Was there a urinalysis performed at the same time as the blood panel? If not I would request one, kidney function can't be fully determined without a urine test.
If the urine was tested I would ask for that & the T4 panel to be retested again in about 3 weeks for a comparison to the one just taken.
Sometimes cats that have beginning stages of kidney insufficiency have good blood results but the Urine Specific Gravity is a tip off that something is going on. Untreated kidney issues can also mask thyroid problems as the kidneys are over working to pump blood through and that makes the thyroid hormone levels look normal.
& let us know what the ultrasound shows