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Hyperthyroidism
Anatomy
Thyroid gland is usually two separate lobes
adjacent to first five of six tracheal rings
Lie very close to the carotid sheath and
vagosympathetic trunk
Accessory tissue commonly found in neck
and trunk
Hyperthyroidism
Most common endocrine disease of cats
Most often thyroid adenoma or hyperplasia
Adenocarcinomas only in ~2% of affected
cats and may metastasize
Dogs usually inactive malignancy
Cats usually functional but benign
Hyperthyroidism
Functional increase in tissue produces
excess thyroid hormone
See increased energy metabolism
Body burns through fuel faster and systems
function at higher rate
Multisystemic effects so most have clinical
signs that reflect dysfunction of several
organ systems.
Signalment
Usually middle aged to older cats
No breed or sex predilection
Most often slowly progressive
~70% cats have bilateral lobe enlargement
Clinically
Weight loss, unkempt,
restless, can’t cope
with stress, increased
appetite, vomiting,
diarrhea, PU/PD
Can palpate
enlargement in ~80%
Systolic murmur,
gallop rhythm,
tachycardic
Lab Work
CBC – direct effect on the erythroid marrow
and increased production erythropoietin
Chemistry – slight increase ALT, AST, renal
dysfunction
T3/T4 levels vary – 25% normal T3 but
increased T4, 2% both normal
Free T4 – vary with regular T4 test
Lab Work
Initial evaluation can be off due to
concurrent illness as thyroid hormone may
be high normal or slightly increased
Renal disease, diabetes mellitus, systemic
neoplasia, primary hepatic disease
Repeat basal T4 and rule out concurrent
illness
Radionuclide testing
Depend on the dietary uptake, iodide drugs,
or contrast agent
Relatively insensitive diagnostic
Used to determine dose of therapeutic 131I
Used to detect metastasis
Treatment Options
Life long medical management
Surgical intervention
Radioactive therapy
Medical Management
Aimed to block iodine to tyrosyl group of
thyroglobin and prevent couple into T4 and
T3
Long term goal is to maintain T4 in low
normal range at the lowest possible dose
Short term goal is to lower T4 concentration
before surgical option
Medical Management
Methimazole – manage the problem, not
cure
Dose from 10-15 mg per day so as lower T4
in 2-3 weeks
Can go up to 25-30 mg per day
Recheck every 2-3 weeks to allow
adjustment of the dose
Medical Management
Side effects include
anorexia, vomiting, lethargy, but these usual
transient and resolve
self induced facial excoriation
rare hepatic toxicity
variety hemolytic abnormalities
Surgical Intervention
Most often cures problem but significant
risk due systemic effects of disease process
Can use antithyroid drugs to lower levels
pre-operatively to lower risks
Must leave parathyroid glands to control
calcium homeostasis
Surgical Intervention
Option of intracapsular or extracapsular
removal
Dorsal recumbancy with forelegs pulled
caudal and a ventral midline incision from
larynx to manubrium
Must maintain strict hemostasis or will lose
the parathyroid glands at cranial pole
Surgical Intervention
Extracapsular – remove the entire lobe after
identifying the junction of the external
parathyroid gland and thyroid tissue
Intracapsular – nick incision in capsule to
allow blunt removal of the parenchyma and
removal of as much capsule as can
Surgical Intervention
Side effects include
hypoparathyroidism
Horner’s Syndrome
laryngeal paralysis
hypercalcemia
Radioactive Therapy
Normally in the body iodine only goes to
the thyroid gland
Radioactive iodine concentrates in the
hyperplastic or neoplastic tissue and
destroys it
131I half life of eight days so animal must be
kept isolated
Radioactive Therapy
Cat must be in metabolic cage to collect
waste as is radioactive
Minimal contact
Discharge in 1-3 weeks
No noticed systemic effects
References
Peterson, ME, JF Randolph, and CT Mooney. Endocrine Diseases. The
Cat: Diseases and Clinical Management. 2nd ed vol 2. Ed RG Sherding
DVM. Philidelphia: WB Saunders Co, 1994. 1412Peterson, ME. Hyperthyroidism. Textbook of Veterinary Internal
Medicine. 5th ed vol 2. Ed Ettinger, SJ, and EC Feldman. Philidelphia:
WB Saunders Co, 2000. 1400http://www.vet.purdue.edu/vcs/scottmon/hyperthycat.html
http://www.vin.com/VINDBPub/SearchPB/Proceedings/PR05000
/PR00107.htm
http://www.gcvs.com/imaging/feline_hyperthyroidism.htm