Hello, and welcome.
It's very kind of you to be concerned about your sister's cat. A T4 score of 8.5 is high and indicates hyperthyroidism. This condition is not uncommon in cats of this age and can easily be managed with medication in pill form (methimazole/tapazole), which is a tiny pill, easy to hide in a treat or crush and put into a small amount of a favorite canned cat food. This medication can also be compounded into a liquid suspension with a palatable, flavored base that is tasty to cats. In addition, it can be compounded into a cream or gel to rub inside the pointy part of the cat's ear and absorbed by the skin.
If there is any doubt as to the validity of the test, a Free T4 is recommended as the next step.
If the cat is hyperthyroid, the cat definitely needs to be put on medicine, to lower the thyroid level. Hyperthyroidism can adversely affect the cat's kidneys, heart, and blood pressure and starting the cat on medication now, if hyperthyroidism is suspected, due to the blood results reading, going 2 months until further testing can adversely affect the cat's health.
All medications carry side effects and some cats experience them and some do not. I, personally had a hyperthyroid cat who was also diabetic. He got 1/2 of a methimazole tablet twice daily and experienced no side effects.
If your sister's cat is also exhibiting outward symptoms of hyperthyroidism, like always seeming hungry, yet losing weight, vomiting more than usual, hypervocalizing, mainly at night, having an unkempt looking coat, drinking more than usual, then, the cat needs medication.
There is also a food made by Science Diet, called y/d which is made for hyperthyroid cats.
Here is more information about this disease:
I hope your sister makes the right decision and all will be well with her kitty!
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