thyroid gland physiology Flashcards
1
Q
how many glands does the thyroid gland have
A
2
2
Q
how are the 2 glands joined together
A
by narrow portion of the gland called the isthmus
3
Q
where is the gland located
A
over the trachea just below the larynx
4
Q
what are the major thyroid secretory cells
A
follicular cells
5
Q
what do the follicular cells form
A
follicle
6
Q
what are the functional units of thyroid
A
follicles
7
Q
what is the inner lumen filled part of the follicle called
A
colloid
8
Q
what does the colloid store
A
thyroid hormone
9
Q
is the colloid intracellular or extracellular
A
extracellular
10
Q
what is most of the colloid made of
A
thyroglobulin
11
Q
what 2 hormones do follicular cells produce
A
- tetraiodine thyronine (T4) - tri-iodothyronine (T3)
12
Q
what secretory cells are interspersed between follicles
A
C cells
13
Q
what do C cells secrete
A
calcitonin
14
Q
what does calcitonin play a role in
A
calcium metabolism
15
Q
what are taken up from the blood but the follicular cels
A
- tyrosine - iodine
16
Q
what is tyrosine
A
amino acids
17
Q
how do we acquire iodine
A
dietary intake
18
Q
what is dietary iodine reduced to
A
iodide
19
Q
SYNTHESIS AND STORAGE OF THYROID HORMONE
A
SYNTHESIS AND STORAGE OF THYROID HORMONE
20
Q
where does thyroid hormone synthesis take place
A
within the colloid
21
Q
where is thyroglobulin produced
A
by the endoplasmic reticulum of the thyroid follicular cell
22
Q
what happens to tyrosine containing thyroglobulin once it is exported from follicular cells
A
goes into the colloid by exocytosis
23
Q
how does iodide get into the colloid
A
via the iodide pump
24
Q
how does the iodide pump work
A
symporter driven by Na+ / K+ pump
25
what happens in the iodide pump
transports Na+ into the follicular cells and I- into the cell
26
where is iodide mostly in the body
in the thyroid cells
27
what happens to iodide in the follicular cells
is it oxidised to 'active' iodide
28
how is iodide oxidised
by thryoperoxidase (TPO)
29
how does active iodide get into the colloid
exits through a channel in the luminal membrane to enter the colloid
30
once iodide enters the colloid what happens
TPO attaches iodide to a tyrosine
31
what odes attachment of iodide to tyrosine form
monoiodotyrosine (MIT).
32
what does attachment of two iodine to tyrosine form
di-iodotyrosine (DIT)
33
what happens once MIT and DIT are formed
coupling process occur which forms the thyroid hormones
34
1 MIT + 1 DIT forms
tri-iodothyronine (T3)
35
1 DIT + 1 DIT =
tetraiodothyronine (T4) = thyroxine
36
can 2 MIT molecules join
no
37
where do T3 and T4 remain
stored in the colloid until they are split off and secreted
38
SECRETION OF THYROID HORMONE
SECRETION OF THYROID HORMONE
39
what happens to secrete thyroid hormones from the colloid
the follicular cells internalise a portion of the thyroglobulin complex by phagocytosing part of the colloid
40
what happens when the colloid is internalised
T3 and T4 are split by lysosomes into free T3 and T4 as well as MIT and DIT
41
are thyroid hormones lipophilic
yes so can pass the membrane of follicular cells and into the blood
42
are MIT and DIT of value
no
43
what happens to MIT and DIT
follicular cells have an enzyme called iodinate that removes the iodine from MIT and DIT
44
what happens to the free iodine
they are recycled for synthesis of more hormone
45
what is the majority of T3 and T4 transported by
thyroxine-binding globulin
46
is most T3 or T4 free or bound
bound
47
is bound T3 and T4 able to bind to receptors
no only the free form
48
T4 CONVERTING TO T3
T4 CONVERTING TO T3
49
what is secreted more T4 or T3
T4
50
what is more potent T3 or T4
T3
51
what happens to most of T4
it is converted into T3
52
how is T4 activated to T3
it is stripped of one of its iodide outside the thyroid gland
53
where is T4 converted to T3
liver and kidneys
54
what happens when thyroid hormone crosses the plasma membrane
binds to a nuclear receptor bound to the thyroid-response element of DNA - this alters transcription of mRNA
55
what does the receptor have a higher affinity for T3 or T4
T3
56
does thyroid hormone affect all body cells
yes it can in one way or another
