Hypothyroidism physiology Flashcards
What is primary hypothyroidism and what types are there?
Lack of functional thyroid tissue
Acquired
Iatrogenic
Congenital
What are the causes of acquired primary hypothyroidism?
Lymphocytic thyroiditis
Idiopathic follicular atrophy
what are the causes of iatrogenic primary hypothyroidism?
surgery
radioactive iodine therapy
Anti-thyroid meds
What is secondary hypothyroidism?
Impaired secretion of TSH from pituitary gland
Acquired:
- neoplasia
- pituitary suppression
- illness, malnutrition
Congenital:
- cystic rathke’s pouch
What is tertiary hypothyroidism?
lack of TRH in hypothalamic supraoptic and paraventricular nuclei
what breeds are predisposed to canine hypothyroidism?
Golden retrievers
Cocker spaniels
Terriers
What are the clinical signs of canine hypothyroidism?
Gradual onset of:
- dullness
- lethargy
- exercise intolerance
- obesity without history of polyphagia
- cold intolerance
- skin changes
Why is there a gradual onset of clinical signs due to hypothyroidism?
due to store of thyroid hormones
Describe the effect of hypothyroidism on development
reduced development and maturation of brain cells in foetus and young
Describe the effect of hypothyroidism on growth
Required for normal growth:
- growth retardation
- smaller
- shorter bones
- delayed closure of physes
Describe the metabolic actions of hypothyroidism
Decreases carb and fat metabolism
Decreased BMR (basal metabolic rate)
Increased body weight
=> weight gain
Describe the effects of hypothyroidism on the MSK system
reduced muscle tone and changes in fibre type
Describe the effects of hypothyroidism on the cardiovascular system
Brachycardia, weak apex beat
What is the normal physiological effects of T3 and T4 on the cardiovascular system?
increase blood flow and cardiac output
Increase HR
Increase contractility
What is the effect of hypothyroidism on the nervous system
Neurological anomalies
Neuropathy:
- knuckling and paresis (weak limbs)
- hearing impairment
- slower reflexes
Myopathy:
- paresis
- slow gait
What are thyroid hormones needed for in the nervous system?
normal development
enhances SNS (increase epinephrine receptors)
optimal nerve conduction
What is the effect of hypothyroidism on the CNS?
lethargic and requires more sleep
What is the effect of hypothyroidism on the repro system?
Male:
- infertile
- testicular atrophy
- reduced sperm production
Female:
- infertility
- shortened oestrus
- prolonged oestrual bleeding
- prolonged anoestrus
What is effect of hypothyroidism on the GIT?
decreases motility and secretion of pancreatic enzymes => constipation
Describe the effects of hypothyroidism on integument
Arrests hair growth, hair retained in telogen
Alopecies or failure to regrow hair after clipping
What is the action of thyroid hormone on integument?
initiates and maintains anagen phase
What skin changes can be seen in canine hypothyroidism?
Bilateral symmetrical alopecia
Seborrhea
Lichenification
Comedones
Hyperpigmentation on alopecic areas
Recurrent infections
what can be seen here?
comedones
What can be seen here?
alopecia
hyperpigmentation
What is myxedema in hypothyroidism?
tragic facial expression due to excess mucopolysaccharides and hyaluronic acid in dermis
Describe the complete blood count found in hypothyroidism
Normocytic, normochromic anaemia
Leukocytosis if there is an infection
Describe the biochemistry profile in hypothyroidism
Increased parameters of lipid metabolism - cholesterol, lipids, triglycerides
Increased hepatic enzymes
What baseline thyroid hormones are used in endocrine testing
Total T4
Free T4
TSH
Describe the use of measuring tT4 in endocrine testing
Measures protein bound and free T4
Good sensitivity/screening test
Normal total T4 can exclude hypothyroidism
What reasons other than hypothyroidism can cause a low total T4 result?
daily fluctuations
Non-thyroid illness/euthyroid sick syndrome
Drugs - glucocorticoids, antibiotics
Describe euthyroid sick syndrome
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
Describe use of free T4 in diagnosing hypothyroidism
Conc of fT4 reflects thyroid status at tissue level
Higher specificity/diagnostic test
less affected by external factors
Describe the use of baseline TSH in diagnosis of hypothyroidism
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
