A few illnesses that are characterized by an imbalance or an abnormality in your endocrine glands can also affect your weight. These include hypothyroidism (an underactive thyroid), polycystic ovarian syndrome, and certain unusual tumors of the pituitary gland, adrenal glands, or the pancreas. However, in the vast majority of people, these illnesses are not responsible for weight gain. Most are extremely rare. Hypothyroidism, which is the most common, is seldom the main reason for overweight or obesity. Treatment with thyroid hormone, while medically necessary, does not usually cause a significant weight reduction.
Genetic disorders
Obesity is also a symptom of some rare and complex disorders caused by genetic defects. These obesity syndromes usually appear in early childhood and are tied to several additional medical problems. One such disorder is Prader-Willi syndrome, a form of obesity associated with mild mental retardation that occurs in about 1 in 25,000 people and has been traced to abnormalities in a group of genes on chromosome 15. People with this disorder are unusually short and have primarily upper-body obesity. A less common disorder, Bardet-Biedl syndrome, is similar to Prader-Willi syndrome, but is caused by abnormalities in different genes. Several other rare genetic syndromes cause obesity, but account for only a tiny fraction of all weight disorders.
In particular these two paragraphs:
Illnesses that affect weight
A few illnesses that are characterized by an imbalance or an abnormality in your endocrine glands can also affect your weight. These include hypothyroidism (an underactive thyroid), polycystic ovarian syndrome, and certain unusual tumors of the pituitary gland, adrenal glands, or the pancreas. However, in the vast majority of people, these illnesses are not responsible for weight gain. Most are extremely rare. Hypothyroidism, which is the most common, is seldom the main reason for overweight or obesity. Treatment with thyroid hormone, while medically necessary, does not usually cause a significant weight reduction.
Genetic disorders
Obesity is also a symptom of some rare and complex disorders caused by genetic defects. These obesity syndromes usually appear in early childhood and are tied to several additional medical problems. One such disorder is Prader-Willi syndrome, a form of obesity associated with mild mental retardation that occurs in about 1 in 25,000 people and has been traced to abnormalities in a group of genes on chromosome 15. People with this disorder are unusually short and have primarily upper-body obesity. A less common disorder, Bardet-Biedl syndrome, is similar to Prader-Willi syndrome, but is caused by abnormalities in different genes. Several other rare genetic syndromes cause obesity, but account for only a tiny fraction of all weight disorders.