Methimazole starting dose is 2.5 mg PO q12h, titrated in 2.5 mg increments every 3 weeks to a maximum of 20 mg/day (not exceeding 10 mg per single administration), targeting a serum total T4 (TT4) of 1.0–2.5 mcg/dL. AAHA Clinical G…+2

The AAHA titration approach begins more conservatively — 1.25–2.5 mg q24h for the first week, then advancing to 2.5–5 mg q12–24h — to minimize early adverse effects, though the FDA-approved label for both Felimazole and Felanorm specifies 2.5 mg q12h as the starting dose with reassessment at 3 weeks.AAHA Clinical G…+2 Both approaches converge on the same titration principle: adjust based on individual TT4 response and clinical improvement.

The average maintenance dose in extended use is 2.5 mg q12h (5 mg/day total), with a documented range of 2.5–15 mg/day.FDA DailyMed An… Doses required may be lower now than historically, likely reflecting earlier diagnosis at milder disease stages — the median dose in an earlier era was 10 mg/cat/day (range 2.5–20 mg), compared to a current median of 5 mg/cat/day (range 2.5–20 mg).Journal of Vete… Veterinarians should therefore be conservative with initial dosing.

Monitoring follows a structured schedule: obtain hematology, biochemistry, and TT4 before starting, then recheck at 3 weeks and 6 weeks, then every 3 months thereafter. Cats receiving more than 10 mg/day require more frequent monitoring.FDA DailyMed An…+1 T4 samples may be drawn at any time of day after methimazole administration — timing relative to the last dose does not significantly affect TT4 results in hyperthyroid cats.AAHA Clinical G…+1

Approximately 20% of methimazole-treated cats develop iatrogenic hypothyroidism (IH), defined as a low thyroid hormone concentration combined with an elevated TSH concentration.Journal of Vete… Of cats with a low TT4 or free T4, approximately 70% have a concurrently elevated TSH.Journal of Vete… Development of a low TT4 is dose-related, but azotemia is not — cats with elevated TSH are more likely to be azotemic regardless of TT4 level.Journal of Vete… Avoidance of hypothyroidism is imperative and requires ongoing monitoring.AAHA Clinical G…

Transdermal methimazole (pluronic lecithin organogel applied to the inner pinna) is a safe long-term alternative for cats that are difficult to pill, with clinical improvement observed in all cats and rare adverse effects — mild transient gastrointestinal signs and pinna erythema each occurred in only a small number of cats.Journal of Feli… However, maintaining TT4 consistently within the reference interval is more difficult with transdermal administration, and doses tend to increase after 24–36 months of therapy, with a documented range of 1.0–15.0 mg/day.Journal of Feli… Owner compliance should be assessed regularly, as irregular application was reported by several owners.Journal of Feli… Gastrointestinal adverse effects are mitigated by the transdermal route, but all other serious adverse effects — including bone marrow dyscrasias and skin excoriations — require discontinuation and a change in treatment modality.AAHA Clinical G…

Key drug interactions and contraindications: anticoagulant effect may be potentiated; concurrent phenobarbital may reduce methimazole efficacy; doses of beta-blockers, digitalis glycosides, and theophylline may need reduction once euthyroidism is achieved; methimazole inhibits hepatic oxidation of benzimidazole anthelmintics (e.g., fenbendazole), increasing their plasma concentrations.FDA DailyMed An… Renal function must be carefully evaluated before and during treatment, as reversal of hyperthyroidism can unmask underlying renal disease by reducing glomerular filtration rate.FDA DailyMed An…

Signs requiring immediate drug discontinuation include anorexia, vomiting, head/facial pruritus or edema, depression/lethargy, weight loss, anemia, skin lesions, diarrhea, fever, or lymphadenopathy — these may indicate hepatopathy, immune-mediated anemia, thrombocytopenia, or agranulocytosis.FDA DailyMed An… Adverse effects including vomiting, bone marrow dyscrasias, and skin excoriations can appear up to several months after initiating therapy.AAHA Clinical G…

Would you like guidance on how to manage a cat whose TT4 is controlled but TSH remains elevated on methimazole?