Hypothyroidism physiology

Question 1

What is primary hypothyroidism and what types are there?

Answer 1

Lack of functional thyroid tissue
Acquired
Iatrogenic
Congenital

Question 2

What are the causes of acquired primary hypothyroidism?

Answer 2

Lymphocytic thyroiditis
Idiopathic follicular atrophy

Question 3

what are the causes of iatrogenic primary hypothyroidism?

Answer 3

surgery
radioactive iodine therapy
Anti-thyroid meds

Question 4

What is secondary hypothyroidism?

Answer 4

Impaired secretion of TSH from pituitary gland
Acquired:
- neoplasia
- pituitary suppression
- illness, malnutrition
Congenital:
- cystic rathke’s pouch

Question 5

What is tertiary hypothyroidism?

Answer 5

lack of TRH in hypothalamic supraoptic and paraventricular nuclei

Question 6

what breeds are predisposed to canine hypothyroidism?

Answer 6

Golden retrievers
Cocker spaniels
Terriers

Question 7

What are the clinical signs of canine hypothyroidism?

Answer 7

Gradual onset of:
- dullness
- lethargy
- exercise intolerance
- obesity without history of polyphagia
- cold intolerance
- skin changes

Question 8

Why is there a gradual onset of clinical signs due to hypothyroidism?

Answer 8

due to store of thyroid hormones

Question 9

Describe the effect of hypothyroidism on development

Answer 9

reduced development and maturation of brain cells in foetus and young

Question 10

Describe the effect of hypothyroidism on growth

Answer 10

Required for normal growth:
- growth retardation
- smaller
- shorter bones
- delayed closure of physes

Question 11

Describe the metabolic actions of hypothyroidism

Answer 11

Decreases carb and fat metabolism
Decreased BMR (basal metabolic rate)
Increased body weight
=> weight gain

Question 12

Describe the effects of hypothyroidism on the MSK system

Answer 12

reduced muscle tone and changes in fibre type

Question 13

Describe the effects of hypothyroidism on the cardiovascular system

Answer 13

Brachycardia, weak apex beat

Question 14

What is the normal physiological effects of T3 and T4 on the cardiovascular system?

Answer 14

increase blood flow and cardiac output
Increase HR
Increase contractility

Question 15

What is the effect of hypothyroidism on the nervous system

Answer 15

Neurological anomalies
Neuropathy:
- knuckling and paresis (weak limbs)
- hearing impairment
- slower reflexes
Myopathy:
- paresis
- slow gait

Question 16

What are thyroid hormones needed for in the nervous system?

Answer 16

normal development
enhances SNS (increase epinephrine receptors)
optimal nerve conduction

Question 17

What is the effect of hypothyroidism on the CNS?

Answer 17

lethargic and requires more sleep

Question 18

What is the effect of hypothyroidism on the repro system?

Answer 18

Male:
- infertile
- testicular atrophy
- reduced sperm production
Female:
- infertility
- shortened oestrus
- prolonged oestrual bleeding
- prolonged anoestrus

Question 19

What is effect of hypothyroidism on the GIT?

Answer 19

decreases motility and secretion of pancreatic enzymes => constipation

Question 20

Describe the effects of hypothyroidism on integument

Answer 20

Arrests hair growth, hair retained in telogen
Alopecies or failure to regrow hair after clipping

Question 21

What is the action of thyroid hormone on integument?

Answer 21

initiates and maintains anagen phase

Question 22

What skin changes can be seen in canine hypothyroidism?

Answer 22

Bilateral symmetrical alopecia
Seborrhea
Lichenification
Comedones
Hyperpigmentation on alopecic areas
Recurrent infections

Question 23

what can be seen here?

Answer 23

comedones

Question 24

What can be seen here?

Answer 24

alopecia
hyperpigmentation

Question 25

What is myxedema in hypothyroidism?

Answer 25

tragic facial expression due to excess mucopolysaccharides and hyaluronic acid in dermis

Question 26

Describe the complete blood count found in hypothyroidism

Answer 26

Normocytic, normochromic anaemia Leukocytosis if there is an infection

Question 27

Describe the biochemistry profile in hypothyroidism

Answer 27

Increased parameters of lipid metabolism - cholesterol, lipids, triglycerides Increased hepatic enzymes

Question 28

What baseline thyroid hormones are used in endocrine testing

Answer 28

Total T4 Free T4 TSH

Question 28

Describe the use of measuring tT4 in endocrine testing

Answer 28

Measures protein bound and free T4 Good sensitivity/screening test Normal total T4 can exclude hypothyroidism

Question 29

What reasons other than hypothyroidism can cause a low total T4 result?

Answer 29

daily fluctuations Non-thyroid illness/euthyroid sick syndrome Drugs - glucocorticoids, antibiotics

Question 30

Describe euthyroid sick syndrome

Answer 30

Non-thyroidal illnesses suppress T4 and T3 but patient is not hypothyroid Mechanisms: - decreased binding of T4 and T3 - decreased T4 to T3 conversion - decreased TSH release Low total T4 with low-normal TSH

Question 31

Describe use of free T4 in diagnosing hypothyroidism

Answer 31

Conc of fT4 reflects thyroid status at tissue level Higher specificity/diagnostic test less affected by external factors

Question 32

Describe the use of baseline TSH in diagnosis of hypothyroidism

Answer 32

Primary hypothyroidism => low T4 and high TSH levels High specificity if interpreted with tT4 or fT4 results Low sensitivity - some hypothyroid dogs have normal TSH levels