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ENDO > Physiology of hypothyroidism > Flashcards

Physiology of hypothyroidism Flashcards

1
Q

What impact does hypothyroidism have on development?

A
  • Lack of thyroid during fetal period and first few month after birth causes reduced development and maturation of brain cells in fetus and young
  • Causes cretinism in humans
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2
Q

What causes cretinism?

A

Congenital lack of thyroid gland or iodine deficiency in mother

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3
Q

What impact does hypothyroidism have on growth?

A
  • Growth retardation
  • Smaller
  • Shorter bones
  • Delayed closure of physes
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4
Q

Describe the impacts of hypothyroidism on the metabolism

A
  • Slows carbohydrate metabolism
  • Slows fat metabolism
  • Lowers basal metabolic rate to 50% of normal
  • Results in weight gain
  • Reduced muscle tone
  • Bradycardia
  • Nervous system= slower reflexes, lethargy and need more sleep
  • Constipation
  • Arrests hair growth- stuck at telogen. Resulting in alopecia or failure to regrow after clipping
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5
Q

What affects does thyroid hormone have on carbohydrate metabolism?

A

-Stimulates glucose metabolism: increases glucose uptake, insulin sensitivity, insulin secretion, glycolysis, gluconeogenesis

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6
Q

What affects does thyroid hormone have on the metabolism of fats?

A
  • Thyroid hormone enhances fat metabolism
  • Mobilises lipids from adipose stores
  • Accelerates oxidation of lipids to produce energy (occurs within mitochondria- increase in size/ number of mitochondria)
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7
Q

What affects does thyroid hormone have on the basal metabolic rate?

A
  • Increases BMR in all tissues EXCEPT brain, gonads, spleen
  • Increases heat production
  • Increases oxygen consumption
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8
Q

What induces weight loss?

A

-Levothyroxine

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9
Q

What are the normal affects of thyroid hormone on the cardiovascular system?

A
  • Increases blood flow and cardiac output
  • Increases heart rate
  • Increases contractility
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10
Q

What affects does thyroid hormone have on the nervous system?

A

Aids development, enhances sympathetic nervous system and is needed for optimal nerve conduction

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11
Q

What affects does thyroid hormone normally have on the GI system?

A
  • Increases appetite and feed intake
  • Increases secretion of pancreatic enzymes
  • Increases motility
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12
Q

What impact does thyroid hormone have on the integument?

A

-Initiates and maintains anagen phase

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13
Q

Where is most of the T4 and T3 produced?

A

All of T4 in the thyroid gland

Most of T3 in peripheral tissues

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14
Q

How does thyroid hormone move round the body?

A

Most circulating thyroid hormone is protein bound

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15
Q

Describe tertiary hypothyroidism

A
  • At the level of the hypothalamus

- Lack of TRH

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16
Q

Describe secondary hypothyroidism

A
  • At level of the pituitary
  • Reduced secretion of TSH from pituitary
  • Can be acquired (more common) or congenital (rare)
  • Aquired causes= neoplasia, pituitary suppression (glucocorticoid administration), illness, malnutrition
  • Congenital causes= cystic rathke’s pouch
17
Q

Describe primary hypothyroidism

A
  • Lack of functional thyroid tissue
  • Most common form of hypothyroidism
  • Can be acquired/ iatrogenic or congenital
  • Most common= acquired: lymphocytic thyroiditis, isiopathic follicular atrophy
18
Q

Which breeds are more prone to developing canine hypothyroidism?

A

-Doberman pinschers, golden retrievers, cocker spaniels, irish setter, terriers

19
Q

What age does canine hypothyroidism normally occur?

A

Between ages 4-6

20
Q

What are the 3 types of hypothyroidism and what causes them?

A
  • Primary= lack of functional thyroid tissue
  • Secondary= lack of TSH
  • Tertiary= Lack of TRH
21
Q

What are the clinical signs of canine hypothyroidism?

A 
General appearance= 
Dullness
Lethargy
Exercise intolerance
Obesity
Cold intolerance (seek heat)
'Tragic expression'
'Rat tail'
Bilaterally symmetrical alopecia
Comedomes 
Bradychardia
22
Q

What are the dermatological signs of canine hypothyroidism?

A
  • Bilateral symmetrical alopecia
  • Seborrhea, lichenification, comedones
  • Recurrent infections e.g. otitis externa
23
Q

Discuss the neurological abnormalities involved in canine hypothyroidism

A

-Segmental demylinisation and slow nerve conduction = knuckling, hearing impairment, slow gait

24
Q

What is Myxedema?

A

-Excess mucopolysaccharides and hyaluronic acid in the dermis-> contributes to the tragic facial expression seen in canine hypothyroidism

25
Q

What are the effects of canine hypothyroidism on the reproductive system?

A

In females: infertility, shortened oestrus, prolonged anoestrus
In males: infertility and testicular atrophy

26
Q

What will the blood tests show in a dog with canine hypothyroidism?

A

Complete blood count will show normocytic, normochromic anaemia and leucocytosis if infection is present
Biochem will show increased parameters of lipid metabolism (high cholesterol) and mild/ moderately high liver enzymes

27
Q

How do you test the baseline thyroid hormone level?

A

-You measure the total T4= both membrane bound and free T4
T3 isn’t measured because it doesn’t represent how much the thyroid gland is making but how much the peripheral tissues are converting T4-T3
-A normal T4 level excludes hypothyroidism
-Test has poor specificity

28
Q

What is the normal T4 concentration for a dog?

A

6-15

29
Q

Why does the total T4 test have poor specificity?

A
  • Daily fluctuations within individuals
  • Drugs such as glucocorticoids and antibiotics
  • Eurythyroid sick syndrome
30
Q

What does eurythyroid sick syndrome do?

A
  • Suppresses T4 and T3 but patient isn’t truly hypothyroid

- Mechanisms: decreased protein binding of T3/ T4. Decreases the conversion of T4 to T3. Decreases TSH release.

31
Q

What is the difference between a dog with hypothyroidism and a dog that is eurythyroid sick?

A

Eurythyroid sick= Low tT4 with low-normal TSH

Hypothyroidism= Low tT4 and high TSH

32
Q

What does fT4 mean?

A

Free T4= concentration of fT4 reflects the thyroid status at tissue level

33
Q

Why is total T3 not clinically useful?

A

Doesn’t reflect thyroid gland function because most T3 is produced in peripheral tissue

34
Q

What are the four steps to diagnosing hypothyroidism?

A
  • Compatible clinical signs
  • Total T4 is low-low normal
  • Baseline TSH= low with low T4 (TSH is normal in 20-40% of hypothyroid dogs)
  • Reaction to treatment

ENDO (39 decks)

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