RE: Cat with yo-yo hyperthyroidism

Ian Ramsey — Tue, 22 Jun 2021 22:39:01 GMT

Firstly - do the T4 levels correlate with clinical signs - does the cat behave like a hyperthyroid cat when it is hyper (and vice versa) - are we chasing numbers or is this a real clinical presentation? This is important as assay reliability may be at the heart of this problem.

How are you measuring T4 in this case please? Your figures do not look like a Siemens immulite or other main laboratory assay. At least some of the in house assays tend to have a higher variability as they have been set up primarily for diagnosis rather than monitoring. This sensitivity at one or other end of the normal range can be adjusted in some in house assays to help discriminate between normal and low vs normal and high. The result when used for monitoring will therefore appear to swing around rather a lot as you don't know if you are looking for a low or a high.

If given SID then I would expect greater day to day variability due to greater variation in blood levels day to day and variation in absorption (is the diet constant?, does the cat get diarrhoea??). Not sure what you can do easily about this except perhaps switch to something like Thyronorm that you can dose more accurately whilst keeping to twice daily.

Although I am sure your owners are wonderful, the other main reason for the variability still has to be variable compliance. We know from good quality studies of human medication compliance that even the most diligent patients still manage to miss chronic medications a bit. Then more complex the dosing regimen (frequency or changing dose day to day) then the more likelihood of missing a dose.

Hope this helps and gives you some avenues to explore

Best wishes

Ian Ramsey