physiology of hyperthyroidism

Question 1

what is hyperthyroidism

Answer 1

• 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

Question 2

describe the pathophysiology of hyperthyroidism

Answer 2

• increased metabolic rate
• increased catabolism
• effects multiple organs leading to dysfunction

Question 3

what are the suggested/theorized risk factors for feline hyperthyroidism

Answer 3

• breed
• oncogenesis
• environment or diet

Question 4

what is the signalment of hyperthyroidism

Answer 4

• older cats (> 10)
• no gender predisposition
• rare is himalyans and siamese

Question 5

what are the clinical signs of hyperthyroidism

Answer 5

• weight loss
• polyphagia
• PU/PD
• tachycardia
• diarrhea
• respiratory abnormalities
• vomiting
• haircoat changes

Question 6

why do we see weight loss and polyphagia in hyperthyroid cats

Answer 6

• increased metabolic rate
• increased catabolism
• due to increased T4 production

Question 7

why do we see PU/PD in hyperthyroid cats

Answer 7

• 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

Question 8

why do we see GI signs in hyperthyroid cats (vom and diarrhea)

Answer 8

• 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

Question 9

what are some history signs that could point to hyperthyroidism in cats

Answer 9

• hyperactivity
• vocalisation
• agitation and restlessness
• obsessive grooming
• thermoregulation difficulties

Question 10

what might you find on clinical exam of hyperthyroid cats

Answer 10

• thin
• vervical nodule (goitre) functional or non-functional but couild be something else
• tachycardia +/- murmur, +/- gallop rhythm

Question 11

explain why you might get cardio abnormalities in hyperthyroid cats

Answer 11

• 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)

Question 12

name some uncommon clinical signs of hyperthyroidism in cats

Answer 12

• tremors
• weakness
• dyspnoea
• heat and stress intolerance
• cardiac disease/failure
• systemic hypertension
• fundic exam - renal hemorrhage

Question 13

what might you see on a routine lab result for a hyperthyroid cat

Answer 13

chem:
- alkaline phosphatase
- increased ALT (liver compromise)
- sometimes mild increase in calcium and phosphate
- kidney markers (concurrent illness)

Question 14

how do you evaluate thyroid hormones

Answer 14

• baseline hormone concentration (total T4 and free T4)
• dynamic hormone testing (T3 supression, TRH stim)

Question 15

how is hyperthyroidism diagnosed in cats

Answer 15

• 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

Question 16

what are the limitations to testing totat T4

Answer 16

• daily/hourly fluctuations
• may be normal in early or mild hyperthyroidism
• depressed by non-thyroidal illness (in which case free T4 more helpful)

Question 17

what is a T3 suppression test

Answer 17

• 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

Question 18

what is thyroid scintigrpahy

Answer 18

• 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

Question 19

discuss canine hyperthryoidism

Answer 19

• rare
• thyroid carcinomas (not many thryroid carcinomas are funcitonal)
• highly metastatic
• common in older dogs over 10 years
• goldens, boxers and beagles more commonly

Question 20

how do you treat hyperthyroidism

Answer 20

• 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