Hyperthyroidism in cats (needs clean-up)
Hyperthyroidism
Clinical disease of thyroid gland
excessive production and secretion of...
thyroxine (T4)
tri-iodothyronine (T3
must be included in the ddx of any aged cat
Thyroid Physiology
Thyroxine (T4)
negative feedback on
hypothalamus
pituitary gland
synthesis of TSH
Hyper T4 Signalment
Older (mean = 12.9),
95% > 8yrs
M=F
No breed predisposition
Etiology
Unknown.
First clinically recognized in 1979-80
Incidence ~ 1:300 and rising
Recognition is increasing
Adenomatous hyperplasia or thyroid adenoma
95-98%
benign
3-5% thyroid carcinoma = malignant
Environmental risk factors thought to play a role
Single cat household
Indoor cats
Frequent spraying with flea control products
Feeding canned foods (many + fish flavor)
Carpet
Clinical Features
Insidious disease
slowly progressive
Owners often do not notice early signs
Signs more often associated with health
Polyphagia
Hyperactivity
Signs mimic many other diseases
Historical Complaints
Wt. Loss 93%
Polyphagia 56%
30-40%
Unkempt hair coat/patchy alopecia
PU/PD
Vomiting
Nervous/hyperactive
Less than< 20%
Decreased appetite
Weakness,
Decreased activity/lethargy
Anorexia
“apathetic hyperthyroid”
Physical Examination
Palpable thyroid 89%
required for all cats
Palpating the thyroids
Extend the head and neck, gentle palpation
May be from the larynx to thoracic inlet
Thin 76%
20-40%
Tachycardia
Small kidneys
Heart murmur
Diagnostic Evaluation
CBC
Chemistry profile
Serum Chemistry Changes
Hepatic profile abnormalities
increased ALP 76%
Renal Abnormalities
increased Creatinine 27%*
Hyperbilirubinemia 3%
Blood pressure
Radiographs +/–
SDMA +/- ??
Echocardiogram +/–
Ancillary thyroid diagnostics +/-
Urinalysis
Helps rule in or out various diseases also
Findings
Usg > 1.035 68%
Usg < 1.015 6%
ssociated with PU/PD
Renal disease
Diabetes Mellitus
increased ALT 88%
Elevations = “moderate”
less than 400 IU/L still consistent with and likely just related to T4.
values > 500 IU/L may have another disease concurrently
Consider
Ultrasound/biopsy of liver
increased BUN 34%*
complicating problem in many hyperthyroid cats
Hyperthyroid cats have increased
GFR
RBF
tubular secretory
re-absorptive capacities
Changes may
mask underlying renal diseas
make moderate disease less obvious on chemistry panels
indicated for cats with
respiratory distress
murmurs
muffled
heart sounds
tachycardia
arrhythmias
May reflect
coexisting primary cardiac disease secondary to the hyperthyroidism (most often)
Findings in HyperT4
Cardiomegaly (~ 50%)
Pleural effusion/CHF (< 5%)
Pericardial effusions - rare
Valuable in cats with evidence for cardiac disease and hyperthyroidism
Hyperdynamic function of the myocardium
(Secondary reversible) Hypertrophic cardiomyopathy (usually)
Congestive (dilated) cardiomyopathy (rare)
Basal total T4 concentrations
Highly reliable in diagnosing cats
superior to basal T3
2 to 10% false negative
if highly suspected and "normal" T4 (false negative)...
repeat it over a period of several weeks
rule out concurrent illness
measure "free" T4
Basal T4 measures the protein bound fraction (>99%)
free component (<1%) of the total
Only “free” T4 is able to enter cells
active product
elevated in 96% of cats with borderline total T4 concentrations
equilibrium dialysis technique
⚠ 5% of cats false positive
not a screening test
“Normal” values in hyperthyroid cats
Fluctuations in baseline values occur
Mild early hyperthyroidism
Concurrent illnesses lead to concurrent euthyroid sick syndrome in hyperthyroid cats
Diagnostic Summary
Repeat if not elevated-1-2 weeks
Wait longer if signs still consistent (1-2 months)
Radionuclide scan
Prognosis
Highly variable
Dependent on
coexisting diseases
Pre-existing renal disease = < survival
Ability to tolerate therapeutic options
State at time of diagnosis
Average survival
with therapy with methimazole is 2 years
I131alone = 4 yrs
Free T4 assay
Baseline T4