Hyperthyroidism is the most common hormonal disorder in older cats. When untreated, it causes significant suffering; when managed, most cats regain excellent quality of life.
Feline hyperthyroidism causes significant chronic suffering that owners often misinterpret as normal aging because some signs — increased appetite, apparent energy — seem positive. The underlying reality is that chronically elevated thyroid hormone causes heart strain, hypertension, kidney stress, gastrointestinal distress, and a hyperactivated nervous system that is exhausting rather than energizing. Cats often lose significant muscle mass despite eating well. When properly diagnosed and treated, most hyperthyroid cats experience dramatic quality of life improvement. The challenge is that daily medication requires twice-daily pilling which many cats resist strongly — treatment adherence is a real welfare concern. Radioactive iodine or dietary management may be preferable for cats or owners struggling with daily medication.