Can weight gain be due to underlying health conditions?

Question

According to the NHS (United Kingdom):

Most people put on weight because they eat and drink more calories than they burn through everyday movement and body functions.

But in some cases, your weight gain may be due to an underlying health condition. Here are nine medical reasons that can cause weight gain.

Is it true that most people put on weight because they consume more calories than they burn?

Is it also true that weight gain may be due to an underlying health condition?

Answer 1

Obesity might be caused very rarely by underlying medical conditions as mentioned below.

1. Hypothyroidism (Underactive thyroid): Condition in which the thyroid gland cannot produce optimum levels of thyroid hormone for normal hormonal activity. Harvard med education states "hypothyroidism is seldom the main reason for overweight or obesity. Treatment with thyroid hormone, while medically necessary, does not usually cause a significant weight reduction." In adults, hypothyroidism is associated with obesity, although a cause and effect relationship is not clearly established and thyroid studies may be helpful to determine a diagnosis.

Per Robabeh Ghergherehchi in 2015, "it is presumed that free T4 and thyroid-stimulating hormone (TSH) levels are related to obesity, but the findings are inconsistent."

Although the exact underlying mechanism is not understood, the observation that weight loss leads to decreased TSH levels implies that an increase in TSH in subjects with obesity is reversible and seems to be a consequence rather than a cause of obesity

2. Cushing's syndrome: Condition in which the body's adrenal glands produce high levels of the hormone cortisol. People who have Cushing's syndrome gain weight, have upper-body obesity along with other symptoms such as a rounded face, fat around the neck, and thin arms and legs. Per Kyu Yeon Hur et.al. in 2015,

Prolonged and inappropriately high glucocorticoid levels are associated with systemic abnormalities in metabolic (central obesity, weight gain, hyperglycemia, dyslipidemia, hypokalemia) functions.

3. Polycystic ovarian syndrome (PCOS): PCOS is a condition that affects about approximately 5 to 10 percent of women of childbearing age. Women who have PCOS often are obese, have excess hair growth and reproductive problems or other health issues. These problems are caused by high levels of hormones called androgens. Per Ginger Allen in 2011, "because of the association between obesity and polycystic ovary syndrome (PCOS) in women, a detailed menstrual history and evaluation for hyperandrogenic symptoms is warranted."

However, in the vast majority of people through research, the above mentioned illnesses are directly not responsible for weight gain since they are extremely rare conditions known to cause obesity and underlying metabolic and genetic causes of obesity are uncommon. Per summarization by Bray GA in 1999,

For most patients, however, it is not possible to connect obesity to a specific cause. Leptin deficiency and defects in the leptin receptor both produce human obesity. Defects in the pro-opiomelanocortin receptor system, the peroxisome proliferator-activated receptor-gamma, the agouti-related peptide, and a few other rare genetic syndromes are also associated with human obesity. Of the genetic causes, Prader-Willi syndrome is the most common. Hypothalamic injury following craniopharyngioma is the most common neuroendocrine cause. Endocrine disorders such as Cushing's disease, polycystic ovary disease, and growth-hormone deficiency can lead to increased body fat.

Answer 2

I have done a bit more research on my own question and I think I can answer the question now. Specifically I was interested in:

Is it true that most people put on weight because they consume more calories than they burn?

I got access to some medical journals and found several articles and books on obesity and weight management. The hard thing about the question posted above is that search for information about weight gives a lot of articles about policy making and related issues. But, I have found that in the background section the point is often made that weight gain is ultimately a result of taking more calories in than one uses. To quote a couple of sources:

According to Weighing the Options: Criteria for Evaluating Weight-Management Programs

Obesity is a heterogeneous disease in which genetic, environmental, psychological, and other factors are involved. It occurs when energy intake exceeds the amount of energy expended over time (page 38).

Also, Exercise in weight management of obesity. states

Simply stated, weight reduction depends upon energy intake compared to energy expenditure. (page 463)

Also, Current views on obesity states:

The ultimate cause of obesity is an imbalance between caloric intake and energy expenditure, but the pathologic mechanisms that lead to this imbalance are still not understood.(page 231)

With this in mind the important question for many is how to expend more calories than one intakes in a safe and healthy way. I see other questions dealing with this. For example https://skeptics.stackexchange.com/a/23302/25659 makes the point that drinking water before a meal can make you feel full and therefore can cause you to intake fewer calories.

So with respect to the question

Is it also true that weight gain may be due to an underlying health condition?

the answer is yes. The other answer makes this clear. There are a lot of factors that determine how many calories you expend and how well you absorb and store calories from the food you take in. To add to what the other answer gives, stress can for example being a "cause" of people gaining weight. As

Obesity in Women states:

There is increasing evidence that psychological stress plays a role in the development and maintenance of obesity. ... Individual differences in stress sensitivity and the hypothalamic-pituitary-adrenal (HPA) axis affect stress-induced eating behavior in women. (page 428)

(Not trying to pick on women here.)