Doses - METHIMAZOLE

Cats:

For hyperthyroidism:
a) Initially, 5 mg (not 5 mg/kg) PO q8h. Recheck serum thyroxine in 3 weeks, if little orno decrease, increase dose by 5 mg every 3 weeks until appropriate response occurs.
During first 3 months of therapy, examine cat every 3 weeks to monitor both efficacyand adverse effects. Cats receiving long-term therapy should be monitored every 3-6months. Use lowest effective dose. Some cats may only require once daily administration. (Meric 1989)
b) Initially, 10 - 15 mg/day divided every 8-12 hours PO (depending on the severity ofhyperthyroidism). If little or no decrease in T4 occurs, gradually increase dose in 5 mgincrements. Exclude poor owner compliance or difficulty in administering beforeincreasing dose. If drug is being used as a pre-operative treatment, may perform thyroidectomy when T4 decreases to low or normal levels. When used for long-termtreatment, if T4 levels fall to low or low-normal values, may decrease dose by 2.5 - 5 mguntil lowest effective dosage is reached. Some cats may tolerate once daily dosing.(Peterson, Kintzer, and Hurvitz 1988)
c) Initially 5 mg tid, many cats maintain an euthyroid state at 5 - 7.5 mg bid. (Feldman and Nelson 1987b)
Monitoring Parameters -
During first 3 months of therapy (baseline values and every 2-3 weeks):

1) CBC, platelet count

2) Serum T4

3) If indicated by symptomatology: liver function tests, ANA
After stabilized (at least 3 months of therapy):

1) T4 at 3 - 6 month intervals

2) Other diagnostic tests as dictated by adverse effects
Client Information - It must be stressed to owners that this drug will decrease excessive thyroidhormones, but does not cure the condition and that compliance with the treatment regimen isnecessary for success.
Dosage Forms/Preparations/FDA Approval Status/Withholding Times - Veterinary-Approved Products: None