The possible reasons for a hamster to become smaller (lose weight) are as follows:
Natural Aging
As hamsters age, their gastrointestinal functions gradually deteriorate. Their ability to absorb nutrients declines, causing them to lose weight. Moreover, signs of aging may include a hunched back, hair loss, decreased appetite, and increased sleepiness.

Illness
If a hamster suddenly shows a loss of appetite and weight loss, it may be sick. Common diseases include diabetes and wasting diseases. These ailments can lead to a significant drop in the hamster's weight, often accompanied by symptoms such as abnormal breathing and sluggish movement.
Environmental Changes
A sudden change in environmental temperature, for example, a temperature drop, can impact a hamster's appetite, resulting in weight loss. Additionally, frequently changing items in the living environment, like bedding and food, may make the hamster feel uneasy, thus affecting its appetite and living habits.
Dietary Issues
An unbalanced diet or a lack of proper nutrition can cause a hamster to become thin. Picky - eating hamsters that only eat certain foods and overlook other nutritionally balanced options will also suffer from malnutrition.
Countermeasures
- Adjust the Diet Structure
Make sure the hamster's diet is nutritionally balanced. Provide a diverse range of foods to prevent malnutrition caused by relying on a single type of food. You can try feeding the hamster with rodent food containing probiotics to aid digestion and absorption.
- Keep the Environment Stable
Try to maintain a stable living environment for the hamster. Avoid frequently changing the bedding, food, etc., and create a comfortable living space for them.
- Seek Veterinary Help Promptly
If you notice abnormal symptoms in the hamster, such as a loss of appetite or weight loss, take it to the veterinarian immediately to avoid delaying treatment.
- Pay Attention to Health Conditions
Carefully observe the hamster's health conditions, including its mental state and appetite. Detect and address problems in a timely manner.