A senior cat that eats more and loses weight: what guidelines say about hyperthyroidism — not a second youth

Excess thyroid hormone speeds whole-body metabolism. Cornell and AAFP list weight loss (even with normal or increased appetite), polyuria/polydipsia, restlessness or increased activity, and vomiting or diarrhea among common signs.[1][3] Night vocalizing and pacing sometimes get framed as “getting younger.” That can be the outside of a faster metabolism — not a wellness marker.

Each of those signs has other differentials, including CKD, diabetes, and gastrointestinal disease. AAFP stresses that concurrent kidney and heart disease are common; assessment must be whole-patient, not a single number.[1] Cornell likewise asks clinicians to evaluate the heart and kidneys.[3]

Cornell states that most hyperthyroid cats have elevated blood T4; a small percentage remain within the reference interval and may need additional tests (for example free T4) if clinical suspicion remains high.[3] Cornell’s Animal Health Diagnostic Center also notes that T4 is affected by drugs, concurrent disease, and nutrition — interpretation belongs to the veterinarian.[4]

This page does not discuss drug doses, radioiodine, or surgery. Treatment choice and monitoring intervals belong to a licensed veterinarian, given comorbidities and labs.[1][3]