physiology of hyperthyroidism Flashcards
what is hyperthyroidism
- excessive production of thyroxine (T4)
- due to adenomatous hyperplasia of the thyroid gland
- majority of cats have bilateral disease
- very small number have thyroid carcinoma
describe the pathophysiology of hyperthyroidism
- increased metabolic rate
- increased catabolism
- effects multiple organs leading to dysfunction
what are the suggested/theorized risk factors for feline hyperthyroidism
- breed
- oncogenesis
- environment or diet
what is the signalment of hyperthyroidism
- older cats (> 10)
- no gender predisposition
- rare is himalyans and siamese
what are the clinical signs of hyperthyroidism
- weight loss
- polyphagia
- PU/PD
- tachycardia
- diarrhea
- respiratory abnormalities
- vomiting
- haircoat changes
why do we see weight loss and polyphagia in hyperthyroid cats
- increased metabolic rate
- increased catabolism
- due to increased T4 production
why do we see PU/PD in hyperthyroid cats
- exacts mechanism unclear
- increased cardiac output/glomerular filtration rate and medullary flow
- possible psychogenic component
- T4 may be diuretic
- concurrent renal disease in some cats
why do we see GI signs in hyperthyroid cats (vom and diarrhea)
- vomiting may be due to overeating, activation of the emetic centre or concurrent disease
- diarrhea may be due to hypermotility and volume, dietary indiscretion or malabsorption
what are some history signs that could point to hyperthyroidism in cats
- hyperactivity
- vocalisation
- agitation and restlessness
- obsessive grooming
- thermoregulation difficulties
what might you find on clinical exam of hyperthyroid cats
- thin
- vervical nodule (goitre) functional or non-functional but couild be something else
- tachycardia +/- murmur, +/- gallop rhythm
explain why you might get cardio abnormalities in hyperthyroid cats
- hypertrophic cardiomyopathy is direct effect of thyroid hormones on myocytes
- indirect effect of adrenergic nervous system
- indirect compensatory changes for altered peripheral perfusion
- tachycardic
- systolic murmur
- dysrhythmias (APC/VPC)
name some uncommon clinical signs of hyperthyroidism in cats
- tremors
- weakness
- dyspnoea
- heat and stress intolerance
- cardiac disease/failure
- systemic hypertension
- fundic exam - renal hemorrhage
what might you see on a routine lab result for a hyperthyroid cat
chem:
- alkaline phosphatase
- increased ALT (liver compromise)
- sometimes mild increase in calcium and phosphate
- kidney markers (concurrent illness)
how do you evaluate thyroid hormones
- baseline hormone concentration (total T4 and free T4)
- dynamic hormone testing (T3 supression, TRH stim)
how is hyperthyroidism diagnosed in cats
- in most cats a single serum total T4 is a diagnostic for hyperthyroidism
- should repeat T4
- maybe a free T4 by equilibrium dialysis
- uncommonly: T3 supression, TRH response and scintigraphy
what are the limitations to testing totat T4
- daily/hourly fluctuations
- may be normal in early or mild hyperthyroidism
- depressed by non-thyroidal illness (in which case free T4 more helpful)
what is a T3 suppression test
- in normal cats administration of exogenous T3 causes a decrease in TSH and T$
- in diseased cat, T4 and TSH cant be downregulated further, so will see minimal decrease in T4 with hyperthyroid cats
- used to rule out hyperthyroidism
what is thyroid scintigrpahy
- radioactive marker identifies functional thyroid tissue
- IV admin of 99mtechnetium or 123iodine isotope
- specific counts via gamma camera to determine thyroid/salvary gland ratio
- needs special facilities
- confirms dx of hyperthyroidism
- localises tissue
- determines benigns vs malignant
- identifies ectopic tissue
- identifies metastatic disease
discuss canine hyperthryoidism
- rare
- thyroid carcinomas (not many thryroid carcinomas are funcitonal)
- highly metastatic
- common in older dogs over 10 years
- goldens, boxers and beagles more commonly
how do you treat hyperthyroidism
- medical: inhibit thyroid hormone synthesis (therapy for life)
- surgical: remove enlarged thryoid gland(s) (requires care not to damage or remove parathyroids as will lead to hypocalcemia)
- radiotherapy: local destruction of tissue
- dietary control: exclusive diet of food deficient in iodine and selenium so body cant make thryoid hormone
