Blood testing cats on Felimazole

RE: Blood testing cats on Felimazole

Ian Ramsey — Tue, 12 Nov 2019 09:55:29 GMT

Dear all,

I also was unaware of any specific time that was needed and read this thread with interest. So I did a bit of research......

Fortunately there was one article published which backs up this emerging consensus on this discussion thread regarding timing of sampling. This paper shows no effect of time post dosing on T4 when methimazole is given orally. A later paper looked at transdermal application and in this case also there was no effect of time after administration on T4 concentrations.

Both abstracts are below if you are interested. Hope this helps with any discussion with colleagues about this.

Best wishes

Ian

J Vet Intern Med. 2009 Sep-Oct;23(5):1025-30. doi: 10.1111/j.1939-1676.2009.0370.x. Epub 2009 Aug 11.

Optimal testing for thyroid hormone concentration after treatment with methimazole in healthy and hyperthyroid cats.

Rutland BE1, Nachreiner RF, Kruger JM.

Author information

1: Department of Small Animal Clinical Sciences, Michigan State University, East Lansing, MI 48824-1314, USA. rutlandb@cvm.msu.edu

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 < .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.

J Vet Intern Med. 2013 Mar-Apr;27(2):377-81. doi: 10.1111/jvim.12040. Epub 2013 Feb 9.

Duration of t4 suppression in hyperthyroid cats treated once and twice daily with transdermal methimazole.

Boretti FS1, Sieber-Ruckstuhl NS, Schäfer S, Baumgartner C, Riond B, Hofmann-Lehmann R, Reusch CE.

Author information

1: Clinic for Small Animal Internal Medicine, Vetsuisse Faculty, University of Zurich, Zurich, Switzerland. fboretti@vetclinics.uzh.ch

Abstract

BACKGROUND:

Transdermal methimazole is an acceptable alternative to oral treatment for hyperthyroid cats. There are, however, no studies evaluating the duration of T4 suppression after transdermal methimazole application. Such information would be valuable for therapeutic monitoring.

OBJECTIVE:

To assess variation in serum T4 concentration in hyperthyroid cats after once- and twice-daily transdermal methimazole administration.

ANIMALS:

Twenty client-owned cats with newly diagnosed hyperthyroidism.

METHODS:

Methimazole was formulated in a pluronic lecithin organogel-based vehicle and applied to the pinna of the inner ear at a starting dose of 2.5 mg/cat q12h (BID group, 10 cats) and 5 mg/cat q24h (SID group, 10 cats). One and 3 weeks after starting treatment, T4 concentrations were measured immediately before and every 2 hours after gel application over a period of up to 10 hours.

RESULTS:

Significantly decreased T4 concentrations were observed in week 1 and 3 compared with pretreatment concentrations in both groups. All cats showed sustained suppression of T4 concentration during the 10-hour period, and T4 concentrations immediately before the next methimazole treatment were not significantly different compared with any time point after application, either in the BID or SID groups.

CONCLUSIONS:

Because transdermal methimazole application led to prolonged T4 suppression in both the BID and SID groups, timing of blood sampling does not seem to be critical when assessing treatment response.

RE: Blood testing cats on Felimazole

Andreia Dias — Mon, 11 Nov 2019 14:54:35 GMT

I was told with Felimazole you can do it at any time during the day. With Thyronorm we do it between doses, but not too close to when patient is due the next dose, but honestly I don't know if there is a scientific reason for that!!!

RE: Blood testing cats on Felimazole

Kate Richardson — Mon, 11 Nov 2019 13:24:53 GMT

I test any time of day

Fasted samples seem popular with some vets also, unless specifically checking glucose I don’t insist on fasting

RE: Blood testing cats on Felimazole

Bob Russell — Fri, 08 Nov 2019 09:41:41 GMT

I cannot see any scientific logic to the 4 hours post pill.

The medication works by selectively suppressing thyroid tissue which is not likely to increase significantly over a few hours.

It also does not matter if the T4 is 25, 26 or 30 because treatment requirements will be the same.

I blood test whenever it is convenient

RE: Blood testing cats on Felimazole

Thomas Johnson — Fri, 08 Nov 2019 09:09:41 GMT

[quote user="Clive Ansell"]

[quote user="Sarah Keir"] Perhaps they are getting mixed up with when to measure T4 for hypothyroid dogs which is time critical?[/quote]

that was my thought initially, but they are adamant all cats on thyroid medication should be bled 4 hours post pill

[/quote]

I've got memories of this being the dogma 10-15 years ago based on the plasma half life of methimazole, but since it has been shown that methimazole maintains concentrations for significantly longer in the thyroid tissue it's no longer considered necessary.

RE: Blood testing cats on Felimazole

Clive Ansell — Fri, 08 Nov 2019 08:25:30 GMT

[quote user="Sarah Keir"] Perhaps they are getting mixed up with when to measure T4 for hypothyroid dogs which is time critical?[/quote]

that was my thought initially, but they are adamant all cats on thyroid medication should be bled 4 hours post pill

RE: Blood testing cats on Felimazole

Sarah Keir — Thu, 07 Nov 2019 21:58:30 GMT

I thought if at steady state and had the treatment regularly for the previous few weeks then it didn't matter when during the day T4 was measured. Perhaps they are getting mixed up with when to measure T4 for hypothyroid dogs which is time critical?

RE: Blood testing cats on Felimazole

Sara Ramsey — Thu, 07 Nov 2019 19:20:03 GMT

I had a colleague who insisted this was correct, I asked Dr Sarah Caney and she didn't stipulate timings for monitoring bloods.

My thoughts are that as we aim for the lower end of the reference range and clinical resolution, that timing doesn't matter too much? I also like to make things as easy for owners to consent to blood tests. I do them quite regularly in these cats. More to try spot CKD early than to monitor T4 in stable cats.

Also: I wondered if, because it's tumour secreting thyroxine diurnal rhythms might not be so relevant.