Lots of cats with hyperthyroidism also have kidney disease. In many cases, this is not apparent on initial blood testing and only reveals itself once the thyroid is brought under control. The drop in blood pressure back to normal with thyroid treatment reduces excessive blood flow through the kidneys.
These diseased kidneys may have been performing ok with plenty of blood passing through them at high pressure. However, they begin to struggle once this is brought down to normal levels and flags may start to appear on your cat’s bloodwork indicating a problem. This is referred to as ‘unmasking’ underlying kidney disease. This is not caused by anti-thyroid treatments; none are kidney damaging. The kidney disease was always there but was disguised by the cat’s elevated blood pressure (Carney et al, 2016).
However, there are known damaging effects of high blood pressure on healthy kidneys, so untreated hyperthyroidism also has the potential to contribute to kidney disease in the first place.
Your vet will usually monitor your cat’s kidneys as part of their blood screening.