Hyperthyroidism physiology Flashcards
What are the most common endocrinopathies seen in cats?
hyperthyroidism
diabetes mellitus
Describe the pathology of hyperthyroidism
Excessive T4 production
Adenomatous hyperplasia of thyroid glands - more functional cells in gland
Describe the pathophysiology of hyperthyroidism
Increased metabolic rate
Increases catabolism as uses up energy reserves
Effects thermoregulation
Effects interaction with CNS (increased sympathetic drive)
Effects emetic centre - makes animal feel nauseous
What are the clinical signs of hyperthyroidism
Weight loss
Polyphagia
PU/PD
Tachycardia
Diarrhoea
Resp abnormalities
vomiting
Haircoat changes (alopecia, matting, seborrhea, thin skin)
Why does hyperthyroidism cause weight loss and polyphagia?
Increased metabolic rate
Increased catabolism
Why does hyperthyroidism cause PU/PD
increase CO, GFR and medullary blood flow
Psychogenic component?
Why does hyperthyroidism cause vomiting?
overeating
activation of emetic centre
Why does hyperthyroidism cuase diarrhoea?
hypermotility
dietary indiscretion - eating odd things
Malabsorption due to excessive eating
What behavioural changes are associated with hyperthyroidism and what causes them?
Hyperactivity
Vocalisation
Agitation and restlessness
Obsessive grooming
Caused by:
- thermoregulation
- increased sympathetic drive
What would you on a clinical exam of a cat with hyperthyroidism?
thin
cervical nodule (goitre) - enlarged lymph node
Tachycardia
Systolic murmur
Dysrhythmias (premature contractions)
What causes hypertrophic cardiomyopathy in hyperthyroidism?
direct effect of thyroid hormones on myocytes
Indirect effect of adrenergic nervous system
Indirect compensatory changes for altered peripheral effusion
What can be seen in clinical chemistry of hyperthyroidism?
Alkaline phosphatase
ALT - some liver compromise due to metabolic effects
Mild Ca and P increase due to bone turnover
Describe methods of diagnosing hyperthyroidism
Baseline hormone conc:
- Total T4
- Free T4
Dynamic hormone tests:
- T3 suppression test
- TRH stim test
Scintigraphy
What are the limitation of total T4 measurement in diagnosing hyperthyroidism?
normal fluctuations
may be normal in early or mild hyperthyroidism
Depressed by non-thyroidal illness
Describe the T3 suppression test in hyperthyroidism diagnosis
In normal cats exogenous T3 => decrease in TSH and T4
Hyperthyroid cats:
- minimal decrease in T4
Used to rule out hyperthyroidism (screening test)
Describe thyroid scintigraphy in hyperthyroidism diagnosis
Radioactive marker identifies functional thyroid tissue
Density of thyroid glands can be compared
Used to confirm hyperthyroidism
Determines benign vs malignant disease
Identifies metastatic disease
