physiology of hypothyroidism

Question 1

where is most of T3 produced

Answer 1

peripheral tissues

Question 2

what are the types of hypothyroidism and where do they each originate

Answer 2

• primary: thyroid disease
• secondary: pituitary origin
• tertiary: hypothalamus

Question 3

discuss primary hypothyroidism

Answer 3

• lack of functional thyroid tissue
• most common form
• can be acquired, iatrogenic or congenital
• acquired usually due to: lyphocytic thyroiditis, idiopathic follicular atrophy or secondary to neoplasia
• iatrogenic usually due to surgery, radioactive iodine therapy or anti-thyroid medications
• congenital usually due to cretinism (early death), thyroid gland agenesis or hysgenesis, thyroid peroxidase deficicy or dificient dietary iodine/ingestion of goitrogens

Question 4

discuss secondary hypothyroidism

Answer 4

• impaired secretion of TSH from pituitary
• uncommon
• acquired usually due to neoplasia, pituitary suppression, illness or malnutrition
• congenital is rare but usually due to cystic rathke’s pouch or accompanied by other pituitary hormone deficiencies

Question 5

discuss tertiary hypothyroidism

Answer 5

• lack of TRH in the hypothalamic supraoptic and paraventricular nuclei
• rarely documented in the dog

Question 6

what is the most common cause of hypothyroidism in dogs

Answer 6

lymphocytic thyroiditis
or
idiopathic follicular atrophy

Question 7

what is the signalment of canine hypothyroidism

Answer 7

• breeds: dobermans, goldens, cocker spaniels, irish setters, terrier
• no sex predisposition
• peak incidence 4-6 years

Question 8

list clinical signs of canine hypothyroidism

Answer 8

• vague, diffuse and gradual onset
• not pathognomic for the disease
• most common signs are dermatologic or metabolic
• appear dull
• lethargy
• exercise intolerance
• obesity without history of polyphagia
• cold intolerance
• skin changes

Question 9

how does hypothyroidism affect development

Answer 9

• reduced development and maturation of brain cells in fetus and young
• important to have thyroid during fetal period and first few months after bitth

Question 10

how does hypothyroidism affect growth

Answer 10

• growth retardation
• smaller
• shorter bones
• delayed closure of physes (growth plates)

Question 11

what are the metabolic actions of thyroid hormones

Answer 11

• increase carbohydrate metabolism
• increase fat metabolism
• increase basal metabolic rate
• decrease body weight

Question 12

how does hypothyroidism affect carbohydrate metabolism

Answer 12

• thyroid hormone stimulates glucose metabolism, increasing glucose uptake, insulin sensitivity, insulin secretion, glycolysis and gluconeogenesis
• therefore, hyporhtyroidism slows carbohydrate metabolism leading to weight gain

Question 13

how does hypothyroidism affect metabolism of fats

Answer 13

• thryoid hormone enhances fat metabolism
• mobolizes lipids from adipose stores
• accelerates oxidation of lipids to produce energy
• therefore hypothyroidism slows fat metabolism leading to weight gain

Question 14

how does hypothyroidism affect basal metabolic rate

Answer 14

• increased BMR in all tissues except the brain, gonads and spleen
• leads to increased heat production and oxygen consumption
• hypothyroidism leads to lower BMR (50% of normal) leading to weight gain

Question 15

how does hypothyroidism affect musculoskeletal

Answer 15

• reduces muscle tone and changes in fibre type

Question 16

how does hypothyroidism affect cardiovascular system

Answer 16

normal physiological effects on the heart:
- increase blood flow and cardiac output
- increase heart rate
- increase contractility

hypothyroidism = bradycardia, weak apex beat, electrocardiogram = low voltage complexes, echo = decreased fractional shortening

Question 17

how does hypothyroidism affect the respiratory system

Answer 17

• resp system indirectly affected by T3 and T4
• normal action = increase in basal metabolic rate (caused by increased demand for oxygen and excretion of CO2)

Question 18

how does hypothyroidism affect the nervous system

Answer 18

• thyroid hormones normally needed for normal development, enhances SNS and optimal nerve conduction
• in hypo, neurological abnormalities such as peripheral neuropathy (knuckling, paresis, hearing impairment, slower reflexes) and myopathy (paresis and slow gait)

Question 19

how does hypothyroidism affect the CNS

Answer 19

• mentally slower (hard to quantify in animals)
• lethargic and require more sleep

Question 20

how does hypothyroidism affect the reproductive system

Answer 20

• reduced sexual drive
• infertility
• shortened oestrus
• prolonged eostrual bleeding
• prolonged anoestrus
• testicular atrophy
• reduced spem production

Question 21

how does hypothyroidism affect the GI system

Answer 21

• thyroid hormone increases appetite and feed intake, increases secretion of pancreatic enzymes and increases motility in normal animals
• leads to consipation in hypothyroid

Question 22

how does hypothyroidism affect the integument

Answer 22

• thyroid growth initiates and maintains anagen phase normally
• in hypo, get arrests of hair growth, hair is retained in telogen and alopecia or failure to regrow hair after clipping

Question 23

what do canine hypothyroid skin changes appear as

Answer 23

• bilateral symmetrical lopecia (esp in areas of wear or pressure points, not pruritic unless pyoderm underlying)
• seborrhea, lichenification and comedones
• hyperpigmentation on alopecic areas
• recurrent infections (otitis externa and pyoderma)
• rat tail of bald bridge of nose

Question 24

what is myxedema and how does it relate to hypothyroidism in dogs

Answer 24

• excess mucopolysaccharides and hyaluronic acid in dermis
• tragic facial expression

Question 25

what would you see in the CBC and chem of a dog with hypothyroidism

Answer 25

CBC:
- normocytic, normochromic anemia
- leukocytosis if there is an infection

chem:
- increased parameters of lipid metabolism (cholesterol, lipids and triglycerides - while fasting)
- mild-moderate increase hepatic enzymes

Question 26

what are the most common clinical signs of hypothyroidism

Answer 26

dermatologic and metabolic

Question 27

how do you diagnose hypothyroidism

Answer 27

• measure total T4 (high sensitivity, good for screening) should have low or low-normal T4
• free T4
• baseline TSH

Question 28

why does total T4 have poor specificity

Answer 28

can have low result if:
- daily fluctuations within individuals
- non-thyroidal illness/euthyroid sick syndrome
- drugs (glucocorticoids and antibiotics)

Question 29

what is euthyroid sick syndrome

Answer 29

• non thyroidal illnesses supress T4 nd T3 but patient is not truly hypothyroid
• works by decreasing protein binding of T4 and T3, decreasing T4 to T3 conversion and decreasing TSH release

Question 30

how do you idfferentiate euthyroid sick patients from truly hypothyroid patients

Answer 30

euthyroid pateints should have low total T4 with low-normal TSH
hypothyroid should have low total T4 and high TSH

Question 31

explain how measuring free T4 can diagnose hypothyroidism

Answer 31

• only free/unbound T4 which enters the cell is measured
• concentration free T4 reflects thyroid status at tissue level
• less affected by external factors
• mor accurate at diagnosing hypo

Question 32

explain how baseline TSH can diagnose hypothyroidism

Answer 32

• dogs with primary hypo have low T4 and high TSH
• specific if interpreted with tT4 or freeT4
• low sensitivity

Question 33

what are the steps to diagnosing hypothyroidism

Answer 33

• compatible clinical signs = most important
• total T4 low to low normal (better if can get free T4)
• TSH > 0.5 micrograms with low T4
• response to therapeutic trial