If you have a work permit in Thailand or you have a 1 year marriage visa or a retirement visa then you cannot leave Thailand without a re-entry permit. This is a permit that is placed in your passport to ensure that when you leave Thailand you do no invalidate you visa status. With the re-entry permit in your passport you can enter Thailand and continue on the ‘old’ visa you had before you left.
The single entry re-entry permit costs 1,000 Baht but if you are going to leave Thailand more than once in the space of a year then you are best served with a multiple entry re-entry permit which costs 3,800 Baht. Consider the options and you will note that it is better to obtain the re-entry permit. This can be issued at the border or at the airport but it is always best to have it done in Bangkok as it takes about 3 hours to have it done and you don’t want to miss your flight.