My cat was recently diagnosed with feline hyperthyroidism. I've been giving her the prescription of 2 pills/day (morning and night). The last couple of days she's been shaking her back legs like they're wet (but they're not). I've looked to see if there's something there (sticky, etc.), but there isn't. Now she's hiding under the loveseat which is not normal behavior. Could she have another problem like a heart issue or renal failure? Her appetite is also depressed. Thank you for any input or advice.
Your cat could certainly have a heart issue or kidney failure. Hyperthyroidism in cats causes a thickening of the lining of the heart, and the treatment of feline hyperthyroidism can cause kidney insufficiency as the thyroid levels become regulated.
In my experience, 2 tablets per day is a high dosage for a recently diagnosed feline hyperthyroid cat. Regardless, did your veterinarian have you schedule a recheck after 3 weeks on the medication? This is very important, as your cat's kidney values need to be checked regularly in addition to monitoring her thyroid levels.
Please make an appointment to recheck your cat's bloodwork for feline hyperthyroidism as soon as possible. A cat in kidney failure can die quickly if the issue is not addressed.
All the best,