I wouldn’t expect T4 levels to actually increase during treatment with Vidalta but although some cats do well on it, it certainly isn’t unknown for others to fail to respond. When it works its brilliant but there are technical reasons why it doesn’t always do so (see later explanation if you’re interested) and in your situation I would be inclined to ask your vet if you could try Felimazole (no more than 2.5 mg twice daily as a starting dose), instead of persevering with the Vidalta. You have nothing to lose by doing this because it is an accepted and respected alternative.
Techy bit:
Conventional tablets are designed to break down in the stomach and release all of the active ingredient which is then absorbed into the system. Vidalta is different in that it is a sustained-release tablet, which is designed to release its active ingredient slowly during its passage though the entire gastro-intestinal tract. The theory is that this results in more even blood levels of the active ingredient throughout the day, rather than the peaks and troughs that you get with a conventional tablet. That’s ideal when it works as it’s supposed to do, but unfortunately there is a lot of variation in the way in which bodies, be they animal or human, process sustained-release formulations. It is quite possible for the tablets to pass through the body without releasing the full dose and, in a worst case scenario, they might not release any of the dose.
Thyroid hormones speed up most body processes, including gastro-intestinal transit time (i.e. the length of time from swallowing something to it being excreted in the faeces) and in a hyperthyroid cat (or person), the tablets may simply not be in the body long enough for the medication to be released as it should be. That’s bad enough but what happens next can be even worse. You haven’t seen an adequate therapeutic response so you increase the dose (possibly more than once) and eventually you reach the point where the T4 levels fall to within the normal range. You think that’s it, job done, but it isn’t. When the T4 levels fall the speeded-up body processes, including the GI transit time, also slow down, meaning that the tablets now remain in the system for longer and have the potential to release more of the active ingredient than they were doing previously. The result can be a catastrophic overdose of medication.
You’ve probably already realised that I am not a fan of Vidalta. There are other reasons in addition to those I have already mentioned. The ‘active ingredient’ is carbimazole, which has to be metabolised in the body to methimazole (also known as thiamazole), before it can do its job. Without getting too technical, metabolic rates are also likely to be affected by thyroid hormones which adds yet another complication when attempting dosage adjustment.
