physiology of hypothyroidism Flashcards
where is most of T3 produced
peripheral tissues
what are the types of hypothyroidism and where do they each originate
- primary: thyroid disease
- secondary: pituitary origin
- tertiary: hypothalamus
discuss primary hypothyroidism
- 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
discuss secondary hypothyroidism
- 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
discuss tertiary hypothyroidism
- lack of TRH in the hypothalamic supraoptic and paraventricular nuclei
- rarely documented in the dog
what is the most common cause of hypothyroidism in dogs
lymphocytic thyroiditis
or
idiopathic follicular atrophy
what is the signalment of canine hypothyroidism
- breeds: dobermans, goldens, cocker spaniels, irish setters, terrier
- no sex predisposition
- peak incidence 4-6 years
list clinical signs of canine hypothyroidism
- 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
how does hypothyroidism affect development
- reduced development and maturation of brain cells in fetus and young
- important to have thyroid during fetal period and first few months after bitth
how does hypothyroidism affect growth
- growth retardation
- smaller
- shorter bones
- delayed closure of physes (growth plates)
what are the metabolic actions of thyroid hormones
- increase carbohydrate metabolism
- increase fat metabolism
- increase basal metabolic rate
- decrease body weight
how does hypothyroidism affect carbohydrate metabolism
- thyroid hormone stimulates glucose metabolism, increasing glucose uptake, insulin sensitivity, insulin secretion, glycolysis and gluconeogenesis
- therefore, hyporhtyroidism slows carbohydrate metabolism leading to weight gain
how does hypothyroidism affect metabolism of fats
- 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
how does hypothyroidism affect basal metabolic rate
- 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
how does hypothyroidism affect musculoskeletal
- reduces muscle tone and changes in fibre type
how does hypothyroidism affect cardiovascular system
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
how does hypothyroidism affect the respiratory system
- 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)
how does hypothyroidism affect the nervous system
- 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)
how does hypothyroidism affect the CNS
- mentally slower (hard to quantify in animals)
- lethargic and require more sleep
how does hypothyroidism affect the reproductive system
- reduced sexual drive
- infertility
- shortened oestrus
- prolonged eostrual bleeding
- prolonged anoestrus
- testicular atrophy
- reduced spem production
how does hypothyroidism affect the GI system
- thyroid hormone increases appetite and feed intake, increases secretion of pancreatic enzymes and increases motility in normal animals
- leads to consipation in hypothyroid
how does hypothyroidism affect the integument
- 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
what do canine hypothyroid skin changes appear as
- 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
what is myxedema and how does it relate to hypothyroidism in dogs
- excess mucopolysaccharides and hyaluronic acid in dermis
- tragic facial expression
what would you see in the CBC and chem of a dog with hypothyroidism
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
what are the most common clinical signs of hypothyroidism
dermatologic and metabolic
how do you diagnose hypothyroidism
- measure total T4 (high sensitivity, good for screening) should have low or low-normal T4
- free T4
- baseline TSH
why does total T4 have poor specificity
can have low result if:
- daily fluctuations within individuals
- non-thyroidal illness/euthyroid sick syndrome
- drugs (glucocorticoids and antibiotics)
what is euthyroid sick syndrome
- 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
how do you idfferentiate euthyroid sick patients from truly hypothyroid patients
euthyroid pateints should have low total T4 with low-normal TSH
hypothyroid should have low total T4 and high TSH
explain how measuring free T4 can diagnose hypothyroidism
- 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
explain how baseline TSH can diagnose hypothyroidism
- dogs with primary hypo have low T4 and high TSH
- specific if interpreted with tT4 or freeT4
- low sensitivity
what are the steps to diagnosing hypothyroidism
- 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