57
how dose thyroid hormone affect metabolic rate
increases it
58
what does thyroid hormone do to catecholamines
stimulates them
59
effect of thyroid hormone on cardiovascular status
increase heart rates and force of contraction therefore increase cardiac output
60
is thyroid hormone essential for normal growth
yes
61
does thyroid hormone stimulate growth hormone secretion
yes
62
does excess thyroid produce excess growth
no
63
does thyroid hormone play an essential role in normal development of nervous system
yes
64
REGULATION OF THYROID HORMONE
REGULATION OF THYROID HORMONE
65
what its het most important regulator of thyroid hormone secretion
thyroid stimulating hormone
66
what produces thyroid stimulating hormone
the anterior pituitary
67
what does thyroid stimulating hormone increase
cAMP in the thyrotropes
68
what is every step of thyroid hormone synthesis stimulated by
TSH
69
other than enhancing thyroid hormone secretion what else does TSH have a role in
maintaining the structural integrity of the thyroid gland
70
what happens to the thyroid gland in the absence of TSH
atrophies (decreases in size)
71
how does excess TSH affect the thyroid gland
hypertrophy and hyperplasia i
72
what turns on TSH secretion
thyrotropin releasing hormone
73
what turn off TSH secretion
thyroid hormone
74
how does TRH function via
the IP3/DAG/ Ca2+ pathway
75
what is the only factor that increase TRH secretion
exposure to cold in newborn infants
76
what factors inhibit TSH and thyroid hormone secretion
stress starvation infection
77
ABNORMALITIES IN THYROID FUNCTION
ABNORMALITIES IN THYROID FUNCTION
78
what are the 2 main thyroid disorders
- hypothyroidism - hyperthyroidism
79
how can hypothyroidism occur
- primary failure of thyroid gland - secondary to a deficient TRH, TSH or both - inadequate dietary supply of iodine
80
symptoms of hypothyroidism
- reduced BMR - poor tolerance to cold - gain excessive weight - easily fatigued - slow, weak pulse - slow reflexes - diminished alertness - slow speech - poor memory - puffy appearance in face and hands (myxoedema)
81
what develops if someone has hypothyroidism from both
cretinism
82
what is cretinism characterised by
- dwarfism - mental retardation
83
treatment of hypothyroidism
replacement therapy by administering thyroid hormone
84
what is the most common cause of hyperthyroidism
Graves disease
85
what is Graves disease
autoimmune disease in which the body produces excess thyroid stimulating immunoglobulin (TSI)
86
what does TSI stimulate
secretion and growth of the thyroid
87
does TSI affect the negative feedback loop
nope it has no effect on negative feedback
88
symptoms of hyperthyroidism
- elevated BMR - increased heat production - excessive perspiration - poor tolerance to heat - weight loss - muscle weakness - palpitations - irritable - tense - anxious
89
prominent features of Graves disease
- exophthalmos
90
what is exopthalamos
bulging eyes - the eye may bulge so much that they cannot close their eyes
91
treatment of hyperthyroidism
- antithyroid drugs - surgical removal of a portion of the over secreting thyroid gland - administration of radioactive iodine
92
what happens when radioactive iodine is administered
destroys thyroid glandular tissue
93
GOITER
GOITER
94
what is a goiter
enlarged thyroid gland
95
is a goitre palpable
yes
96
can goitre be present in hypothyroidism and hyperthyroidism
yes
97
when is a goitre present in hypothyroidism
- thyroid gland failure - lack of iodine
98
why is there a goitre in hypothyroidism
as its a negative feedback system there is little thyroid hormone so TSH is being produced this increases the size of the thyroid gland
99
if there is still TSH why may there not be thyroid hormone
due to lack of iodine or enzymes
100
why is there a goitre in hyperthyroidism
- excessive TSH secretion - Graves disease (TSI promotes growth)
101
when would there be hyperthyroidism but no goitre
thyroid tumour
102
why is there no goitre in thyroid tumour
as there is excessive amounts of T3 and T4 it inhibits TSH so there is no input to promote growth of the thyroid
103
look at diagram of anatomy and histology
104
diagram
105
diagram
