Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Endo > The thyroid gland > Flashcards

The thyroid gland Flashcards

1
Q

What is the embryological origin of the thyroid gland?

A
  • Downgrowth from pharyngeal endoderm of developing tongue (follicular cells)
  • C cells are from ultimobranchial body
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

Describe the structure of the thyroid glands

A
  • Thyroid follicles
  • Single layer of follicular cells around lumen filled with protein rich colloid where hormones are stored
  • Microvilli on colloid side
  • Cell cells large, pink cytoplasm, interspersed between follicular cells
  • Lots of contact with capillaries on basal side
  • Tight junctions between neighboruing cells
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

What is the function of the parafollicular cells?

A
  • Connective tissue near follicles

- secrete calcitonin

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What is the colloid?

A
  • Stored within follicles
  • Primarily thyroglobulin
  • T3 and T4 bound to thyroglobulin
  • Normally only 10% of tyrosines in thryroglobulin are iodinated
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

What is the function of iodine in the body?

A

Thyroid hormone synthesis, this is its only role

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

Describe the synthesis of the thyroid hormones

A
  • Iodide pumped from blood into follicular cells using secondary active transport with sodium, down sodium concentration gradient (is against iodine gradient)
  • Stimulates by TSH
  • Once iodide in, diffuses through cell to colloid end
  • Thyroperoxidase (TPO) at colloid end
  • Golgi in cell produce thyroglobulin (with 120 tyrosine residues), transported into colloid via exocytosis
  • Thyroperoxidase catalyses iodination of thyroglobulin using the iodide
  • Iodide oxidised to iodine as H2O2 is reduced
  • Iodine added to tyrosine within thyroglobulin, catalysed by iodinase enzyme
  • This coupling reaction produces monoiodotyrosinase or diiodotyrosinase
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

How are T3 and T4 produced from monoiodo and diiodotyrosinase?

A
  • Mono and diiodo combine to form T3

- 2x diiodo to form T4

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

Describe the process of thyroid hormone secretion

A
  • Get from colloid to basal membrane and into capillary network
  • Endocytosis for colloid uptake
  • Intracellular vesicles fuse with lysosomes
  • Lysosomal enzymes split thryoid hormones from thyroglobulin
  • Hormones diffuse across basal plasma membrane into interstitium (lipid soluble so bound to thyroid hormone binding globulin or albumin)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

Compare the blood concentrations of the thyroid hormones

A
  • 50-60 times higher blood levels of T4 compared to T3 in circulating blood concentrations
  • Very small amount of free hormones (0.03% T4, 0.3% T3)
  • Equilibrium between bound and free
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

Describe the deiodination of thyroxine

A
  • Deiodinated to triiodothyronine within cells of many tissues, esp liver and kidneys
  • Free hormones enter cells
  • Can be converted to T3 or rT3
  • Therefore deiodination pathways are mechanism for regulation
  • Active T3 using 5’-deiodinase
  • rT3 using 5-deiodinase
  • Different position of iodine removed
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is the function of rT3?

A
  • No action and so will slow metabolism

- Adaptation to starvation or illness

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

How are thyroid hormones excreted?

A
  • Catabolised by liver and kidney
  • Further deiodinated to diiodotyrosine and monoiodotyrosine
  • Produces conjugated excretory products
  • Iodide either recycled or excreted via urine
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What are the actions of thyroid hormones?

A
  • Development and growth
  • Increase metabolic actions
  • Increase breakdown of glycogen in MSK system
  • Increased efficiency of oxygen delivery to tissues
  • Respiratory system effects
  • Maintain normal function of adult nervous system
  • Reproductive system
  • Gastrointestinal system
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

What are the effects of thyrotropin hormone (TSH)?

A
  • Increased endocytosis and proteolysis of thyroglobulin from colloid
  • Increased activity of Na+/I- symport
  • Increased iodination of tyrosine
  • Increased size and secretory activity of thyroid follicular cels
  • Increased number of follicular cells
  • Goitre (enlarged from TSH stimulation)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

Describe the regulation of TSH and TRH

A
  • Hypothalamus secretes TRH
  • Stimulates TSH release from anterior pituitary (water soluble)
  • Acts on thyroid gland
  • Adenyl cyclase signal transduction pathway
  • Negative feedback from thyroid hormones in long and short loops
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

Where is most T3 produced?

