Are diabetes, thyroid diseases, child births common causes of obesity?

Obesity. Obesity is the main cause of diabetes type 2. Thyroid disease can cause weight gain or weight loss, depending on what it's doing. Obesity does not cause child births.
No. It is the other way around: obesity can lead to type 2 diabetes, and is associated with a higher risk for developing type 1 diabetes. Hypothyrodism is not responsible for obesity, especially when thyroid function tests become normal with levothyroxine treatment.