Is It Sass…or a Symptom? Understanding Behavioral Changes in Cats with Hyperthyroidism
When your cat’s mood changes, it could be more than a phase.
Of course your cat has an attitude. That’s part of their charm! But when the sass turns into midnight zoomies, random outbursts, or sudden clinginess, it might be more than personality.
Behavioral changes in cats with hyperthyroidism are often one of the first signs something’s off, yet they’re some of the most overlooked. Increased irritability, nervousness, and vocalization are well-documented symptoms, but easy to chalk up to aging or quirks.
This blog is for the curious, concerned, and cat-obsessed. Whether you’re a pet parent, veterinary professional, or just wondering why your senior cat is suddenly acting strange.
At the Center for Advanced Thyroid Treatment (CATT), we specialize in I-131 therapy for hyperthyroid cats. But we also understand that hyperthyroidism in cats behavior changes everything, including how your cat feels, interacts, and connects.
This blog covers:
Emotional and behavioral signs of thyroid issues in cats
How treatment can improve your cat’s mood and behavior
What you can do at home to support their emotional health
If something feels off, trust your gut. Let’s take a closer look at what your cat’s mood might be trying to tell you!
Common Behavioral Changes in Cats with Hyperthyroidism
When cats don’t feel well, they often don’t show it the way we expect. Instead of limping or whining, they may hide, act jumpy, or even lash out. That’s especially true for cats with hyperthyroidism, where a surge in thyroid hormones can affect both the body and the brain.
Here are some of the most common ways hyperthyroidism can alter your cat’s behavior:
Increased vocalization – Many cats become more talkative, especially at night. This can stem from anxiety, confusion, or restlessness.
Irritability or aggression – A normally sweet cat may swat or hiss when touched. This isn’t a personality change—it’s a symptom.
Hyperactivity or nervous energy – You may notice pacing, zooming, or a general inability to settle.
Sudden clinginess or withdrawal – Some cats follow their owners obsessively, while others hide or avoid contact.
Changes in litter box habits or grooming – While these can be physical, they also reflect discomfort or emotional distress.
If your cat’s mood or habits have shifted and you can’t identify a clear cause, a thyroid issue may be the root. Let’s take a closer look at the emotional side of feline hyperthyroidism and how to recognize what’s really going on.
Emotional Symptoms of Hyperthyroidism You Might Miss
Cats with hyperthyroidism may hide, withdraw, or seem less interested in interaction.
Not all symptoms of feline hyperthyroidism show up in lab results or even in the physical body. Many cats experience emotional and behavioral changes that are just as real but much easier to overlook.
Here are a few of the most commonly missed emotional signs of hyperthyroidism in cats:
Anxiety or skittishness – Your cat may jump at small sounds, seem hyper-alert, or pace without resting.
Disrupted sleep – Restlessness or nighttime activity can be a sign that your cat is physically uncomfortable or emotionally unsettled.
Neediness or separation anxiety – Some cats become suddenly clingy, meowing for attention or following you from room to room.
Depression-like behavior – Others may retreat, hide, or seem less interested in interaction, even with their favorite people.
These shifts often get chalked up to “getting older” or “being in a mood,” but they can be early indicators that your cat’s thyroid is out of balance.
Just like humans, cats respond emotionally to internal imbalances. When their hormones are off, their sense of safety and stability can be too. That’s why it’s so important to trust your gut when you feel something is off, even if your cat doesn’t seem “sick.”
Why Hyperthyroidism Messes With Your Cat’s Mood
Feline hyperthyroidism makes your cat feel overstimulated and it starts with hormone overproduction in the brain.
Hyperthyroidism causes your cat’s thyroid gland to produce too much thyroid hormone (T4), which speeds up their metabolism and overstimulates multiple systems in the body, including the brain.
The result? More adrenaline, more cortisol, and a nervous system that’s stuck in overdrive.
That internal chaos often shows up in behavior and it’s easy to mistake it for mischief or moodiness. Their body is telling them to fight or flee, even if there’s no threat around.
Here’s the bright side: Understanding this can be a relief. It means your cat’s behavior isn’t bad or the result of aging. It’s a treatable symptom of a real medical condition.
How Treating Hyperthyroidism Can Improve Mood & Behavior
One of the most rewarding parts of treating hyperthyroidism is watching a cat’s true personality re-emerge.
As thyroid hormone levels stabilize, especially after I-131 therapy, those emotional and behavioral shifts often begin to fade. Cats who were once restless or withdrawn frequently become more relaxed, affectionate, and engaged.
You might notice:
Nighttime pacing and yowling subside
Cuddling and physical touch become welcome again
Familiar habits like sunbathing or bird-watching start to return
A sense of calm and emotional balance settles in
That’s because effective treatment doesn’t just improve lab results, it gives the whole body a break. The nervous system calms. The body stops working against itself. And the brain finally finds quiet after weeks or months of overstimulation.
At CATT, we’ve seen it firsthand. Anxious, unsettled cats transform into calm, connected companions. I-131 radioiodine therapy can seem like magic. But it’s simply medicine that works.
Why Radioactive Iodine Therapy Helps Your Cat Feel Like Themselves Again
Radioiodine therapy can help restore your cat’s health, mood, and quality of life.
Radioactive iodine therapy (I-131) is the only treatment that targets the source of feline hyperthyroidism: the overactive thyroid tissue. Unlike daily medications or prescription diets, I-131 is a one-time injection that restores normal hormone levels without ongoing stress or side effects.
So what does that mean for your cat’s emotional well-being?
No more daily pilling or battles at mealtime
No more rollercoaster mood swings from fluctuating thyroid levels
And most importantly, a cat who feels safe, steady, and at peace again
Many pet parents report their cat seems happier, more affectionate, and back to their usual habits within weeks of treatment. That’s because I-131 allows your cat’s entire system (body, brain, and behavior) to reset and heal.
At CATT, this is what we do every day. We’ve helped hundreds of hyperthyroid cats reclaim their health—and their personalities.
How to Reverse Behavioral Changes in Your Cat
Mood changes, strange behaviors, and emotional shifts can be powerful signals that something deeper is going on. Hyperthyroidism doesn’t just affect your cat’s body, it changes how they experience the world.
But you don’t have to settle for temporary fixes or daily struggles. Radioactive iodine (I-131) therapy is the safest, most effective way to treat hyperthyroidism at the source and help your cat get their spark back. We’ll guide you through every step, answer your questions, and help your cat reclaim the comfort, calm, and connection they deserve.
Contact us today if you’re seeing signs of cat behavioral change and suspect thyroid disease.
