Thank you Jo for all the information,
I absolutely understand your distress over Amber's struggle to regulate her thyroid. Now while it is a bit difficult to visualize the erratic swings in her thyroid values without having a timeline (with dates, thyroid readings, drug doses at the time, etc), her results do sound a wee bit odd. The reason why is because while we can see thyroid levels appear depressed when the cat is unwell (what we call "sick euthyroid"), but we'd not expect spikes in samples (provided her drug dose was stable --not changed by anyone and doses not missed by Amber spitting out tablets) in between samples. So, just as you feel, this is a strange and very uncommon scenario to see in a cat.
Now in regards XXXXX XXXXX here, if this was my patient, my first question would be whether I trust the in-practice blood machine. In house blood machines are usually alright but with the thyroid one (especially if its an Idexx "snap" based machine), it isn't going to be as sensitive as the vet lab machines. Furthermore, while vet nurses to a good job taking care of their in house blood machines, they are not nearly as qualified to manage the blood machines as the professional personnel that a vet lab would employ. Therefore, for the sake of ensuring the machine isn't to blame here, I'd consider testing her thyroid using a professional veterinary lab from now on.
If the phenomena continues, then one would have to question if there is another underlying disease that has not been detected yet that is causing intermittent sick euthyroid when she is stable on her medication. And if that is the case, we'd have to look to Amber's vet to see if there was any other disease suspicion here that they might suspect (since that would guide any diagnostics like further bloods or an ultrasound, etc).
Further to that, since Amber has you in a situation where you have a cat showing hyperthyroid signs but is difficult to medicate due to spikes and dips in her thyroid hormone levels, I would suggest considering using an alternative to traditional tablet treatment for her. Specifically, she may be a good candidate for Hill's Y/D, the thyroid diet. The reason I would suggest this is because the diet is used for cats at any level of hyperthyroidism (since it just limits the iodine the thyroid needs to overproduce her thyroid hormone). Therefore, it wouldn't theoretically matter how high her thyroid was for her to be on the diet. And even if she dipped low, the ramifications of being low on the diet would not be as worrisome as it'd be if she was on a dose of felimazole. If you were keen to read about this diet, you can have a wee peek HERE. And this may be a good option to see if she can be stabilized (without risk of overdose when her levels crash) and to give you a hint if something else may be influencing her thyroid levels.
Overall, Amber is giving you quite a challenge here with her radical changes in thyroid levels on blood sample. Therefore, my first query in this mystery is whether we can trust the blood machine. If you swap to having samples sent to the lab and this settles, then that was likely the source of this blip. If not, and Amber is just being challenging, then the diet treatment for thyroid disease would be the safest treatment option to use with a kitty who's thyroid hormone levels are like a roller coaster. And if she continues to have these spikes/dips, then I would say that we'd want to speak to her vet about whether there could be something else playing a role and influencing this for her.
I hope this information is helpful.
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All the best,
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