Exactly, as Rosella said just because the kidney ( especially ) values were OK on initial testing DOESN'T mean that they ARE in reality always fine!
Hyperthyroidism actually increases blood supply to them (one of the 'good' effects of the conditon) thus masking problems. For example, in a cat without this condition kidney problems would be apparent. So then as a result of the meds, when the metabolism ( and the blood supply ) is slowed down, due to to lowering of the T4 levels this 'benefitial 'effect is removed and pre-existing ( if applicable ) kidney problems becomes evident.
In these cases, the trick is to find an optimun level of dosing which lowers the T4 while effectively retaining good kidney function. Unfortunately, in some cases this isn't always achieved and renal failure results owing to 'over-medicating' in the initial stages.
Faced with the fact that MOST older cats have compromised kidney function, it's something vets should be mindful of when prescribing anti-thyroid meds and sadly this isn't always the case.
Of course, Tily may have genuinely good kidney function but best to be forewarned! Like Rosella said, it's best to get tests sooner rather than later.