Genetic: Obesity tends to run in families. This is caused both by genes and by shared diet and lifestyle habits. However, having obese relatives does not mean that you have to be obese. You may still prevent obesity by adhering to healthy diet and exercise.
Thyroid disesase: Underactive thyroid can cause overweight; overactive thyroid causes underweight. Make sure to check a screening blood test. Start with blood TSH level (Thyroid Stimulating Hormone) and consult your doctor. You will never know if your thyroid is normal unless you check this simple blood test. This is an often overlooked condition.
Diabetes can cause weight changes. Check fasting blood sugar and consult your doctor.
Depression can lead to overeating and obesity.
Certain medications: Corticosteroids and tricyclic antidepressants, in particular, can lead to weight gain. So can some high blood pressure and antipsychotic medications.
Some medical problems, such as arthritis, can lead to decreased activity, which may result in weight gain.
Other very rare medical causes include: Cushing’s syndrome or excess production of hormones by the adrenal glands; increased growth hormone secretion by pituitary gland; polycystic ovarian syndrome etc.. These are really extremely rare conditions and not worth considering for most people. Your doctor will be able to pick these up if you have it.