Treatment strategies for hyperthyroid cats (Proceedings)

a. Medical - antithyroid medications - thiourelene compounds

(1) mechanism of action

(a) prevent iodine incorporation into tyrosyl groups

(b) prevent coupling of MIT and DIT into T3 and T4

(c) may directly interact with the thyroglobulin molecule

(2) examples

(a) methimazole (Tapazole)

(b) propyluracil (PTU) - don't use - too many side effects in cats

(c) carbimazole - not available in the US

(3) advantages for use

(a) inexpensive - less initial outlay of $ for owners

(b) can control the amount of thyroid hormone produced - especially important if there is suspected renal compromise

(c) stabilize animals for surgery

(4) preparations

(a) pill

(b) liquid – compounding pharmacy

(c) transdermal ointment – PLO vehicle

(5) disadvantages

(a) owners have to pill or apply ointment to animal daily

(b) side effects - usually see within the first 3 months

i. transient anorexia, vomiting, lethargy (15% of cats treated) - usually resolves quickly - can still use drug

ii. facial excoriations - resolves, but need to stop drug

iii. hepatic toxicity - resolves-sometimes takes several weeks, but need to stop drug immune-mediated problems

-hemolytic anemia -leukopenia

-thrombocytopenia

-positive ANA

-resolve after stopping methimazole, but can't use the drug again

(c) transdermal ointment- less GI side effects; less effective.

(6) treatment regime

(a) if no signs of renal disease begin at 2.5-5 mg bid

(b) if renal insufficiency or failure present, begin at 1.25-2.5 mg bid

(c) monitor in 1-2 weeks and make dose adjustments; also monitor for adverse effects

(d) should check a CBC+platelets, BUN/Cr and T4, minimally

(e) increase methimazole slowly if necessary

(f) monitor every 2-4 weeks for first 3 months; then every 3-6 months as necessary

b. Medical - organic iodine compounds – ipodate (Orografin) or iopanoic acid (Telepaque)

(1) mechanism of action

(a) inhibition of outer-ring 5' deiodination of T4 to T3

(b) direct inhibition of thyroid hormone secretion

(2) advantages

(a) similar to methimazole

(b) should be used in cats in which surgery or 131I is not an option and who cannot tolerate methimazole

(3) disadvantages

(a) ipodate is off the market and is not widely available, iopanoic acid (Telepaque) is available, but studies done with ipodate

(b) works best in cats with mild or moderate disease

(c) cats become refractory after approximately 3 months

(d) monitor by serum T3 levels and primarily by improvement in clinical signs

c. Surgery

(1) "a chance to cut is a chance to cure"

(2) usually remove both glands

(3) animals usually euthyroid

(4) advantages

(a) hopefully curative

(b) no lifelong administration of medication

(c) no monitoring of blood levels unless signs recur

(5) disadvantages

(a) cost

(b) invasiveness

(c) requires hospitalization

(d) complicated post-surgical monitoring

(e) post-surgical hypocalcemia - usually resolves

(f) won't get ectopic hyperthyroid tissue; animals may relapse with disease (uncommon complication)

(g) may precipitate renal failure

d. 131I treatment

(1) 131I is concentrated in the thyroid gland in the hyperfunctioning follicles

(2) 131I is primarily a emitter - causes local tissue damage- kills hyperfunctional follicle cells; T1/2 = 8 days

(3) advantages

(a) one subcutaneous injection is curative in 95% of cats

(b) negligible side effects

(c) not invasive

(d) will also affect ectopic thyroid tissue

(e) will kill malignant cells

(4) disadvantages

(a) hospitalization according to state law for radioactive clearance; usually 3 days-2 weeks - animals must be away from owners

(b) cost

(c) may precipitate renal failure

(d) optimally animals should be off methimazole for 48 hours prior to treatment

(5) dose

(a) 4-6 mCi for benign disease depending on the severity of disease, thyroid size, and initial T4 level

(b) 10-30 mCi for adenocarcinomas

e. Percutaneous ethanol injection

(1) ultrasound-guided injection of ethanol into the thyroid gland

(2) complication - laryngeal paralysis

(3) not recommended

f. Adjunct therapies

(1) blocker - propanolol or atenolol may need to decrease dose in hyperthyroid animals

(2) thiamine