Thyroid Gland Flashcards
(175 cards)
1
Q
Develops as an endodermal outgrowth or an evagination from the midline of the floor of the pharynx, between tuberculum impar and copula
A
Thyroid gland
2
Q
The thickening becomes what duct
A
Thyroglossal duct
- wc elongates and later be bilobed
3
Q
The duct becomes a solid cord and migrates down the neck passing thru the developing
A
Hyoid bone
4
Q
Site of origin of the thyroglossal duct on the tongue remajns as a pit called the
A
Foramen cecum
5
Q
Thyroid gland location
A
Below larynx on either side of and the anterior to trachea
6
Q
Thyroid gland and relation to larynx, trachea, esophagus
A
Below larynx on either side
Anterior to trachea
Trachea lies inferior to larynx, esophagus is posterior to it
Surgical removal of thyroid may lead to injury of these organs
7
Q
Thyroid gland and relation to parathyroid glands
A
Parathyroid glands - embedded posterior to thyroid gland, superior & inferior pole
Inadvertent removal during thyroidectomy causes hypoparathyroidism
8
Q
Thyroid gland and relation to recurrent laryngeal nerve
A
These bilateral branches of the vagus nerve pass posteromedially along the groove between the esophagus and the trachea
Tumor invasion or injury results to hoarseness
9
Q
Thyroid gland and relation to neck muscles & great vessels
A
Common carotid artery lies immediately lateral to the thyroid
Tumor invasion or injury may cause injury to any or all adjacent structures
10
Q
Relation to lobes: ANTEROLATERALLY
A
The sternothyroid, superior belly of omohyoid, and anterior border of the sternocleidomastoid
11
Q
Relation to lobes: POSTEROLATERALLY
A
Carotid sheath w the common carotid artery, the internal jugular vein, and vagus nerve
12
Q
Relation to lobes: MEDIALLY
A
Larynx, trachea, pharynx and esophagus
Associated w these are cricothyroid muscle and its nerve supply, external laryngeal nerve
Between esophagus & trachea = recurrent laryngeal nerve
Rounded posterior border of each lobe is posterior to the superior and inferior parathyroid glands and the anastomosis between the superior and inferior thyroid arteries
13
Q
Relations of the Isthmus
A
Anteriorly: the sternothyroids, sternohyoids, anterior jugular veins, fascia, and skin
Posteriorly: the second, third, and fourth rings of the trachea
The terminal branches of the superior thyroid arteries anastomose along its upper border
14
Q
Thyroid gland is firmly attached to the laryngotracheal skeleton via the
A
Visceral or pretracheal fascia
15
Q
When swallowing, what moves? Thyroid masses or brachial cysts and dermoid cyst
A
Thyroid masses
16
Q
What lvl of the thyroid does the base lie?
A
4th and 5th tracheal ring
Others sources: attached to travhea from the midlvl of thyroid cartilage to the 5th and 6th travheal cartilage
17
Q
Thyroid wt
A
Approx 15-20g
Some source: 16 +/- 6 g (10-22)
18
Q
Each lobe is approx what thickness and length
A
2 to 2.5cm in thickness and width at its largest diameter and 4cm length
19
Q
Thyroid lobes are connected by a narrow bridge of tissue called
A
Isthmus
20
Q
A vascular organ surrounded by a sheath derived from the
A
pretracheal layer of deep fascia
- sheath attaches the gland to the larynx and trachea
21
Q
Isthmus extends across the midline in front of the
A
2nd, 3rd or 4th tracheal rings
22
Q
A pyrimidal lobe projects upward from the isthmus, usually to
A
the left of the midline
23
Q
A fibrous or muscular band frequently connects the pyramidal lobe to the hyood bone; if it’s musculare it is referred to as
A
Levator glandulae thyroideae
24
Q
Abnormalities in the pattern of thyroid dev, s/a persistence of this tract leads to a
A
thyroglossal duct cyst
25
Protrusion of tongue causes movement of thyroglossal duct cyst as opposed to
True thyroid masses (e.g goiter) wc move on deglutition
26
Fxnal unit of thyroid gland
Follicle (acinus) surrounded by a rich capillary plexus
27
The ht of the follicular epithelium varies w the degree of stimulation of
TSH
28
The ht of the follicular epithelium varies w the degree of stimulation of
TSH
29
Glandular epi veries w the degree of stimulation
Columnar: active
Flat: inactive
30
