5 Causes of Weight Loss in Cats (And What to Do)

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Here are 5 causes of weight loss in cats:

  • Ageing
  • Hyperthyroidism
  • Stress
  • Plant based diet
  • Constipation

Being underweight is a sign of underlying illness, which may need addressing with vet support.

Thankfully, there’s a few easy ways to get your cat eating. Find out more in our guide below to help your cat gain weight.

I am not a veterinarian and I recommend seeking the advice of a vet for any further questions. This article is not intended as a replacement for medical advice.

5 Causes of Weight Loss in Cats

1. Ageing

Old cats tend to lose weight.

This is due to digestive and appetite changes. 12 is the age cats tend to experience decline.


Try the following:

  • Energy dense wet cat food
  • Increasing protein (aim for >40% dry matter)
  • More food bowls
  • Easier access to bowls
  • Warming food to improve taste

Learn more:

A senior cat
Senior cats tend to lose weight and need dietary support

2. Hyperthyroidism

Hyperthyroidism is a problem where cats make too much thyroid hormone.

Thyroid is involved in metabolic rate.

The more thyroid, the higher the metabolism – which causes weight loss.

Other symptoms include:

  • Increased appetite
  • Unkempt appearance
  • Rapid breathing
  • Increased urination

Diagnosis is done with bloodwork.


If diagnosed, vets will prescribe medication and/or hyperthyroid cat food.

The latter treatment is done with iodine restricted cat food. This helps reduce thyroid hormone.

Cat food
Iodine restriction is recommended for hyperthyroidism in cats

3. Stress

Like humans, cats get stressed.

Changing environment or being in a shelter are stressful experiences for cats.

Signs of stress include:

  • Excess grooming
  • Loss of appetite
  • Toileting outside the litter box


A predictable routine helps with stress management in cats.

Having consistent feeding times, keeping a clean litter box, and avoiding boredom help.

Cat toys help with boredom.

Researchers also tested offering cats hiding boxes, which helped lower stress. A simple cardboard box with bedding can help offer a ‘safe space’.

A cat in a cardboard box
Boxes are a great way to help lower stress in cats

4. Plant Based Diet

These days, plant based diets are becoming more popular.

Some cat owners might opt to put their cats on a vegan or plant based diet for ethical reasons.

Cats are carnivores though, and have a harder time getting in the nutrients they need with a plant based diet.

Case studies report cats with weight loss on home prepared vegan diets.


At this point, I don’t recommend vegan diets.

There’s not enough research on best practices and how to best create a diet that meets a cats needs.

Read more:

A cat with vegetables
Vegan diets can cause weight loss in cats due to a lack of nutrition

5. Constipation

Constipation is a difficulty passing feces.

It’s common in middle-age male cats and can result in loss of appetite.


Vets may prescribe medications like laxatives and/or enemas to treat the problem.

Long term, dietary change is worth investigating.

Research is mixed on what to do, but the idea is to trial different fiber types to help. Insoluble fiber adds bulk, whilst soluble fiber softens feces.

Soluble fiber seems most promising, using psyllium powder.

1-4 tsp psyllium with low residue wet cat food helped a group of 14 cats enter remission in one study.

Learn more:

A cat with a litter tray
Constipation can cause weight loss in cats due to a lack of appetite


There are many causes of weight loss in cats.

In this article, I covered the following 5:

  • Ageing
  • Hyperthyroidism
  • Stress
  • Plant based diet
  • Constipation

The best first step is to go see a vet if adjusting diet doesn’t help. This will help diagnose the problem and get the right solution for your cat.

Ageing is the most likely cause of weight loss in cats after the age of 12.

Support ageing cats with more food bowls and easy-to-digest high calorie cat food.

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I'm qualified dietitian that's turned their attention to cat nutrition. My goal is to help tease out the science on how best to feed your cat.