Feline hyperthyroidism is the increased production of the thyroid hormone. It is almost always caused by a cancer, benign or malignant. It can set off a chain reaction which could cause blindness.
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The cat’s heart should be carefully evaluated as it could be damaged by the increased metabolic rate causes by the increased production of thyroid hormones.
The heart, working harder, may cause high blood pressure (hypertension). High blood pressure can cause retinal bleeding. This in turn can cause the retina of the eye to become detached. If this is left untreated blindness can occur.
It is a long chain of events but there is a connection between hyperthyroidism and blindness in cats.
I am not a veterinarian but I’ll guess that it is quite rare. The possibility of blindness in cats with hyperthyroidism is not mentioned in the best book for home treatments and care1. But VCA hospitals (author: Ernest Ward, DVM & Robin Downing, DVM, CVPP, CCRP, DAAPM) do refer to it on the internet.
Note: 1 – Cat Owner’s Home Veterinary Handbook 3rd Ed.