Lumen of the follicle filled w a clear, amber proteinaceous fluid called
Also a major constituent of thyroid mass
Colloid
31
Extend to colloid from the apical (adluminal) border, wc is the site od iodination rxn
Microvilli
32
Also occurs in the apical border
Initial phase of thyroid hormone secretion (i.e resorption of the colloid by endocytosis)
33
Parafollicular (C) cells secrete
Do not border on the follicular lumen
Calcitonin
34
Major constituent of colloid is the large glycoprotein that contains thyroid hormones within its molecule
Thyroglobulin
35
When the gland is inactive
The colloid is abundant
Follicles are large
Cells lining them are flat
36
When gland is active
Follicle are smol
Cells cuboid or columnar
Edge of colloid is scalloped, forming many smol "reabsorption lacunae"
37
Thyroid gland arteries
Superior thyroid artery
Inferior thyroid artery
Thyroidea ima
38
A branch of the external carotid artery
Descends to the upper pole of each lobe, accompanied by external laryngeal nerve
Superior thyroid artery
39
A branch of the thyrocervical trunk, ascends behind the gland to the lvl of the cricoid cartilage
It then turns medially and downward to reavh the posterior border of the gland
The recurrent laryngeal nerve crosses either in front or behind the artery or pass between its branches
Inferior thyroid artery
40
If present, may arise from the brachiocephalic artery or arch of aorta
Ascends in front of trachea to the isthmus
Thyroidea ima
41
Thyroid gland veins
Superior thyroid - drains into internal jugular vein
Middle thyroid - drains into internal jugular vein
Inferior thyroid - receives its tributaries from the isthmus and the lower poles of the gland
42
Veins of the two sides anastomose w one another as they descend in front of the trachea
They drain into the left brachiocephalic vein in the thorax
Inferior thyroid vein
43
Lymphatic drainage of thyroid gland
Mainly laterally into the deep cervical LN
| Few descend to the paratracheal nodes
44
Thyroid innervation
Both adrenergic and cholinergic NS via fivers arising from the cervical ganglia and vagus nerve, respectively
45
Afferent fibers pass thru the laryngealnerves and regulate an
Active vasomotor system
46
One fxn of neurogenic stimuli is to
regulate blood flow of thyroid
47
Major secretory product of thyroid gland is
3,5,3',5' - tetraiodothyronine (thyroxine) T4
48
Other thyroid hormone
T3
| 3,5,3'-triiodothyronine
49
Molar activity
T3 to T4
| 3-5:1
50
Secretory ratio
T4 to T3
| 10-20:1
51
Plasma conc ratio
Free T4 to free T3
| 2:1
52
Most T3 in plasma is derived from the monodeiodination of T4 by action of
Monodeiodinase (5'-deiodinase)
| - found in peripheral tissues
53
Biologically inactive thyronine formed by peripheral conversion catalyzed by 5-deiodinase
rT3
54
Each thyroglobulin molecule contains approximately how many tyrosine residues?
120
55
Thyroid accumulates what from the plasma?
Inorganic iodide
56
In the US, Daily dietary iodine intake is about
500ug
57
How much iodide is required per week to maintain euthyroidism?
1mg of iodide or 150ug/day
58
Thyroid gland stored enuff thyroid hormone to maintain a euthyroid state for how many months w/o hormone synthesis?
3 months
59
Throid gland contains how much iodide?
5-7mg iodide
60
2/3 of total iodide content in the colloid is in the form of
biologically inactive iodothyrosines
61
1/3 of the colloid iodide content is in form of
biologically active thyronines t3t4
62
Also formed in the peripheral tissues by deiodination of T4
T3
63
More active hoe
T3 ayt
64
The naturally occurring forms of T4 and its congeners with asymmetric C atom are the
L isomers
65
Has only smol fraction of the activity of D form
D-thyroxine
66
Smol amts of rT3 are also found in
Thyroid venouse bld
67
Thyroglobulin glycoprotein is made up of two subunitsand has a MW of
660,000
10% carbs by wt
123 tyrosine residues (4-8 lang incorporated)
68
3 fxns of thyroid cells
1. Collect abd transport iodine
2. Synthesize Tg, secrete it into the colloid
3. Remove thyroid hormones from Tg and secrete to circ
69
The normal plasma iodine lvl
0.3ug/dL
70
How much iodine enter the thyroid at normal rates?
