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This is Dr. KV. I'm sorry to hear Zion isn't feeling well. Vocalizing can be a sign of hyperthyroidism, but it is also possible that she is crying due to pain in her mouth from bad teeth. Crying in the litter box usually connects with constipation....many older kitties suffer from kidney disease which leads to dehydration and constipation.
It is a very tough decision to decide to euthanize your sweet kitty, but sometimes it is the most humane option. I recommend taking her in to your vet for a "quality of life" exam. Your vet may be able to tell you if they feel it is time to put her down, or they may be able to suggest inexpensive medications and supplements to keep her comfortable and improve her quality of life.
Feel free to reach out if I can assist you further.