A

In the tissues, by deiodination of T4

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Which of the thyroid hormones is the primary biologically active hormone?

A

T3

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

What are the effects of thyroid hormones on development?

A
  • Required in foetal period and first few months

- Lack leads to reduced development and maturation of brain cells (permanent, cretinism in humans)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

What are the effects of hypothyroidism on growth?

A
  • Required for normal growth

- In young animal: growth retardation, smaller, shorter bones, dealyed closured of physes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Describe the role of thyroid hormones in carbohydrate metabolism

A
  • Stimulate glucose metabolism
  • Increase glucose uptake into cells
  • Insulin sensitivity
  • Insulin secretion
  • Glycolysis in liver and skeletal muscle
  • Gluconeogenesis
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Describe the role of thyroid hormones in fat metabolism

A
  • Enhances fat metbaolism
  • Mobilise lipids fom adipose stores
  • Accelerate oxidation of lipids to produce energy (by increasing size and number of mitochondria as this is where beta-oxidation takes place)
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Describe the role of thyroid hormones in basal metabolic rate

A
  • Increase BMR in all tissues except brain, gonads nad spleen
  • Increase heat production
  • Increase oxygen consumption
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

What is the effect of hyperthyroidism on basal metabolic rate and body weight?

A
  • BMR twice normal level

- Weight loss

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

What is the effect of hypothyroidism on basal metabolic rate and body weight?

A
  • BMR 50% of normal

- Weight gain

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

Describe the effect of hypothyroidism on the musculoskeletal system

A
  • Reduced muscle tone, changes in fibre type

- Not seen clinically, very little cahnge in MSK system seen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

Describe the effect of hyperthyroidism on the musculoskeletal system

A
  • Protein depletion
  • AAs used for gluconeogenesis
  • Muscle tremors
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

What are the normal physiological effects of the thyroid hormones on the cardiovascular system?

A
  • Increased blood flow and cardiac output
  • Increased heart rate
  • Increased contractility
  • Aims to increase oxygen supply to the body
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

What are the effects of hypothyroidism and hyperthyroidism on the cardiovascular system?

A

Hypo: bradycardia
Hyper: tachycardia

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

What is the normal action of the thyroid hormones on the respiratory system?

A
  • Increased basal metabolic rate

- Increased demand for oxygen thus increased excretion of carbon dioxide (increased ventilation)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

What are the effect of hypo and hyperthryoidism on respiration rate?

A
  • Hypo: little change seen

- Hyper: increased respiration rate

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

What is the normal action of the thyroid hormones on the nervous system?

A
  • Development
  • Enhances sympathetic nervous system (increases epinephrine receptors)
  • Needed for optimal nerve conduction
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

What is the effect of hypothyroidism on the nervous system?

A
  • Slower reflexes
  • Lethargic
  • Mentally slower
  • Require more sleep
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

What is the effect of hyperthyroidism on the nervous system?

A
  • Hyperexcitable

- Tired becuase of increased nerovus and muscular activity, but difficulty sleeping

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

What is the effect of hypothyroidism on the reproductive system

A
  • Reduced sexual drive
  • Reduced sperm production
  • Irregular or absent cycling
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

What is the normal action of the thyroid hormones in the gastrointestinal system?

A
  • Increases appetite and feed intake
  • increases secretion of pancreatic enzymes
  • Increases motility
36
Q

What are the effects of hypo and hyperthyroidism in the gastrointestinal system?

A
  • In humans, not always the case in animals, may be seen in some but not others
  • Hypo: constipation
  • Hyper: diarrhoea
37
Q

What is the normal role of the thyroid hormones in the integument?

A

Initiates and maintains anagen phase of hair growth

38
Q

What is the effect of hypothyroidism in the integument?