120ug/d
71
Thyoid secretes how much iodine in T3 and 4
80ug/d
72
How much iodine per day diffuses into the ECF?
40 micrograms
73
The secreted t3&4 are metabolized in the liver and other tissues, w the release of how much in the ECF
60ug/d
74
Net loss of iodine in the stool
20ug/d
75
Normal thyroid iodide to plasma iodide conc ratio
25-40:1
76
Biosynthetic steps that are stimulated by TSH
1. Iodine uptake (Iodide Trapping)
2. Oxidation of iodide; the role of thyroperoxidase
3. Iodination of Tyrosine (Organification)
4. Coupling (Condensation)
77
Luminal iodide to follicular cell iodide conc ratio
5:1
78
A useful therapeutic index of the fxnal statys of the thyroid gland
Radioactive iodide uptake by the thyroid gland
79
A 24-hr uptake normally ranges between
10-35% of the administered dose
80
In a nirmal gland, the iodide pump concentrates the iodide to abt
30x its conc in bld
81
Active iodide forms
Iodinum ion (I+)
A free radixal of iodine (IO3-)
Iodine (I2)
82
Denotes tge addition of iodide to the tyrosyl residues of thyroglobulin
Iodination of active iodide
83
Substrate for iodination
Thyroglobulin
84
Requires the fusion of two diiodotyrosine molecs
T4 synthesis
85
Requires the condensation of monoiodotyrosine with a diiodotyrosine
T3 synthesis
86
Also mediates the coupling rxn
Thyroid peroxidase
87
The release of hormones involves the following rxns
1. Hydrolysis of Tg by thyroid protease and peptidases
2. Secretion of iodothyronines
3. Pseudopod extensions --> pinocytic vesicles
4. 3/4 of iodinated tyrosine in the Tg never become thyroid hormones but remain MIT abd DIT
88
Freed from the Tg but they're not secreted into the bld, instead their iodine is cleaved from them by a deiodinase enz (iodotyrosine deiodinase)
Iodinated tyrosines
89
99.95% of T4 is bound to
Plasma proteins
90
Binds about 75% of plasma T4
TBG (Thyroxine-binding globulin)
91
In normal indivs, less than half of the available binding sites on TBH are saturated with
T4
92
Binds abt 15-20% of the circulating T4
Thyroxine-binding prealbumin (TBPA or transthyretin)
93
Abt 9% of T4 is bound to
Albumin
94
Almost all (99.5%) of T3 is transported bound to
TBG
95
Very little T3 us bound to
Albumin
96
None is bound to
TBPA :
97
Thyroid hormone is metabolized by
Deiodination
Deamination
By conjugation w glucoronic acid
98
Is secreted via the bile duct into the intestine
Conjugate
99
Catalysing the formation of T3 and rT3 (inactive)
5'-deiodinase
100
In the liver, T4 & T3 are conjugated to form
Sulfates and glucoronides
101
The thyroid conjugates are
Hydrolyzed
Reabsorbed (enterohepatic circ)
Excreted in the stool
102
TSH secretion is infuenced by 4 factors:
1. TRH secretion from the median eminence
2. Bld lvl of unbound T4
3. Bld lvl of unbound T3 generated by peripheral conversion of T4 to T3
4. Peripheral conversion of T4 to T3 within the pituitary gland
103
A tripeptide synthesized by the parvicellular peptidergic neurons in the hypothalamus
TRH
104
Plays a major role in negative feedback in pituitary lvl
T3
105
High conc of intrathyroidal inorganic iodide lead to
Inhibition of thyroid release
106
High conc of organic iodide (thyroid hormone) lead to
Decrease in iodide uptake
107
cAMP synthesis
TSH receptors -activate-> adenylcyclase -to form-> cAMP -activate-> protein kinase ---> PHOSPHORYLATION
108
It inhibits TSH secretion and the response to TRH
Somatostatin
109
Decrease the basal secretion of TSH
Dihydroxyphenylethylamine (dopamine)
Dopa
Bromocriptine
110
Cause competitive inhibition of iodide transport into the cell, that is, inhibition of iodide-trapping mechanism
Thiocyanate in high enuff conc
111
Deficiency of thyroid hormones leads to increased secretion of TSH by ant pit gland wc causes
Overgrowth of thyroid gland
112
Block of thyroid secretion can lead to the ions to block thyroid secretion can lead to the development of an enlarged thyroid gland wc is called
Goiter
113
