Person standing on bathroom scale wondering about the causes of weight gain

Weight Gain Explained: Causes and More

Table of contents

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.

Causes of Weight Gain

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].
  • 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. Take our thyroid 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.
  • 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

Weight Management Tips

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 [14].

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 [15].

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.

Everlywell Weight Loss Support

Common Questions About Weight Gain

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.

Why Am I Gaining Weight When I Barely Eat?

What Causes Obesity in America?

8 Possible Reasons Why You’re Experiencing an Increased Appetite


  1. Scott KA, Melhorn SJ, Sakai RR. Effects of Chronic Social Stress on Obesity. Curr Obes Rep. 2012;1(1):16-25. doi:10.1007/s13679-011-0006-3
  2. Mullur R, Liu YY, Brent GA. Thyroid hormone regulation of metabolism. Physiol Rev. 2014;94(2):355-382. doi:10.1152/physrev.00030.2013
  3. Vanderpump MP, Tunbridge WM. Epidemiology and prevention of clinical and subclinical hypothyroidism. Thyroid. 2002;12(10):839-847. doi:10.1089/105072502761016458
  4. Hypothyroidism (Underactive Thyroid). National Insititute of Diabetes and Digestive and Kidney Disease. Accessed February 1, 2024.
  5. Ogilvie RP, Patel SR. The epidemiology of sleep and obesity. Sleep Health. 2017;3(5):383-388. doi:10.1016/j.sleh.2017.07.013
  6. Hall KD, Ayuketah A, Brychta R, et al. Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain: An Inpatient Randomized Controlled Trial of Ad Libitum Food Intake [published correction appears in Cell Metab. 2019 Jul 2;30(1):226] [published correction appears in Cell Metab. 2020 Oct 6;32(4):690]. Cell Metab. 2019;30(1):67-77.e3. doi:10.1016/j.cmet.2019.05.008
  7. NIH study finds heavily processed foods cause overeating and weight gain. National Institutes of Health. Accessed February 1, 2024.
  8. Curtis J, Watkins A, Rosenbaum S, et al. Evaluating an individualized lifestyle and life skills intervention to prevent antipsychotic-induced weight gain in first-episode psychosis. Early Interv Psychiatry. 2016;10(3):267-276. doi:10.1111/eip.12230
  9. Wharton S, Raiber L, Serodio KJ, Lee J, Christensen RA. Medications that cause weight gain and alternatives in Canada: a narrative review. Diabetes Metab Syndr Obes. 2018;11:427-438. Published 2018 Aug 21. doi:10.2147/DMSO.S171365
  10. Grossmann M. Hypogonadism and male obesity: Focus on unresolved questions. Clin Endocrinol (Oxf). 2018;89(1):11-21. doi:10.1111/cen.13723
  11. Hargens TA, Kaleth AS, Edwards ES, Butner KL. Association between sleep disorders, obesity, and exercise: a review. Nat Sci Sleep. 2013;5:27-35. Published 2013 Mar 1. doi:10.2147/NSS.S34838
  12. Goldenberg K. Weight Change. In: Walker HK, Hall WD, Hurst JW, editors. Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd edition. Boston: Butterworths; 1990. Chapter 210. Available from:
  13. Panuganti KK, Nguyen M, Kshirsagar RK. Obesity. [Updated 2023 Aug 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from:
  14. Top 10 Things to Know About the Second Edition of the Physical Activity Guidelines for Americans. Accessed February 1, 2024.
  15. Institute of Medicine (US) Subcommittee on Military Weight Management. Weight Management: State of the Science and Opportunities for Military Programs. Washington (DC): National Academies Press (US); 2004. 4, Weight-Loss and Maintenance Strategies. Available from:
Everlywell makes lab testing easy and convenient with at-home collection and digital results in days. Learn More