoter region of thyroid regulated gene
| Thyroid hormone response elements
53
How does the thyroid hormone receptor affect the thyroid regulated gene?
It represses transcription of the gene
54
When thyroid hormone binds its receptor, what happens to the receptor?
Undergoes conformational change
| Becomes transcriptional activator
55
How does the changed thyroid hormone receptor affect the thyroid regulated gene?
Relieves its repression
| Results in the gene being transcribed, translated, protein produced
56
What are some examples of proteins produced by thyroid hormone activated genes?
Ca2+ ATPase
Na+ K+ ATPase
Cytochrome oxidase
PEPCK
57
How do thyroid hormones affect cells?
Affect cell differentiation
cell development
both linked to metabolism
58
What are the general effects of thyroid hormone?
Increase in BMR, heat production
Stimulation of metabolic pathways
Sympathomimetic effects
59
How does thyroid hormone increase BMR, heat production?
Increasing number AND size of mitochondria
Stimulating synthesis of enzymes in electron transport chain
60
Which tissues do not undergo and increase in BMR, heat production by thyroid hormone?
Brain
Spleen
Testes - for sperm production
61
What general type of metabolic pathway do thyroid hormones tend to stimulate?
Catabolic
62
How does thyroid hormone affect carbohydrate metabolism?
Increase in GLUT4 transporters, insulin dependent entry of glucose
Gluconeogenesis
Glycogenolysis
63
How does thyroid hormone affect lipid metabolism?
Stimulates lipolysis
B-oxidation of fatty acids
64
How do thyroid hormones have sympathomimetic effects?
Increase catecholamine receptors on cells
| larger response to catecholamines
65
Which tissues are specifically affected by thyroid hormone?
CVS
| Nervous sustem
66
How is the CVS affected by thyroid hormones?
Increased response to catecholamines gives...
-increased heart rate
-increased force of contraction
=increased cardiac output
-peripheral vasodilation
to carry extra heat to body surface
67
How is the nervous system affected by thyroid hormones?
Required for development and function of neruones
| e.g. their myelination
68
How is the release of thyroid hormone regulated?
HPT axis
69
Outline the HPT axis
Hypothalamus releases....
Thyrotropin releasing hormone (TRH) .... which stimulates....
Anterior pituitary ...which releases....
Thyroid stimulating hormone ...which stimulates...
Thyroid gland ...which releases...
T3 and T4 ...which inhibit the anterior pituitary, inhibit the hypothalamus - long loop
TSH inhibits the hypothalamus via the short loop
70
What type of hormone is TSH?
Glycoprotein hormone
71
What is the structure of TSH?
Two subunits
a and B
joined by covalent bonds
72
What other hormones is the a subunit present in?
FSH
| LH
73
What does the B subunit confer?
Specifity
74
What type of receptor is the TSH receptor?
GPCR
75
What G proteins is the TSH GPCR coupled to? What are the final outcomes of these G proteins?
Gas - gives PKA
| Gaq - gives PKC, Ca2+ increase
76
What is the outcome of TSH binding to its receptor?
Stimulate thyroid hormone synthesis and release
77
How exactly does TSH binding to its receptor stimulate thyroid hormone synthesis and release?
```
Iodide uptake
Iodide oxidation to iodine
Thyroglobulin synthesis
Thyroglobulin iodination
Thyroglobulin pinocytosis
Thyroglobulin proteolysis
```
78
Why might free T3 and T4 levels show variation that's not due to disease?
Factors to do with method
Changes in albumin levels - also carries thyroid hormones in blood, but to lesser extent
Dilution effects
79
Why might free TSH levels show variation that's not due to disease?
Variation depending on time of day
Pulsatile secretion
Medications
80
What is the first endocrine gland to develop in the embryo?
Thyroid gland
81
When does the thyroid gland begin to develop?
3-4 weeks
82
How does the thyroid gland first appear?
Epithelial proliferation
83
Where does the thyroid gland first appear?
Floor of pharynx
| Base of tongue
84
What direction does the developing thyroid gland move in?
Descends downwards
85
What does the developing thyroid gland descend as?
Thyroid diverticulum
86
What does the descending thyroid gland move through?
The thyroglossal duct
87
How does the developing thyroid gland move in relation to the hyoid bone?
In front of the hyoid bone
| Then behind the hyoid bone
88
What happens to the thyroglossal duct?
It degenerates
89
What is lingual thyroid?
Remnant of thyroid tissue on tongue
90
What is a thyroglossal duct cyst?
Remnant of thyroglossal duct
| Shows up as swelling in anterior neck
91
How does a thyroglossal duct cyst move when the tongue us protruded? Why?
Moves inwards and upwards
| because is connected to tongue
92
How many cells in the body does thyroid hormone affect?
All of them!
93
Where is a thyroglossal duct cyst located?
Near hyoid bone
| Always in the midline