Prevents formation of thyroid hormone from iodides and tyrosine
Propylthiouracil
| Also methimazole and carbimazole
114
The mild abd transient inhibition of organic binding of iodide and hence of hormone synthesis is known as the
Wolff-Chalkoff effect
115
Vegetables of Brassicacae family, particularly rutabagas, cabbage and turnips, contain
Progoitrin
116
Attached to the DNA genetic strands or in proximity to them
Thyroid hormone receptors
117
Total membrane surface area of the mitochondria increases almost directly in proportion to the
Increased metabolic rate of the whole animal
118
Enz that becomes increased in response to thyroid hormone is
Na-K-ATPase
119
Thyroid hormone causes cell mem to be
Leaky to Na ions
| - wc further activates the Na pump and further increase heat prodxn
120
Normal BMR for adult euthyroid males is
35-40kcal/m^2 body surface/hr
121
Normal BMR
6-10% lower in euthyroid females
122
Marked decrease in the myelination abd arborization of neurons in the brain
Hypothyroidism
123
What occurs if hypothyroidism it is untreated?
Mental retardation
124
Thyroid hormone is necessary for
Normal lactation
125
Thyroid hormone potentiates the glycogenolytic effevt of epinephrine, causing
Glycogen depletion
126
In physiologic amounts, thyroid hormones has a
Potent protein anabolic effect
127
In large doses, thyroid hormone has a
Protein catabolic effect
128
Increased thyroid hormone
decreases the concentrations of cholesterol, phospholipids, and triglycerides in the plasma, even though it increases the free fatty acids.
129
The large increase in circulating plasma cholesterol in prolonged hypothyroidism is often associated with
severe atherosclerosis
130
One of the mechanisms by which thyroid hormone decreases plasma cholesterol concentration is to
Increase significantly cholesterol secretion in the bile and consequent loss in the feces
131
A possible mechanism for the increased cholesterol secretion is that thyroid hormone induces increased numbers of
low-density lipoprotein
receptors on the liver cells, leading to rapid removal of low-density lipoproteins from the plasma by the liver and
subsequent secretion of cholesterol in these lipoproteins by the liver cells
132
relative vitamin deficiency can occur when
excess thyroid hormone is secreted
133
A greatly increased amount of thyroid hormone almost always
decreases body weight
134
mean arterial pressure usually remains normal. Because of
increased blood flow through the tissues between heartbeats, the pulse pressure is often increased, with the systolic pressure elevated 10 to 15 mm Hg in hyperthyroidism and the diastolic pressure reduced a corresponding amount.
135
lack of thyroid hormone can cause
constipation
136
diarrhea
Hyperthyroidism
137
lack of thyroid hormone
decreases rapidity of cerebration
138
extremely nervous and have many psychoneurotic tendencies, such as anxiety complexes, extreme worry, and paranoia
hyperthyroidism
139
makes the muscles react with vigor, but when the quantity of hormone becomes excessive, the muscles become weakened because of excess protein catabolism
hyperthyroidism
140
it causes the muscles to become sluggish, and they relax slowly after a contraction.
hypothyroidism
141
resting hand tremors, wt loss, palpitations, DoB, complaining of lack of sleep
hyperthyroidism
142
feeling of constant tiredness, but because of the excitable effects of thyroid hormone on the synapses, it is difficult to sleep
hyperthyroidism
143
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extreme somnolence (Sleepiness, the state of feeling drowsy, ready to fall asleep) is
characteristic of
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hypothyroidism
144
is likely to cause loss of libido
lack of thyroid hormone
145
sometimes causes impotence
a great excess of the hormone
146
menorrhagia and polymenorrhea
| amenorrhea
lack of thyroid hormone
147
the production of heat via the digestion of food, or by the action of hormones.
