Canine Hypothyroidism Flashcards
What is the only source of T4?
Thyroid glands
How is thyroid hormone production regulated?
TRH from the hypothalamus
TSH from the pituitary gland
What do thyroid hormones do?
Crucial for fetal and juvenile development
Drive calorigenesis
Boost protein and enzyme synthesis
Impact carbohydrate and lipid metabolism
Stimulate erythropoiesis
Supports normal immune function
Supports healthy skin and hair
Every organ is influenced in some way by thyroid activity
What do thyroid hormone levels tend to do with illness?
Drop
What is the biologically active form?
T3
What is the most commonly diagnosed endocrine disease in dogs?
Hypothyroidism
What type of hypothyroidism accounts for 95% of cases?
Primary
What are the 3 causes of primary hypothyroidism?
Lymphocytic thyroiditis (immune mediated)
Idiopathic thyroid atrophy
Iatrogenic
What happens with idiopathic thyroid atrophy?
Non-inflammatory replacement with adipose and fibrous tissue
What occurs in secondary hypothyroidism?
Problem specific to anterior pituitary gland resulting in depressed TSH levels
Thyroid gland becomes atrophies
What occurs in secondary hypothyroidism?
Problem specific to anterior pituitary gland resulting in depressed TSH levels
Thyroid gland becomes atrophied
What is tertiary hypothyroidism due to?
Problem specific to the hypothalamus resulting in depressed TRH levels
Malformation or destruction
What causes congenital hypothyroidism?
Iodine deficiency, thyroid dysgenesis, dyshormonogenesis, faulty thyroid peroxidase activity
What breeds may be predisposed to hypothyroidism?
Golden Retriever
Dobermans
Irish Setters
Great Danes
Beagles
What are clinical signs of hypothyroidism?
Lethargy and general ‘blah’
Weight gain
Dermatological issues - alopecia, seborrhea, etc
Pyoderma
Myxedema (tragic face)
Gall bladder mucocele
What chemistry findings would you expect in a hypothyroid dog?
Hypercholesterolemia - can be dramatic
Hypertriglyceridemia
Mild increase in ALP
How does neonatal T4 levels compare to adults?
Neonates normally have higher T4 levels
How do sighthound T4 levels compare to other breeds?
Sighthounds have lower T4 levels
What do barbiturates do to thyroid hormones?
Decrease tT4 and fT4
TSH may be normal or high
What do glucocorticoids do to thyroid hormones?
Decrease tT4 and fT4
Inhibits TRH/TSH release
What do NSAIDs do to thyroid hormones?
Decrease tT4
Mild effect
What do sulphonamides do to thyroid hormones?
Decrease tT4 and fT4
Increase TSH
Recovery expected within 3 weeks
Is a low tT4 test value enough evidence to make a diagnosis of hypothyroidism?
No
What measurement is a useful way to establish a diagnosis of hypothyroidism?
TSH
What is the expected thyroid test results for a primary hypothyroidism dog?
low tT4, low fT4, increased TSH
(25% of affected dogs have a normal TSH)
What is the expected thyroid test results for a secondary hypothyroidism dog?
low tT4, low fT4, low TSH
What is the expected thyroid test results for euthyroid sick syndrome?
low tT4, normal fT4, normal TSH
(fT4 will drop in extreme illness)
Does the measurement of autoantibodies predict eventual clinical hypothyroidism?
No
What is the recommended treatment for hypothyroidism?
Levothyroxine
0.02 mg/kg PO BID
Make sure you are consistent
Need increased dosing if given while being fed
How should you monitor a hypothyroid dog?
Check tT4 after 2-4 weeks of therapy, adjust dose as needed to keep tT4 at upper end of reference range or even just above, check every 4-6 months longterm
What would you expect to see with congenital hypothyroidism in a cat?
Kitten
Disproportionate dwarfism
Constipation
What should you always consider in a kitten with recurrent constipation?
Feline hypothyroidism