A
  • Arrests hair growth, hair retained in telogen phase

- Alopecia or failure to regrow after clipping

39
Q

What are the different types of hypothyroidism with respect tothe HPT axis?

A
  • Primary
  • Secondary
  • Tertiary
40
Q

Outline primary hypothyroidism

A
  • Disease of thyroid gland itself
  • Lack of functional thyroid tissue
  • Most common form
  • Can be acuired, iatrogenic or congenital
41
Q

What are the potential causes of acquired primary hypothyroidism?

A
  • Lymphocytic thyroiditis ~50% (autoimmune)
  • Idiopathic follicular atrophy
  • Secondary to neoplasia (least likely)
  • Clinically unimportant to kow which one of these as long as can tell if is neoplasia or not
42
Q

What are the potential causes of iatrogenic primary hypothyroidism?

A
  • Surgery, radioactive iodine therapy, anti-thyroid medications
  • Treatment of hyperthyroidism is common cause
43
Q

What are the potential causes of congenital primary hypothyroidism?

A
  • Rare, cretinism and early death
  • Thyroid gland agenesis or dysgenesis
  • Thyroid peroxidase deficiency
  • Deficient dietary iodine, ingestion of goitrogens
44
Q

What is secondary hypothyroidism?

A
  • Disease of pituitary affecting the thyroid (no TSH)
  • Uncommon
  • Acquired
45
Q

What are potential causes of acquired secondary hypothyroidism?

A
  • Neoplasia
  • Pituitary suppression e.g glucocorticoid administration
  • Illness, malnutrition
46
Q

What are potential causes of congenital secondary hypothyroidism?

A

Cystic Rathke’s pouch (accompanied by other pituitary hormone deficiencies e.g. ADH)

47
Q

Outline tertiary hypothyroidism

A
  • Disease of hypothalamus affecting the thyroid (no TRH)
  • Lack of RH in hypothalamic supraoptic and paraventricular nuclei
  • Very rare
48
Q

Describe the signalment for hypothyroidism

A
  • Breed disposition: Doberman Pinschers, Golden retriever, Cocker spaniels, Irish setters, terriers
  • No sex or neutering predisposition
  • Peak incidence 4-6
49
Q

Describe the clinical signs of hypothyroidism

A
  • Vague, diffuse gradual onset
  • non-pathognomic
  • Most commonly dermatological and metabolic signs
  • Cardiovascular, neurological and reproductive signs
50
Q

Describe the metabolic signs of hypothyroidism

A
  • Dullness
  • Lethargy
  • Obesity without history of polyphagia
  • Exercise intolerance
  • Cold intolerance
  • Tragic appearance
51
Q

Describe the dermatologic signs fo hypothyroidism

A
  • Bilaterally symmetrical alopecia on areas of wear/pressure points flanks, down backs of limbs, non-pruritic
  • Seborrhea, lichenification, comedones
  • Hyperpigmentation of alopecic areas
  • recurrent infections
  • Dry skin and haircoat
  • alopecia on bridge of nose and “rat tail” on dogs
  • Myxedema: excess mucopolysaccharides and hyaluronic acid in dermis leading to skin folds and haning(tragic) facial expression
52
Q

Describe the clinical cardiovascular signs of hypothyroidism

A
  • Sinus bradycardia
  • Weak apex beat
  • Electrocardiograph: ow voltage complexes
  • Echocardiogram: decreased fractional shortening
  • Decreased contractility
53
Q

Describe the clinical neurologic abnormalities of hypothyroidism

A
  • Segmental demyelinisation and nerve conduction
  • Peripheral neuropathy: knuckling, paresis, hearing impairment
  • Myopathy (paresis, slow gait)
  • Myxednea coma
54
Q

Describe the clinical reproductive signs of hypothyroidism

A
  • Female: infertility, shortened oestrus, prolongeed oestrual bleeding, prolonged anoestrus
  • Male: infertility, testicular atrophy
55
Q

Outline the complete blood count results for hypothyroidism

A
  • Nomocytic, normochromic anaemia (non-regenernative)
56
Q

Outline the biochemistry profile in hypothyroidism

A
  • Increased parameters of lipid metabolism: cholesterol, lipids, triglycerides (TAGs)
  • Mild-moderately increased hepatic enzymes
  • Fasting hypercholasterolemia
57
Q

What is euthyroid sick syndrome?