Calorigenesis
148
the basal metabolic rate falls to almost one-half normal
when no thyroid hormone is produced
149
Chronotropic effevt on heart
Increase number and affinity of B-adrenergic receptors
150
Inotropic effect on heart ❤️
Enhance responses to circulating catecholamines
| Increase proportion to a-myosin heavy chain (w highger ATPase activity)
151
Effect of adipose and muscle
Catabolic
152
Metabolic effects of the gut
Increase rate of carbohydrate absorption
153
Metabolic effects og lipoprotein
Stimulate formation of LDL receptors
154
Stimulate O2 consumption by metabolically active tissues and inc metabolic rate
Tissue exceptions: testes, uterus, LN, spleen, ant pit
Calorigenic
155
Characterized by nervousness, wt loss, hyperphagia, heat tolerance, increased pulse p, fine tremor of outstretched fingers, warm soft skin, sweating, BMR +10 to as high as +100
Hyperthyroidism (thyrotoxicosis)
156
Hyperthyroidism most common cause is
Graves' dse (exophthalmic goiter)
157
Autoimmune dse in wc circulating Ab formed against the TSH receptor activate the receptor, making glands hyperactive
Graves' dse
158
Immunity that destroys the gland rather than stimulates it
Hypothyroidism
159
Hypothyroidism pt first have this
Thyroiditis - inflammation
160
Goitrous glands may secrete normal quantities of thyroid hormones, but more frequently, the secretion of hormone is depressed
Idiopathic nontoxic colloid goiter
161
Iodine is not pumped adequately into the thyroid cells
Deficient iodide-trapping mechanism
162
Iodides aren't oxidized to the iodine state
Deficient peroxidase system
163
Final thyroid hormones cannot be formed
Deficient coupling of iodinated tyrosines in the thyroglobulin molecule
164
Prevents recovery of ipdine from the iodinated tyrosines that are not coupled to form the thyroid hormones thus leading to iodine deficiency
Enz deiodinase deficiency
165
Prevents recovery of ipdine from the iodinated tyrosines that are not coupled to form the thyroid hormones thus leading to iodine deficiency
Enz deiodinase deficiency
166
Are substance s that block the synthesis of thyroid hormone
They hav a propylthiouracil-type of antithyroid activity
TSH secretion is enhanced
Goitrogens
167
Goiterogenic agents include
Perchiorate, thiocyanate, pertechnetate
Thionamides (propylthiouracil and methimazole)
Iodide def
Excess iodide
Turnips, rutabagas, cabbages, veggies of fam Brassicacae
168
Monovalent anions that block iodide-trapping
Perchiorate, thiocyanate, pertechnetate
169
Block coupling of iodotyrosines
Thionamides (prophylthiouracil and methimazole)
170
Physiologic characteristics of hypothyroidism
Depressed growth of hair and scaliness of skin
Frog like husky voice
Edematous appearance throughout the body (myxedema)
171
Wc can result in peripheral vascular dse, deafness, coronary artery dse w consequent early death
D/t increase in quantity of blood CHOL bcs of altered fat and CHOL metabolism and diminished liver excretion of cholesterol in bile
Atherosclerosis
172
Hypothyroidism tx
Daily ingestion of tablet or more containing thyroxine to maintain a steady lvl if thyroid hormone activity
173
Conc of binding proteins in hyperthyroidism and hypothyroidism
Normal
174
Conc of binding proteins in Euthyroid: estrogens, methadone, heroin, major tranquilizers, clofibrate
High
Also high total plasma T4, T3 abd RT3
Normal: fT3 fT4 and frT3 also plasma TSH
175
Conc of binding proteins in Euthyroid: glucocorticoids, androgens, danazol, asparaginase
Low
Also low total plasma T4, T3 abd RT3
Normal: fT3 fT4 and frT3 also plasma TSH
