Exam 5: feline Hyperthyroidism

how is feline hyperthyroidism generally defined

• excessive production and secretion of T4 and T3 by the thyroid gland

• most common endocrine disorder in cats

• one of the most common geriatric disorders in cats

what is the pathology of hyperthyroidism

• primary disease

• adenmtous hyperplasia

• human - Plummer’s dx

• benign

• bilateral usually

• rarely carcinoma

what is the signalment typically associated with hyperthyroidism

• older cats

• females

  • decreased risk in siamese and himalayans

what are the clinical signs of hyperthyroidism

• weight loss

• polyphagia

• PU/PD

• hyperactivity

• vomiting

• apathetic

• may not be noticable early in disease

how might hyperthyroidism present on physical exam

• thin

• palpable thyroid aka thyroid slip

• hyperactive

• poor hair coat

• dehydration

• attitude

• cervical ventroflexion

• tachycardia

• murmur

• gallop rhythm

what is thyroxic cardiomyopathy

• similar to HCM

• can cause murmur

• usually resolves with therapy

how is blood pressure affected by hyperthyroidism

• often hypertensive

• older cats with hyperthyroidism, cardiac disease, renal disease, hyperaldosteronism

• fundic exam

• neurologic

which fundic exam is hypertensive

right

what is the diagnostic process of hyperthyroidism

• clinical signs

• CBC/chem/UA

• concentrations of total T4, free T4, TSH

• suppression testing

how will hyperthyroidism present on minimum database

• dehydration

• azotemia either from dehydration or concurrent renal disease

• increased ALT

• usosthenuria common

what is the screening test of choice for diagnosing hyperthyroidism

tT4

how is normal T4 in hyperT4 cats described

• daily fuctuation

• euthyroid sick syndrome

• hyper T4 cats may have normal in top ½ of range

if everything else fits with hyperthyroidism, but total T4 is still within normal limits, what do you do?

• repeat total T4

• free T4 and TSH

• T3 suppression

• utimately scintigraphy

describe free T4 by equilibrium dialysis

• $$

• more sensitive than total T4

• more false positives (different from dogs)

• may be increased in sick patients

• most useful in strong suspects with normal T4

describe TSH testing

• expected to be very low with hyperthyroidism

• canine TSH is not feline TSH is not human TSH

• feline closer to canine

what is the theory of the T3 suppression test

• T3 should inhibit TSH production and should decrease T4

• following T3 administration, T4 should be <50% baseline in normal cats, minimal suppression in hyperthyroid cats

how is nuclear scintigraphy performed

• radioactive isotope administered I-123 (half life 8.1 days) or Pertechnetate (HL 6 hrs )

• gamma emission then quantified

what is the use of nuclear scintigraphy in diagnosing hyperthyroidism

• confirms

• unilateral vs biateral

• identifies ectopic tissue (surgery is not an option)

• helps differentiate adenoma from carcinoma

describe the scintigraphy

normal

describe the scintigraphy

unilateral adenoma

describe the scintigraphy

bilateral adenomas

describe the scintigraphy

thyroid carcinoma

describe the scintingraphy

ectopic thyroid adenoma