A
  • Non-thyroidal illnesses suppress T4 and T3
  • Patient not truly hypothyroid
  • Mechanisms: decreased protein binding of T4 and T3, decreased T4 to T3 conversion, decreased TSH release
58
Q

What tests are used to diagnose hypothyroidism?

A
  • Free and total T4
  • TSH
  • Total T3
59
Q

Describe total T4 measurements in hypothyroidism diagnosis

A
  • Measure protein bound and free T4
  • Good screening test
  • Hypothyroid have low or low-normal total T4
  • tT4<6nmol/l = very likely
  • tT4>20nmol/l = very unlikely
  • Normal total T4 can exclude hypothyroisim
  • Good for ruling out, but low specificity due to daily fluctuations, euthyroid sick syndrom, drug (e.g. glucocorticoids, antibiotics)
60
Q

What will the tests results for total T4 and TSH be if euthyroid sick syndrome is occuring?

A

Low tT4, low-normal TSH

61
Q

What will the test results be for total T4 and TSH in hypothyroidism?

A

Low tTH, high TSH

- For diagnosis use all 4 tests together

62
Q

Describe free T4 test in diagnosin hypothyroidism

A
  • Only free/unbound T4 can enter cells
  • Concentration of fT4 reflects thyroid status at tissue level
  • Less affected by external factors
  • 90% accuracy in hypothyroidism diagnosis
63
Q

Describe total T3 in diagnosing hypothyroidism

A
  • Does not reflect thyroid gland function
  • Most will be from deiodination of T4 at extra thyroidal sites
  • Will be conserveed in hypothyroid states and so is not that clinically useful
64
Q

Describe baseline TSH in diagnosing hypothyroidism

A
  • 90% specificity
  • Lower sensitivity
  • Low or high Sh depends on primary or secondary hypothyroidism
  • Primary: high TSH, no negative feedback
  • Secondary: low TSH leading to decreased T4 production
65
Q

In order to accurately diagnose hypothyroidism, what is required?

A
  • All 4 tests: tT4, free T4, free T3, baseline TSH
  • Compatible clinical signs
  • Response to therpeutic trial
  • Usually see: total T4 low to low-normal (<6nmol/L)
  • TSH>0.5ug/L with low T4
66
Q

What is the most common cause of hyperthyroidism?

A

Adenomatous hyperplasia of the thyroid glands

- Small number of thyroid carcinoma

67
Q

What is the histological appearance of a hyperthyroid gland?

A
  • Lots of small follicles and extra tissue between them
  • Looks like “normal” tissue but a lot more of it and bigger
  • Can be cystic
68
Q

What is the effect of hyperthyroidism on the HPT axis?

A
  • Increased production of thyroid hormones

- Negative feedback, decrease TSH production

69
Q

What is the signalment for feline hyperthyroidism?

A
  • Older cats (>10 years)
  • No gender difference
  • Rare in himalayans and siamese
70
Q

Outline the common history for a cat with hyperthyroidism

A
  • Wet canned food more likely than dry good
  • Fish flavoured more at risk
  • Pedigree cats more resistant than domestic shorthairs
  • Indoor cats more at risk
  • Behavioural changes: hyperactivity, vocalisaion, agitation and restlessness, changes to grooming (over or under), poor thermoregulation
71
Q

What are the common clinical signs of feline hyperthyroidism?

A
  • Insidious progressive signs
  • Weight loss, polyphagia, PUPD are main ones
  • Diarrhoea, respiratory abnormalities, vomiting, haircoat changes
  • Thin cat
  • Cervical nodule/goitre
  • Tachycardia(>240bpm) +/- murmur +/- gallop rhythm
  • Less commonly: tremors, weakness, dyspnoea, heat and stress intolerance, cardiac disease/failure, systemic hypertension, blindness (due to haemorrhage of retinas)
72
Q

Outline canine hyperthyroidism

A
  • Rare
  • Thyroid carcinomas (only small proportion of thyroid carcinomas are functional)
  • Highly metastatic
  • Older dogs >10 years
  • Golden retrievers, boxers, beagles
73
Q

How does hyperthyroidism cause weight loss and polyphagia?

