What Diseases Cause Rapid Weight Gain?

In most cases, patients will gain weight from eating or drinking more calories than are burned throughout the day. In some cases, however, weight gain may the result of an underlying health condition. This article will review some of the most common medical issues and diseased which can cause unexpected weight gain.

Underactive Thyroid

An underactive thyroid, or hypothyroidism, means the thyroid gland is not producing an adequate amount of thyroid hormones. These hormones play a critical role in regulating a patient’s metabolism. While underactive thyroids are possible for patients of any age or sex, they are most common in older women.


It is common for patients who are taking insulin to manage diabetes to experience weight gain. Insulin aids in controlling blood sugar levels. Some people who have diabetes eat more than is needed in order to avoid low blood sugar. This is also called "hypo" or hypoglycaemia.

Weight Gain with Age

As people get older, they start to lose small amounts of muscle. This is typically occurs because the patient is less active. Muscles are quite efficient at burning calories. The loss of muscle mass often means fewer calories are being burned. When patients continue to eat and drink the same amount and reduce their physical activity, it will most likely result in weight gain. In order to reduce muscle loss and minimize weight gain, it is important to remain active and continue to perform muscle-strengthening activities.


Steroids, which are also called corticosteroids, are commonly used to treat asthma, arthritis, and other conditions. Extended use of steroids has demonstrated to cause an increase appetite in some patients which often leads to a weight gain. Patients should not reduce or stop taking steroids midway through the treatment. If there is a concern with weight gain, be sure to discuss this with your doctor.


The results from some studies found that people who had less than 7 hours of sleep each day were more likely to be overweight compared to those who slept for 9+ hours. While the association is unclear, people who are sleep-deprived have be found to have lower levels of leptin. This is the chemical which makes people feel full. In addition, the sleep deprived patients had higher levels of ghrelin, which is the hormone which stimulates hunger. People who are chronically tired are more likely to snack and make poor food choices. They are also less likely to be physically active, meaning they burn fewer calories. All of which, can result in weight gain.

Fluid retention

Fluid retention, or oedema, results in certain parts of the body to become swollen. This will ultimately result in weight gain. The weight gain is caused by an accumulation of fluid in the body. In some cases, fluid retention may be common and normal. This can include situations when you are standing for extended periods of time or prior to having a menstrual cycle. With oedema or more chronic fluid retention, the swelling may occur in a specific are of the body, or may be more general and widespread. When the fluid retention is more severe, it may also cause breathlessness.

Polycystic Ovary Syndrome (PCOS)

PCOS is a condition which impacts the function of the ovaries. Common symptoms of the condition include irregular periods, difficulty in getting pregnant, excess hair growth and unexpected weight gain. While the specific cause of PCOS is unknown, it may be related to hormone levels such as a surplus of insulin and testosterone. It is common for women with PCOS to specifically gain weight around their mid section.

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