Unexplained weight gain can be frustrating and difficult to understand. Because there are many possible causes, such as hormone imbalances, thyroid disorders, chronic stress, and lack of sleep, it can be challenging to know why it’s happening—especially if there are multiple factors that could be causing you to gain weight.

Here are some common causes of unexplained weight gain that might help you find out what’s going on:

• Chronic stress - Long-term or chronic stress can be related to unexplained weight gain. One possible reason? Stress can change your food preferences—making you crave foods high in fats and sugar. Many factors can contribute to chronic stress, but social stressors—like interpersonal conflicts or toxic work environments—are some of the most common drivers [1]. Test for indicators of chronic stress.

• Thyroid disorders - Hypothyroidism—a condition involving an  underactive thyroid gland—has been linked to weight gain [2]. This thyroid disorder is especially common in women [3] and in people over 60 [4]. The most common cause of hypothyroidism in U.S. adults is Hashimoto’s disease. Test your thyroid with our test.

• Lack of sleep - Not getting enough sleep and poor sleep quality are both potentially linked to weight gain [5]. One common cause of low sleep quality is artificial lighting. Exposure to light affects your body’s sleep-regulating hormones—like melatonin—so scrolling through your phone or watching TV just before bedtime can make it challenging to fall asleep and/or disrupt your sleep quality. Check in on your sleep hormone levels.

• Diet - Ultra-processed foods like refined sugar, corn syrup, and hydrogenated fats are connected to weight gain [6, 7]. These foods can promote overeating and are packed with calories that often aren’t burned with regular exercise. When you don’t burn as many calories as you take in, weight gain can occur.

• Medication side effects - Antipsychotic medications can also lead to changes in your weight or rapid weight gain [8]. Other medications that can sometimes induce weight gain include some antidepressants, mood stabilizers, and corticosteroids [9]. (Note: if you believe a medicine you’re taking may be contributing to weight gain, be sure to talk with your healthcare provider.)

Weight gain and obesity have been connected with the following health conditions:

• Testosterone deficiency in men [10]

• Hypothyroidism (underactive thyroid)

• Chronic stress

• Sleep disorders [11]

• Fluid retention (also called edema, which can result from kidney dysfunction) [12]

• High blood pressure (also known as hypertension) [13]

• Heart disease

• Diabetes

• High cholesterol

Not sure where to start? Here are some helpful weight management tips.

Increase physical activity

Consistent exercise is key—and can even be something you look forward to. Take walks, play team sports, do yoga, or whatever activity you genuinely enjoy. 

The Physical Activity Guidelines for Americans recommends a minimum of 150 minutes of moderate intensity aerobic activity per week. Muscle-strengthening activities—like push-ups or weight lifting—are also important to do at least two days a week because they help support and maintain your muscle mass [15].

Eat a well-balanced diet

Eating a nutritionally balanced diet that isn’t too high in calories may help with weight management. A balanced diet primarily consists of grain products—like bread, pasta, cereal, and rice—plus fruits and vegetables, moderate amounts of meat and dairy products, and only limited amounts of foods that are high in fat or sugar [16]. 

How many calories you should consume each day depends on several factors, such as your weight loss goal, so it’s a great idea to talk about calories and other dietary recommendations with a registered dietitian.

What causes sudden weight gain? 

Rapid or unexplained weight gain can be caused by several kinds of prescription medications—including antipsychotics, mood stabilizers, and antidepressants. Other causes of weight gain—such as chronic stress or eating a diet high in ultra-processed foods—may not result in sudden weight increases but instead affect your weight in the long-term.

What causes unintentional weight gain?

Unintentional weight gain can have several potential causes—including some thyroid disorders, lack of sleep, and chronic stress. 

Determining the reasons for unexplained weight gain can be hard, but lab tests can help—so consider taking an Everlywell at-home test that measures markers connected to weight gain.