A
  • Increased metabolic rate
  • Increased catabolism of stores
  • Making up for lost stores (may lead to regurgitation as are eating more than are able to digest)
74
Q

How does hyperthyroidism lead to vomiting?

A
  • Overeating
  • Activation of emetic centre
  • Concurrent disease
75
Q

Why does diarrhoea occur in hyperthyroidism?

A
  • Pancreas unable to cope with quantitiy of food
  • Food not digested properly due to insufficient pancreatic secretions
  • Leads to maldigestion and thus diarrhoea
  • Hypermotility and thus malabsorption
76
Q

How does hyperthyroidism cause the cardiovascular signs seen?

A
  • Sensitisation of cardiactissue to catecholamines (as thyroid hormoens enhance SNS)
  • More response to catecholamines leading to tachycardia
  • Disturbed cardiac activity and flow
  • Hypertrophic cardiomyopathy leading to murmurs
77
Q

How does hyperthyroidism cause PU/PD?

A
  • Exact mechanism unclear
  • Increased cardiac output, glomerular filtration rate and medullary blood flow
  • Possibly psychogenic component
  • Concurrent renal disease in some cats as is mostly old cat disease
78
Q

How does hyperthyroidism lead to behavioural changes?

A
  • Interaction with CNS
  • Leads to increased SNS activity, thus increased activity in general
  • Thermoregulation changes
79
Q

How does hyperthyroidism lead to hypertrophic cardiomyopathy?

A
  • Direct effect of thyroid hormones on myocytes
  • Indirect effect of adrenergic nervous system
  • Indirect compensatory changes for altered peripheral perfusion
80
Q

What tests are used to diagnose hyperthyroidism?

A
  • Baseline hormone concentration (total and free T4)

- Dynamic hormone testing: T3 suppression test, TRH stimulation test

81
Q

Rank the tests used to diagnose hyperthyroidism in order of preference

A
  • Serum total T4 (usually diagnostic, rare to have other reason for raised hyperthyroidism)
  • Repeat T4
  • Free T4 by equilibrium dialysis
  • T3 suppression test
  • TRH response test
  • Scintigraphy
82
Q

What are the limitations of Total T4 testing in diagnosis of hyperthyroidism?

A
  • Daily/hourly fluctuations
  • May be normal in early or mild hyperthyroidism
  • Depressed by non-thyroidal illness (euthyroid sick mechanism)
  • Addition of free T4 may be helpful
83
Q

Describe the T3 suppression test in hyperthyroidism diagnosis

A
  • Admin of exogenous T3 causes decrease in TSH and thus decrease in T4
  • Measure T4: thyroid gland response, in hyper are autonomously producing T4 and TSH already suppressed so more T3 will not lead to further suppression of TSH or T4
  • Measure T3 to check T3 has been absorbed/given. If there is no cahnge in T3 then shows it has not been absorbed
84
Q

Outline the use of thyroid scintigraphy in the diagnosis of hyperthyroidism

A
  • Radioactive marker identifies functional thyroid tissue
  • Technetium or iodine isotope
  • Specific counts via gamma camera to determine thyroid/salivary gland ratio
  • Need specific facilities, not commonly available
  • Confirms and localises
  • Identifies ectopic tissues and metastatic disease
85
Q

Decribe the appearance of hyper and hyperthyroid glands on thyroid scintigraphy

A
  • Hyperthyroid darkened more intensely

- Hypothyroid very pale/alnost invisible

86
Q

Outline the treatment of hyperthyroidism

A
  • Administration of radioactive iodine, will concentrate in thyroid gland to a level where it is able to destroy the thyroid tissue, thus preventing production of T4
  • Drugs inhibiting thyroid peroxidase (needed to make iodide into iodine)
  • Starving cats of iodine using special diet

Endo (30 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions