Optimal Testing for Thyroid Hormone Concentration after Treatment with Methimazole in Healthy and Hyperthyroid Cats

Abstract

Background: Methimazole suppresses thyroid hormone synthesis and is commonly used to treat feline hyperthyroidism. The degree of variation in thyroid hormone concentrations 24 hours after administration of methimazole and optimal time for blood sampling to monitor therapeutic efficacy have not been determined.Objective: To assess thyroid hormone concentration variation in serum of normal and hyperthyroid cats after administration of methimazole.Animals: Four healthy cats and 889 retrospectively acquired feline thyroid hormone profiles. Methods: Crossover and retrospective studies. In the crossover study, healthy cats were treated with increasing doses of oral methimazole until steady state of thyroid suppression was achieved. Thyroid hormones and thyroid stimulating hormone (TSH) were serially and randomly monitored after methimazole. Paired t-tests and a 3-factor analysis of variance were used to determine differences between thyroid hormone concentrations in treated and untreated cats in the crossover study. Thyroid profiles from methimazole-treated hyperthyroid cats were retrieved from the Diagnostic Center for Population and Animal Health database and reviewed. Linear regression analysis evaluated relationships of dosage (mg/kg), dosing interval (q24h versus q12h), and time after methimazole to all thyroid hormone concentrations.Results: All serum concentrations of thyroid hormones were significantly suppressed and TSH was significantly increased for 24 hours after administration of oral methimazole in healthy cats (P o .005). In hyperthyroid cats, there were no significant relationships between thyroid hormone concentrations and time postpill or dosing interval.Conclusions: Timing of blood sampling after oral methimazole administration does not appear to be a significant factor when assessing response to methimazole treatment.

The Efficacy and Safety of a Novel Lipophilic Formulation of Methimazole for the Once Daily Transdermal Treatment of Cats with Hyperthyroidism

“…Pharmacokinetic studies on cats with hyperthyroidism and normal cats have shown methimazole to have a short half‐life in the plasma (2.3 ± 0.4 hours for cats with hyperthyroidism and 4.7 ± 1.4 hours for normal cats). Moreover, a recent study has shown that there is no relationship between the timing of blood sampling after oral methimazole and the TT4 concentration . Therefore, because methimazole concentrates in the thyroid, plasma methimazole concentrations are unlikely to be correlated with plasma TT4 concentrations.…”

“…We found that TT4 concentrations remained suppressed for 18–20 hours after (Fig ) treatment with transdermal methimazole, indicating that for this treatment, once daily application is sufficient. These results are similar to a recent study of healthy cats treated with once daily oral methimazole that showed significant suppression of thyroid hormone concentrations for 24 hours …”

Feline hyperthyroidism: diagnosis and treatment

“…Even though canine thyroid stimulating hormone has a 96% homology with the hormone in cats, a distinction between normal and undetectable values was not found to be always reliable. Today, the value of thyroid stimulating hormone is limited to the exclusion of the disease when a detectable concentration is measured (Wakeling et al, 2008, Rutland et al, 2009.…”

“…It begins after two to three weeks and includes measurement of a complete blood count, alkaline phosphatase, alanino-aminotransferase, blood urea nitrogen concentration and creatinine concentration, along with total thyroxine blood concentrations (Trepanier, 2007). Timing of blood sampling is not a significant factor for total thyroxine blood concentration measurement (Rutland et al, 2009;Borreti et al, 2013). If side effects are noted, the drug should be discontinued and another therapeutic option should be pursued (Luyre, 2006).…”

Pharmacokinetics and Relative Bioavailability of Ambroxol Hydrochloride Aerosol and Injection

“…The bidirectional one-side t test suggests that the bioavailabilities of AH aerosol and injection were identical [7,8].…”