Hyperthyroidism happens when thyroid tissue becomes overactive and produces too much thyroid hormone. In most cats, this is caused by benign enlargement of the thyroid tissue rather than cancer, but the effect on the body can still be significant.
Because thyroid hormone influences metabolism, appetite, heart function, blood pressure, and many other body systems, an overactive thyroid can place ongoing strain on the whole body if left untreated.
The thyroid glands sit in the neck and help regulate the body’s metabolic rate. When too much thyroid hormone is produced, the body is effectively pushed into overdrive.
This means the cat may burn energy too quickly, behave differently, eat more while still losing weight, and place extra strain on organs such as the heart and kidneys.
Many owners first notice that an older cat is losing weight despite eating well, or even eating more than usual. Others notice increased thirst, more urination, restlessness, vomiting, a rougher coat, more vocal behaviour, or a cat that simply seems more unsettled and less like themselves than before.
Some cats become more active and demanding. Others seem more fragile and harder to keep in good condition.
Hyperthyroidism can make a cat look both hungry and unwell at the same time. Because some affected cats seem bright, food-driven, and active, owners do not always realise that the weight loss and behaviour change reflect a medical problem.
The signs can also overlap with other age-related conditions, which is one reason diagnosis matters rather than assumption.
Untreated hyperthyroidism places ongoing strain on the heart, kidneys, blood pressure, and general metabolic stability of the cat. Over time, this can worsen quality of life and contribute to more serious complications.
Treatment is not only about controlling weight loss. It is about removing or managing the overactive thyroid tissue so the whole body is no longer being pushed harder than it should be.
Diagnosis usually involves clinical history, physical examination, and blood testing. In many cats, the diagnosis becomes clear from the combination of weight loss, appetite change, age, and thyroid hormone levels on blood work.
Further assessment may also be useful to understand how the condition is affecting the kidneys, heart, blood pressure, and overall health before treatment decisions are made.
Hyperthyroidism in cats can be managed in different ways, including medication, prescription diet in selected situations, or radioactive iodine treatment.
Medication and diet may help control the condition, but they do not remove the abnormal thyroid tissue itself. Radioactive iodine, also known as I-131, directly targets the overactive thyroid tissue and is considered the gold-standard treatment because it offers a safe and often permanent cure in most cases.
I-131 treats the disease at its source rather than relying on long-term daily control. For many cats, that means one treatment, no surgery, no anaesthesia, and the possibility of resolving the condition without ongoing medication for life.
That is one reason many owners and referring veterinarians consider it such an important option.
If your older cat is losing weight, eating differently, drinking more water, or has been diagnosed with hyperthyroidism, we can help talk through what the condition means and whether treatment options such as I-131 are likely to be appropriate.
Learn how radioactive iodine treatment works, what the process involves, and why it is considered the gold-standard treatment for feline hyperthyroidism.
Learn why weight loss in older cats is an important sign and how thyroid disease can be one of the underlying causes.
Learn why increased thirst in older pets can be an important clue to underlying disease, including conditions such as hyperthyroidism.
