Can excessive sweating and weight gain be due to a thyroid disease ?

Unlikely: Excessive sweating can be seen with an overactive thyroid, but associated with other signs/symptons. Weight gain is more common w/hypothyroidism, again associated with other signs/symptoms. Your doctor can reassure you with a simple blood test if you're concerned.

Hypothyroidism : Yes. It would be a good idea to see your primary care physician and have a routine thyroid screening. It is a simple blood test. There are many symptoms associated with thyroid disease. If you have noticed weight gain without actively trying to do so, or without a significant change in your exercise, or eating habits, it could potentially be related to a thyroid issue.